Introduce AccordExecutorSignalLoop that aims to reduce lock contention:

- consumer and producer threads wait signal without acquiring the lock first
 - lock owners may prepare more work than they need, moving some dynamically-adjusted portion of the work from the prioritised lock-managed structures onto a non-blocking queue, so that other threads may consume work from there; the portion is continually micro-adjusted to target some available work whenever the lock is acquired.
 - adopts/supports some features of the SEPExecutor:
  - threads may auto-adjust the number of running threads based on how much time is collectively spent waiting, to minimise time spent signalling
  - consumers may (timed-sleep) poll rather than await a signal, reducing the number of kernel interactions needed; relying on the auto-adjustment to bound the time the scheduler spends waking threads with no work to do
Also Fix:
 - Client Result should not be persisted or included in Command object at any time

patch by Benedict; reviewed by Alex Petrov and Ariel Weisberg for CASSANDRA-21375

.build/checkstyle.xml

@@ -63,6 +63,12 @@
        <property name="influenceFormat" value="0"/>
     </module>
 
+    <module name="SuppressWithNearbyCommentFilter">
+      <property name="commentFormat" value="checkstyle: permit this invocation"/>
+      <property name="idFormat" value="blockInstantNow"/>
+      <property name="influenceFormat" value="0"/>
+    </module>
+
     <module name="RegexpSinglelineJava">
       <!-- block system time -->
       <property name="id" value="blockSystemClock"/>

modules/accord

@@ -1 +1 @@
-Subproject commit e8c70e097f305a5097084079f136d8be7c085b29
+Subproject commit a081e19e33ddeb91040f2ff8f0bef119f57d11fb

src/java/org/apache/cassandra/config/AccordConfig.java

@@ -66,11 +66,15 @@ public class AccordConfig
         /**
          * Same number of threads as queue shards, but the shard lock is held only while managing the queue,
          * so that submitting threads may queue load/save work.
+         *
+         * This is incompatible with QueueSubmissionModel.SIGNAL
          */
         THREAD_PER_SHARD,
 
         /**
          * Same number of threads as shards, and the shard lock is held for the duration of serving requests.
+         *
+         * This is incompatible with QueueSubmissionModel.SIGNAL
          */
         THREAD_PER_SHARD_SYNC_QUEUE,
 
@@ -103,6 +107,14 @@ public class AccordConfig
          */
         ASYNC,
 
+        /**
+         * Queue workers try to avoid competing for the lock, with the lock owner distributing work to any waiting threads
+         * and signalling them without them taking the lock
+         *
+         * NOTE: EXPERIMENTAL
+         */
+        SIGNAL,
+
         /**
          * The queue is backed by submission to a single-threaded plain executor.
          * This implementation does not honor the sharding model option.
@@ -151,8 +163,30 @@ public class AccordConfig
      */
     public volatile OptionaldPositiveInt queue_shard_count = OptionaldPositiveInt.UNDEFINED;
 
+    /**
+     * The total number of threads to share between queue shards
+     */
+    public volatile OptionaldPositiveInt queue_thread_count = OptionaldPositiveInt.UNDEFINED;
+
     public QueuePriorityModel queue_priority_model = HLC_FIFO;
 
+    /**
+     * If set, the signal loop does not match park/unpark pairs, but instead consumers perform timed-park spin waits
+     */
+    public DurationSpec.LongMicrosecondsBound queue_spin_interval;
+
+    /**
+     * If set, the signal loop reduces the number of threads it is using when the time spent parked exceeds real-time
+     * by this interval.
+     */
+    public DurationSpec.LongMicrosecondsBound queue_stop_check_interval;
+
+    /**
+     * If set, the signal loop reduces the number of threads it is using when the time spent parked exceeds real-time
+     * by this interval.
+     */
+    public DurationSpec.LongMicrosecondsBound queue_signal_stop_check_interval_credit;
+
     // yield to other executor threads after executing this many tasks in a row, if there are waiting threads and tasks
     public int queue_yield_interval = 100;
 
@@ -456,4 +490,9 @@ public class AccordConfig
             return version.version;
         }
     }
+
+    public int commandStoreShardCount()
+    {
+        return command_store_shard_count.or(DatabaseDescriptor::getAvailableProcessors);
+    }
 }

src/java/org/apache/cassandra/config/Config.java

@@ -203,7 +203,6 @@ public class Config
 
     public int concurrent_reads = 32;
     public int concurrent_writes = 32;
-    public int concurrent_accord_operations = 32;
     public int concurrent_counter_writes = 32;
     public int concurrent_materialized_view_writes = 32;
     public OptionaldPositiveInt available_processors = new OptionaldPositiveInt(CASSANDRA_AVAILABLE_PROCESSORS.getInt(OptionaldPositiveInt.UNDEFINED_VALUE));

src/java/org/apache/cassandra/config/DatabaseDescriptor.java

@@ -700,9 +700,6 @@ public class DatabaseDescriptor
         if (conf.concurrent_counter_writes < 2)
             throw new ConfigurationException("concurrent_counter_writes must be at least 2, but was " + conf.concurrent_counter_writes, false);
 
-        if (conf.concurrent_accord_operations < 1)
-            throw new ConfigurationException("concurrent_accord_operations must be at least 1, but was " + conf.concurrent_accord_operations, false);
-
         if (conf.networking_cache_size == null)
             conf.networking_cache_size = new DataStorageSpec.IntMebibytesBound(Math.min(128, (int) (Runtime.getRuntime().maxMemory() / (16 * 1048576))));
 
@@ -2900,7 +2897,7 @@ public class DatabaseDescriptor
 
     public static int getAccordConcurrentOps()
     {
-        return conf.concurrent_accord_operations;
+        return conf.accord.queue_thread_count.or(2 * FBUtilities.getAvailableProcessors());
     }
 
     public static void setConcurrentAccordOps(int concurrent_operations)
@@ -2909,7 +2906,7 @@ public class DatabaseDescriptor
         {
             throw new IllegalArgumentException("Concurrent accord operations must be non-negative");
         }
-        conf.concurrent_accord_operations = concurrent_operations;
+        conf.accord.queue_thread_count = new OptionaldPositiveInt(concurrent_operations);
     }
 
     public static int getFlushWriters()
@@ -5635,6 +5632,7 @@ public class DatabaseDescriptor
         return conf.accord;
     }
 
+    // TODO (expected): move all getAccordX into AccordConfig
     public static AccordConfig.TransactionalRangeMigration getTransactionalRangeMigration()
     {
         return conf.accord.range_migration;
@@ -5665,44 +5663,6 @@ public class DatabaseDescriptor
         conf.accord.enabled = b;
     }
 
-    public static AccordConfig.QueueShardModel getAccordQueueShardModel()
-    {
-        return conf.accord.queue_shard_model;
-    }
-
-    public static AccordConfig.QueueSubmissionModel getAccordQueueSubmissionModel()
-    {
-        return conf.accord.queue_submission_model;
-    }
-
-    public static int getAccordQueueShardCount()
-    {
-        switch (getAccordQueueShardModel())
-        {
-            default: throw new AssertionError("Unhandled queue_shard_model: " + conf.accord.queue_shard_model);
-            case THREAD_PER_SHARD:
-            case THREAD_PER_SHARD_SYNC_QUEUE:
-                return conf.accord.queue_shard_count.or(DatabaseDescriptor::getAvailableProcessors);
-            case THREAD_POOL_PER_SHARD:
-                return conf.accord.queue_shard_count.or(DatabaseDescriptor.getAvailableProcessors()/4);
-        }
-    }
-
-    public static int getAccordCommandStoreShardCount()
-    {
-        return conf.accord.command_store_shard_count.or(DatabaseDescriptor::getAvailableProcessors);
-    }
-
-    public static int getAccordMaxQueuedLoadCount()
-    {
-        return conf.accord.max_queued_loads.or(getAccordConcurrentOps());
-    }
-
-    public static int getAccordMaxQueuedRangeLoadCount()
-    {
-        return conf.accord.max_queued_range_loads.or(Math.max(4, getAccordConcurrentOps() / 4));
-    }
-
     public static DefaultProgressLog.Config getAccordProgressLogConfig()
     {
         return accordProgressLogConfig;

src/java/org/apache/cassandra/service/accord/AccordCommandStore.java

@@ -245,26 +245,6 @@ public class AccordCommandStore extends CommandStore
         if (this.progressLog instanceof DefaultProgressLog)
             ((DefaultProgressLog)this.progressLog).unsafeSetConfig(DatabaseDescriptor.getAccordProgressLogConfig());
 
-        final AccordCache.Type<TxnId, Command, AccordSafeCommand>.Instance commands;
-        final AccordCache.Type<RoutingKey, CommandsForKey, AccordSafeCommandsForKey>.Instance commandsForKey;
-        try (AccordExecutor.ExclusiveGlobalCaches exclusive = sharedExecutor.lockCaches())
-        {
-            commands = exclusive.commands.newInstance(this);
-            commandsForKey = exclusive.commandsForKey.newInstance(this);
-            this.caches = new ExclusiveCaches(sharedExecutor.unsafeLock(), exclusive.global, commands, commandsForKey);
-        }
-
-        this.exclusiveExecutor = sharedExecutor.executor(id);
-        {
-            AccordConfig.RangeIndexMode mode = getAccord().range_index_mode;
-            switch (mode)
-            {
-                default: throw new UnhandledEnum(mode);
-                case journal_sai: rangeIndex = new JournalRangeIndex(this); break;
-                case in_memory: rangeIndex = new InMemoryRangeIndex(this); break;
-            }
-        }
-
         maybeLoadRedundantBefore(journal.loadRedundantBefore(id()));
         maybeLoadBootstrapBeganAt(journal.loadBootstrapBeganAt(id()));
         maybeLoadSafeToRead(journal.loadSafeToRead(id()));
@@ -284,6 +264,26 @@ public class AccordCommandStore extends CommandStore
             return a;
         }).orElseThrow(() -> Invariants.illegalState("CommandStore %d created with no ranges", id));
 
+        final AccordCache.Type<TxnId, Command, AccordSafeCommand>.Instance commands;
+        final AccordCache.Type<RoutingKey, CommandsForKey, AccordSafeCommandsForKey>.Instance commandsForKey;
+        try (AccordExecutor.ExclusiveGlobalCaches exclusive = sharedExecutor.lockCaches())
+        {
+            commands = exclusive.commands.newInstance(this);
+            commandsForKey = exclusive.commandsForKey.newInstance(this);
+            this.caches = new ExclusiveCaches(sharedExecutor.unsafeLock(), exclusive.global, commands, commandsForKey);
+        }
+        this.exclusiveExecutor = sharedExecutor.executor(id);
+
+        {
+            AccordConfig.RangeIndexMode mode = getAccord().range_index_mode;
+            switch (mode)
+            {
+                default: throw new UnhandledEnum(mode);
+                case journal_sai: rangeIndex = new JournalRangeIndex(this); break;
+                case in_memory: rangeIndex = new InMemoryRangeIndex(this); break;
+            }
+        }
+
         if (AccordService.isStarted())
             progressLog.unsafeStart();
     }

src/java/org/apache/cassandra/service/accord/AccordCommandStores.java

@@ -48,16 +48,17 @@ import accord.utils.async.AsyncResults;
 import org.apache.cassandra.cache.CacheSize;
 import org.apache.cassandra.concurrent.ScheduledExecutors;
 import org.apache.cassandra.concurrent.Shutdownable;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.AccordConfig.QueueShardModel;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.journal.Descriptor;
 import org.apache.cassandra.schema.TableId;
 import org.apache.cassandra.service.accord.AccordCommandStore.DurablyAppliedTo;
 import org.apache.cassandra.service.accord.AccordExecutor.AccordExecutorFactory;
+import org.apache.cassandra.utils.FBUtilities;
 
 import static org.apache.cassandra.config.AccordConfig.QueueShardModel.THREAD_PER_SHARD;
-import static org.apache.cassandra.config.DatabaseDescriptor.getAccordQueueShardCount;
-import static org.apache.cassandra.config.DatabaseDescriptor.getAccordQueueSubmissionModel;
+import static org.apache.cassandra.config.DatabaseDescriptor.getAccord;
 import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITHOUT_LOCK;
 import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITH_LOCK;
 import static org.apache.cassandra.service.accord.AccordExecutor.constant;
@@ -83,10 +84,12 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
               AccordCommandStore.factory(id -> executors[id % executors.length]));
         this.executors = executors;
         this.mask = Integer.highestOneBit(executors.length) - 1;
+
         cacheSize = DatabaseDescriptor.getAccordCacheSizeInMiB() << 20;
         workingSetSize = DatabaseDescriptor.getAccordWorkingSetSizeInMiB() << 20;
-        maxQueuedLoads = DatabaseDescriptor.getAccordMaxQueuedLoadCount();
-        maxQueuedRangeLoads = DatabaseDescriptor.getAccordMaxQueuedRangeLoadCount();
+        AccordConfig config = DatabaseDescriptor.getAccord();
+        maxQueuedLoads = maxQueuedLoads(config);
+        maxQueuedRangeLoads = maxQueuedRangeLoads(config);
         shrinkingOn = DatabaseDescriptor.getAccordCacheShrinkingOn();
         refreshCapacities();
         ScheduledExecutors.scheduledFastTasks.scheduleWithFixedDelay(() -> {
@@ -102,15 +105,21 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
     static Factory factory()
     {
         return (NodeCommandStoreService time, Agent agent, DataStore store, RandomSource random, Journal journal, ShardDistributor shardDistributor, ProgressLog.Factory progressLogFactory, LocalListeners.Factory listenersFactory) -> {
-            AccordExecutor[] executors = new AccordExecutor[getAccordQueueShardCount()];
+            AccordConfig config = getAccord();
+            AccordExecutor[] executors = new AccordExecutor[executorShards(config)];
             AccordExecutorFactory factory;
             int maxThreads = Integer.MAX_VALUE;
-            switch (getAccordQueueSubmissionModel())
+            switch (config.queue_submission_model)
             {
-                default: throw new AssertionError("Unhandled QueueSubmissionModel: " + getAccordQueueSubmissionModel());
+                default: throw new AssertionError("Unhandled QueueSubmissionModel: " + config.queue_submission_model);
                 case SYNC: factory = AccordExecutorSyncSubmit::new; break;
                 case SEMI_SYNC: factory = AccordExecutorSemiSyncSubmit::new; break;
                 case ASYNC: factory = AccordExecutorAsyncSubmit::new; break;
+                case SIGNAL:
+                    long spinIntervalNanos = config.queue_spin_interval == null ? 0 : config.queue_spin_interval.to(TimeUnit.NANOSECONDS);
+                    long stopCheckIntervalNanos = config.queue_stop_check_interval == null ? 0 : config.queue_stop_check_interval.to(TimeUnit.NANOSECONDS);
+                    factory = (executorId, mode, threads, name, agent0) -> new AccordExecutorSignalLoop(executorId, mode, threads, spinIntervalNanos, stopCheckIntervalNanos, TimeUnit.NANOSECONDS, name, agent0);
+                    break;
                 case EXEC_ST:
                     factory = AccordExecutorSimple::new;
                     maxThreads = 1;
@@ -119,9 +128,8 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
 
             for (int id = 0; id < executors.length; id++)
             {
-                QueueShardModel shardModel = DatabaseDescriptor.getAccordQueueShardModel();
+                QueueShardModel shardModel = config.queue_shard_model;
                 String baseName = AccordExecutor.class.getSimpleName() + '[' + id;
-                int threads = Math.min(maxThreads, Math.max(DatabaseDescriptor.getAccordConcurrentOps() / getAccordQueueShardCount(), 1));
                 switch (shardModel)
                 {
                     case THREAD_PER_SHARD:
@@ -129,6 +137,7 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
                         executors[id] = factory.get(id, shardModel == THREAD_PER_SHARD ? RUN_WITHOUT_LOCK : RUN_WITH_LOCK, 1, constant(baseName + ']'), agent);
                         break;
                     case THREAD_POOL_PER_SHARD:
+                        int threads = Math.min(maxThreads, Math.max(DatabaseDescriptor.getAccordConcurrentOps() / executors.length, 1));
                         executors[id] = factory.get(id, RUN_WITHOUT_LOCK, threads, num -> baseName + ',' + num + ']', agent);
                         break;
                 }
@@ -204,8 +213,8 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
     {
         long capacityPerExecutor = cacheSize / executors.length;
         long workingSetPerExecutor = workingSetSize < 0 ? Long.MAX_VALUE : workingSetSize / executors.length;
-        int maxLoadsPerExecutor = (maxQueuedLoads + executors.length - 1) / executors.length;
-        int maxRangeLoadsPerExecutor = (maxQueuedRangeLoads + executors.length - 1) / executors.length;
+        int maxLoadsPerExecutor = Math.max(1, (maxQueuedLoads + executors.length - 1) / executors.length);
+        int maxRangeLoadsPerExecutor = Math.max(1, (maxQueuedRangeLoads + executors.length - 1) / executors.length);
         for (AccordExecutor executor : executors)
         {
             executor.executeDirectlyWithLock(() -> {
@@ -319,4 +328,34 @@ public class AccordCommandStores extends CommandStores implements CacheSize, Shu
         }
         return AsyncChains.allOf(chains);
     }
+
+    private static int executorShards(AccordConfig config)
+    {
+        switch (config.queue_shard_model)
+        {
+            default: throw new AssertionError("Unhandled queue_shard_model: " + config.queue_shard_model);
+            case THREAD_PER_SHARD:
+            case THREAD_PER_SHARD_SYNC_QUEUE:
+                return config.queue_shard_count.or(DatabaseDescriptor::getAvailableProcessors);
+            case THREAD_POOL_PER_SHARD:
+                return Math.max(1, config.queue_shard_count.or(DatabaseDescriptor.getAvailableProcessors() / 8));
+        }
+    }
+
+    private static int threads(AccordConfig config)
+    {
+        return config.queue_thread_count.or(2 * FBUtilities.getAvailableProcessors());
+    }
+
+    public static int maxQueuedLoads(AccordConfig config)
+    {
+        return config.max_queued_loads.or(FBUtilities.getAvailableProcessors());
+    }
+
+    public static int maxQueuedRangeLoads(AccordConfig config)
+    {
+        return config.max_queued_range_loads.or(maxQueuedLoads(config) / 4);
+    }
+
+
 }

src/java/org/apache/cassandra/service/accord/AccordExecutor.java

@@ -200,7 +200,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     private final TaskQueue<Task> running = new TaskQueue<>(RUNNING);
 
     private final TaskQueue<AccordTask<?>> waitingToLoadRangeTxns = new TaskQueue<>(WAITING_TO_LOAD);
-
     private final TaskQueue<AccordTask<?>> waitingToLoad = new TaskQueue<>(WAITING_TO_LOAD);
     private final TaskQueue<Task> waitingToRun = new TaskQueue<>(WAITING_TO_RUN);
 
@@ -233,7 +232,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     private int activeLoads, activeRangeLoads;
     private boolean hasPausedLoading;
     int tasks;
-    int runningThreads;
     final DebugExecutor debug = DebugExecutor.maybeDebug();
 
     AccordExecutor(Lock lock, int executorId, Agent agent)
@@ -276,7 +274,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     abstract boolean isInLoop();
 
-    final Lock unsafeLock()
+    public final Lock unsafeLock()
     {
         return lock;
     }
@@ -309,7 +307,11 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     final boolean tryLock()
     {
-        boolean result = lock.tryLock();
+       return onTryLock(lock.tryLock());
+    }
+
+    final boolean onTryLock(boolean result)
+    {
         if (DEBUG_EXECUTION && result) debug.onEnterLock();
         if (Invariants.isParanoid())
         {
@@ -347,6 +349,11 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     final boolean hasWaitingToRun()
     {
         updateWaitingToRunExclusive();
+        return hasAlreadyWaitingToRun();
+    }
+
+    final boolean hasAlreadyWaitingToRun()
+    {
         return !waitingToRun.isEmpty();
     }
 
@@ -359,10 +366,15 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     final Task pollWaitingToRunExclusive()
     {
         updateWaitingToRunExclusive();
+        return pollAlreadyWaitingToRunExclusive();
+    }
+
+    final Task pollAlreadyWaitingToRunExclusive()
+    {
         Task next = waitingToRun.poll();
         if (next != null)
         {
-            if (DEBUG_EXECUTION) next.debug.onPolled();
+            if (DEBUG_EXECUTION) DebugTask.get(next).onPolled();
             next.addToQueue(running);
         }
         return next;
@@ -379,7 +391,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         lock();
         try
         {
-            if (tasks == 0 && runningThreads == 0)
+            if (tasks == 0)
                 return;
 
             if (waitingForQuiescence == null)
@@ -414,7 +426,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         lock();
         try
         {
-            if (tasks == 0 && runningThreads == 0)
+            if (tasks == 0)
             {
                 run.run();
                 return;
@@ -571,19 +583,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
     }
 
-    void consumeExclusive(Submittable object)
-    {
-        try
-        {
-            if (object instanceof SubmittableTask) ((SubmittableTask) object).submitExclusive(this);
-            else ((SubmitAsync) object).submitExclusive(this);
-        }
-        catch (Throwable t)
-        {
-            agent.onException(t);
-        }
-    }
-
     private void updateQueue(AccordTask<?> task)
     {
         task.unqueueIfQueued();
@@ -606,13 +605,13 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
     }
 
-    private void waitingToRun(AccordTask task)
+    private void waitingToRun(AccordTask<?> task)
     {
         task.onWaitingToRun();
         task.addToQueue(task.commandStore.exclusiveExecutor);
     }
 
-    private void waitingToRun(SubmittableTask task, @Nullable SequentialExecutor queue)
+    private void waitingToRun(Task task, @Nullable SequentialExecutor queue)
     {
         task.onWaitingToRun();
         task.addToQueue(queue == null ? waitingToRun : queue);
@@ -638,7 +637,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         Invariants.require(task.commandStore.executor() == this,
                               "%s is a wrong command store for %s, should be %s",
                               this, task, task);
-        submit(AccordExecutor::cancelExclusive, CancelAsync::new, task);
+        submit(AccordExecutor::cancelExclusive, CancelTask::new, task);
     }
 
     @Override
@@ -653,27 +652,27 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         return submitPlainExclusive(null, new SaveRunnable(entry, identity, save));
     }
 
-    private <P1> void submit(BiConsumer<AccordExecutor, P1> sync, Function<P1, Submittable> async, P1 p1)
+    private <P1> void submit(BiConsumer<AccordExecutor, P1> sync, Function<P1, Task> async, P1 p1)
     {
         submit((e, c, p1a, p2a, p3) -> c.accept(e, p1a), (f, p1a, p2a, p3) -> f.apply(p1a), sync, async, p1, null, null);
     }
 
-    private <P1, P2> void submit(TriConsumer<AccordExecutor, P1, P2> sync, BiFunction<P1, P2, Submittable> async, P1 p1, P2 p2)
+    private <P1, P2> void submit(TriConsumer<AccordExecutor, P1, P2> sync, BiFunction<P1, P2, Task> async, P1 p1, P2 p2)
     {
         submit((e, c, p1a, p2a, p3) -> c.accept(e, p1a, p2a), (f, p1a, p2a, p3) -> f.apply(p1a, p2a), sync, async, p1, p2, null);
     }
 
-    private <P1, P2, P3> void submit(QuadConsumer<AccordExecutor, P1, P2, P3> sync, TriFunction<P1, P2, P3, Submittable> async, P1 p1, P2 p2, P3 p3)
+    private <P1, P2, P3> void submit(QuadConsumer<AccordExecutor, P1, P2, P3> sync, TriFunction<P1, P2, P3, Task> async, P1 p1, P2 p2, P3 p3)
     {
         submit((e, c, p1a, p2a, p3a) -> c.accept(e, p1a, p2a, p3a), TriFunction::apply, sync, async, p1, p2, p3);
     }
 
-    private <P1, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1, P2, P3, P4> sync, QuadFunction<P1, P2, P3, P4, Submittable> async, P1 p1, P2 p2, P3 p3, P4 p4)
+    private <P1, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1, P2, P3, P4> sync, QuadFunction<P1, P2, P3, P4, Task> async, P1 p1, P2 p2, P3 p3, P4 p4)
     {
         submit(sync, async, p1, p1, p2, p3, p4);
     }
 
-    abstract <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4);
+    abstract <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4);
 
     <R> void submit(AccordTask<R> operation)
     {
@@ -867,7 +866,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
         finally
         {
-            if (DEBUG_EXECUTION) task.debug.onCompleted(task, debug);
             --tasks;
             if (running.contains(task))
                 running.remove(task);
@@ -907,12 +905,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         switch (state)
         {
             default: throw new UnhandledEnum(state);
-            case INITIALIZED:
-                // we could be cancelled before we even reach the queue
-                try { task.cancelExclusive(); }
-                finally { task.cleanupExclusive(this); }
-                break;
-
             case SCANNING_RANGES:
             case LOADING:
             case WAITING_TO_LOAD:
@@ -923,6 +915,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 finally { completeTaskExclusive(task); }
                 break;
 
+            case INITIALIZED: // TODO (expected): preferable to be able to cancel at this stage, even if unlikely to trigger at this phase
             case ASSIGNED:
             case RUNNING:
             case PERSISTING:
@@ -1059,6 +1052,8 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     public void setMaxQueuedLoads(int total, int range)
     {
         Invariants.require(isOwningThread());
+        Invariants.requireArgument(total >= 1, "Must permit at least one load");
+        Invariants.requireArgument(range >= 1, "Must permit at least one range load");
         maxQueuedLoads = total;
         maxQueuedRangeLoads = range;
     }
@@ -1112,25 +1107,71 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     public static abstract class Task extends IntrusivePriorityHeap.Node
     {
+        public final WithResources resources;
+        Task next;
         long queuePosition;
         public long createdAt = nanoTime(), waitingToRunAt, runningAt, cleanupAt;
-        public final DebugTask debug = DebugTask.maybeDebug();
 
         protected Task()
         {
+            resources = DebugTask.maybeDebug(ExecutorLocals.propagate(), this);
         }
 
-        public void onWaitingToRun()
+        public final Task unwrap()
+        {
+            if (this instanceof SequentialQueueTask)
+                return ((SequentialQueueTask) this).queue.task;
+            return this;
+        }
+
+        final void setReadyToCleanup()
+        {
+            queuePosition |= Long.MIN_VALUE;
+        }
+
+        final boolean isReadyToCleanup()
+        {
+            return 0 != (queuePosition & Long.MIN_VALUE);
+        }
+
+        final void onRunning()
+        {
+            runningAt = nanoTime();
+            if (DEBUG_EXECUTION) ((DebugTask)resources).onRunning();
+        }
+
+        final void onRunComplete()
+        {
+            if (DEBUG_EXECUTION) ((DebugTask)resources).onRunComplete();
+        }
+
+        final void onWaitingToRun()
         {
             waitingToRunAt = nanoTime();
         }
 
+        static Task reverse(Task unqueued)
+        {
+            Task prev = null;
+            Task cur = unqueued;
+            while (cur != null)
+            {
+                Task next = cur.next;
+                cur.next = prev;
+                prev = cur;
+                cur = next;
+            }
+            return prev;
+        }
+
         public DebuggableTask debuggable() { return null; }
 
+        abstract void submitExclusive(AccordExecutor owner);
+
         /**
          * Prepare to run while holding the state cache lock
          */
-        abstract protected void preRunExclusive(Thread assigned);
+        abstract protected void preRunExclusive();
 
         /**
          * Run the command; the state cache lock may or may not be held depending on the executor implementation
@@ -1157,6 +1198,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 executor.elapsedRunning.increment(cleanupAt - runningAt, cleanupAt);
                 executor.elapsed.increment(cleanupAt - createdAt, cleanupAt);
             }
+            if (DEBUG_EXECUTION) DebugTask.get(this).onCompleted(executor.debug);
         }
 
         void cancelExclusive(AccordExecutor owner) {}
@@ -1164,16 +1206,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         abstract protected void addToQueue(TaskQueue queue);
     }
 
-    interface Submittable
-    {
-    }
-
-    static abstract class SubmittableTask extends Task implements Submittable
-    {
-        final WithResources locals = ExecutorLocals.propagate();
-        abstract void submitExclusive(AccordExecutor owner);
-    }
-
     // run the task even on a stopped commandStore
     public interface Unstoppable extends PreLoadContext.Empty
     {
@@ -1184,7 +1216,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     {
     }
 
-    static class SequentialQueueTask extends Task
+    static final class SequentialQueueTask extends Task
     {
         private final SequentialExecutor queue;
 
@@ -1194,10 +1226,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             this.queue = queue;
         }
 
+        @Override void submitExclusive(AccordExecutor owner) { throw new UnsupportedOperationException(); }
+
         @Override
-        protected void preRunExclusive(Thread assigned)
+        protected void preRunExclusive()
         {
-            queue.preRunTask(assigned);
+            queue.preRunTask();
         }
 
         @Override
@@ -1224,6 +1258,11 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             Invariants.require(queue.kind == RUNNING);
             queue.append(this);
         }
+
+        protected boolean isInHeap()
+        {
+            return super.isInHeap();
+        }
     }
 
     private static final AtomicReferenceFieldUpdater<SequentialExecutor, Thread> ownerUpdater = AtomicReferenceFieldUpdater.newUpdater(SequentialExecutor.class, Thread.class, "owner");
@@ -1232,7 +1271,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         final int commandStoreId;
         final SequentialQueueTask selfTask;
         private Task task;
-        private Thread assigned;
         private volatile Thread owner, waiting;
         private boolean stopped;
         private volatile boolean visibleStopped;
@@ -1253,31 +1291,31 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             this.debug = DebugSequentialExecutor.maybeDebug(executor.debug, commandStoreId);
         }
 
-        void preRunTask(Thread assigned)
+        void preRunTask()
         {
             Invariants.require(task != null);
-            this.assigned = assigned;
-            task.preRunExclusive(assigned);
+            task.preRunExclusive();
         }
 
         void runTask()
         {
-            if (!ownerUpdater.compareAndSet(this, null, assigned))
+            Thread self = Thread.currentThread();
+            if (!ownerUpdater.compareAndSet(this, null, self))
             {
                 if (DEBUG_EXECUTION) debug.onWaiting();
-                Invariants.require(assigned == Thread.currentThread());
-                waiting = assigned;
+                Invariants.require(self == Thread.currentThread());
+                waiting = self;
                 outer: do
                 {
                     while (true)
                     {
                         Thread owner = this.owner;
-                        if (owner == assigned) break outer;
+                        if (owner == self) break outer;
                         if (owner == null) continue outer;
                         LockSupport.park();
                     }
                 }
-                while (!ownerUpdater.compareAndSet(this, null, assigned));
+                while (!ownerUpdater.compareAndSet(this, null, self));
             }
             waiting = null;
 
@@ -1308,7 +1346,6 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             try { task.cleanupExclusive(AccordExecutor.this); }
             finally
             {
-                assigned = null;
                 owner = null;
                 task = super.poll();
                 if (DEBUG_EXECUTION) debug.onSetTask(task);
@@ -1329,12 +1366,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         {
             if (task != null)
             {
-                Invariants.require(assigned != null || waitingToRun.contains(selfTask));
+                Invariants.require(selfTask.isInHeap());
                 super.append(newTask);
             }
             else
             {
-                Invariants.require(assigned == null && isEmpty());
+                Invariants.require(isEmpty());
                 task = newTask;
                 selfTask.queuePosition = newTask.queuePosition;
                 waitingToRun.append(selfTask);
@@ -1358,7 +1395,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
         private boolean removeCurrentTask(Node remove)
         {
-            if (assigned != null)
+            if (running.contains(selfTask))
                 return false;
 
             Invariants.require(remove == task);
@@ -1427,7 +1464,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected boolean contains(Task contains)
         {
-            return super.contains(contains) || (task == contains && assigned == null);
+            return super.contains(contains) || (task == contains && !running.contains(selfTask));
         }
 
         @Override
@@ -1588,25 +1625,15 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
     }
 
-    private abstract static class SubmitAsync implements Submittable
-    {
-        abstract void submitExclusive(AccordExecutor executor);
-    }
-
-    private static class CancelAsync extends SubmitAsync
+    static class CancelTask extends Task
     {
         final Task cancel;
-
-        private CancelAsync(Task cancel)
-        {
-            this.cancel = cancel;
-        }
-
-        @Override
-        void submitExclusive(AccordExecutor executor)
-        {
-            cancel.cancelExclusive(executor);
-        }
+        private CancelTask(Task cancel) { this.cancel = cancel; }
+        @Override void submitExclusive(AccordExecutor owner) { cancel.cancelExclusive(owner); }
+        @Override protected void preRunExclusive() { throw new UnsupportedOperationException(); }
+        @Override protected void runInternal() { throw new UnsupportedOperationException(); }
+        @Override protected void fail(Throwable fail) { throw new UnsupportedOperationException(); }
+        @Override protected void addToQueue(TaskQueue queue) { throw new UnsupportedOperationException(); }
     }
 
     static <O> IntFunction<O> constant(O out)
@@ -1614,12 +1641,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         return ignore -> out;
     }
 
-    abstract class Plain extends SubmittableTask implements Cancellable
+    abstract class Plain extends Task implements Cancellable
     {
         abstract SequentialExecutor executor();
 
         @Override
-        protected void preRunExclusive(Thread assigned) {}
+        protected void preRunExclusive() {}
 
         @Override
         protected final void addToQueue(TaskQueue queue)
@@ -1631,7 +1658,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void cancel()
         {
-            submit((e, c) -> c.cancelExclusive(e), CancelAsync::new, this);
+            submit((e, c) -> c.cancelExclusive(e), CancelTask::new, this);
         }
 
         void cancelExclusive(AccordExecutor owner)
@@ -1681,14 +1708,14 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
-            runningAt = nanoTime();
-            try (Closeable close = locals.get())
+            onRunning();
+            try (Closeable close = resources.get())
             {
                 run.run();
             }
             if (result != null)
                 result.trySuccess(null);
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
         }
 
         @Override
@@ -1715,7 +1742,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         abstract void postRunExclusive();
 
         @Override
-        protected void preRunExclusive(Thread assigned)
+        protected void preRunExclusive()
         {
             startedAtNanos = MonotonicClock.Global.approxTime.now();
         }
@@ -1784,12 +1811,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void runInternal()
         {
-            runningAt = nanoTime();
-            try (Closeable close = locals.get())
+            onRunning();
+            try (Closeable close = resources.get())
             {
                 result = entry.owner.parent().adapter().load(entry.owner.commandStore, entry.key());
             }
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
         }
 
         @Override
@@ -1831,12 +1858,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
-            runningAt = nanoTime();
-            try (Closeable close = locals.get())
+            onRunning();
+            try (Closeable close = resources.get())
             {
                 wrapped.runInternal();
             }
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
         }
 
         @Override
@@ -1882,12 +1909,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void runInternal()
         {
-            runningAt = nanoTime();
-            try (Closeable close = locals.get())
+            onRunning();
+            try (Closeable close = resources.get())
             {
                 run.run();
             }
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
             failure = null;
         }
 
@@ -1930,8 +1957,8 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
-            runningAt = nanoTime();
-            try (Closeable close = locals.get())
+            onRunning();
+            try (Closeable close = resources.get())
             {
                 runOrFail.run();
             }
@@ -1940,7 +1967,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 // shouldn't throw exceptions
                 agent.onException(t);
             }
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
         }
 
         @Override
@@ -1984,7 +2011,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
 
         @Override
-        protected void preRunExclusive(Thread assigned)
+        protected void preRunExclusive()
         {
             startedAtNanos = MonotonicClock.Global.approxTime.now();
         }

src/java/org/apache/cassandra/service/accord/AccordExecutorAbstractLockLoop.java

@@ -19,25 +19,22 @@
 package org.apache.cassandra.service.accord;
 
 import java.util.concurrent.locks.Lock;
-import java.util.stream.Stream;
 
 import accord.api.Agent;
 import accord.utils.QuadFunction;
 import accord.utils.QuintConsumer;
 
-import org.apache.cassandra.concurrent.DebuggableTask.DebuggableTaskRunner;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.service.accord.AccordExecutorLoops.LoopTask;
 import org.apache.cassandra.service.accord.debug.DebugExecution.DebugExecutorLoop;
-import org.apache.cassandra.utils.concurrent.ConcurrentLinkedStack;
 
 import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITH_LOCK;
 import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
 
-abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
+abstract class AccordExecutorAbstractLockLoop extends AccordExecutorAbstractLoop
 {
     private static final int YIELD_INTERVAL = DatabaseDescriptor.getAccord().queue_yield_interval;
-    final ConcurrentLinkedStack<Submittable> submitted = new ConcurrentLinkedStack<>();
+    int runningThreads;
     boolean shutdown;
 
     AccordExecutorAbstractLockLoop(Lock lock, int executorId, Agent agent)
@@ -49,14 +46,13 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
     abstract void notifyWorkExclusive();
     void loopYieldExclusive() throws InterruptedException {}
     abstract void awaitExclusive() throws InterruptedException;
-    abstract AccordExecutorLoops loops();
-    abstract <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4);
+    abstract <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4);
 
-    <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
     {
         // if we're a loop thread, we will poll the waitingToRun queue when we come around
         // NOTE: this assumes no synchronous blocking tasks are submitted to this executor
-        if (isInLoop() || isOwningThread()) submitted.push(async.apply(p1a, p2, p3, p4));
+        if (isInLoop() || isOwningThread()) push(async.apply(p1a, p2, p3, p4));
         else submitExternal(sync, async, p1s, p1a, p2, p3, p4);
     }
 
@@ -66,7 +62,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         {
             try
             {
-                drainSubmittedExclusive();
+                drainUnqueuedExclusive();
             }
             catch (Throwable t)
             {
@@ -82,33 +78,6 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         }
     }
 
-    public boolean hasTasks()
-    {
-        if (tasks > 0 || !submitted.isEmpty() || runningThreads > 0)
-            return true;
-
-        lock();
-        try
-        {
-            return tasks > 0 || !submitted.isEmpty() || runningThreads > 0;
-        }
-        finally
-        {
-            unlock();
-        }
-    }
-
-    final void updateWaitingToRunExclusive()
-    {
-        drainSubmittedExclusive();
-        super.updateWaitingToRunExclusive();
-    }
-
-    final void drainSubmittedExclusive()
-    {
-        submitted.drain(AccordExecutor::consumeExclusive, this, true);
-    }
-
     final void notifyIfMoreWorkExclusive()
     {
         if (hasWaitingToRun())
@@ -149,7 +118,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         ++runningThreads;
     }
 
-    LoopTask task(String name, Mode mode)
+    LoopTask task(int index, String name, Mode mode)
     {
         return mode == RUN_WITH_LOCK ? runWithLock(name) : runWithoutLock(name);
     }
@@ -178,7 +147,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                                 setRunning(task);
                                 try
                                 {
-                                    task.preRunExclusive(self);
+                                    task.preRunExclusive();
                                     task.runInternal();
                                 }
                                 catch (Throwable t)
@@ -263,7 +232,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                             if (task != null)
                             {
                                 setRunning(task);
-                                task.preRunExclusive(self);
+                                task.preRunExclusive();
                                 if (DEBUG_EXECUTION) debug.onExitLock();
                                 exitLockLoop();
                                 break;
@@ -337,23 +306,10 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         };
     }
 
-    @Override
-    public Stream<? extends DebuggableTaskRunner> active()
-    {
-        return loops().active();
-    }
-
     @Override
     public void shutdown()
     {
         shutdown = true;
         notifyWork();
     }
-
-    @Override
-    public Object shutdownNow()
-    {
-        shutdown();
-        return null;
-    }
 }

src/java/org/apache/cassandra/service/accord/AccordExecutorAbstractLoop.java

@@ -0,0 +1,139 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.service.accord;
+
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
+import java.util.concurrent.locks.Lock;
+import java.util.stream.Stream;
+
+import accord.api.Agent;
+import accord.utils.Invariants;
+
+import org.apache.cassandra.concurrent.DebuggableTask.DebuggableTaskRunner;
+
+abstract class AccordExecutorAbstractLoop extends AccordExecutor
+{
+    private volatile Task unqueued;
+    private static final AtomicReferenceFieldUpdater<AccordExecutorAbstractLoop, Task> unqueuedUpdater = AtomicReferenceFieldUpdater.newUpdater(AccordExecutorAbstractLoop.class, Task.class, "unqueued");
+
+    AccordExecutorAbstractLoop(Lock lock, int executorId, Agent agent)
+    {
+        super(lock, executorId, agent);
+    }
+
+    abstract AccordExecutorLoops loops();
+
+    boolean hasUnqueued()
+    {
+        return unqueued != null;
+    }
+
+    Task unqueued()
+    {
+        return unqueued;
+    }
+
+    final Task push(Task submit)
+    {
+        Invariants.require(submit.next == null);
+        while (true)
+        {
+            Task next = unqueued;
+            submit.next = next;
+            if (unqueuedUpdater.compareAndSet(this, next, submit))
+                return next;
+        }
+    }
+
+    @Override
+    public boolean hasTasks()
+    {
+        if (hasUnqueued() || tasks > 0)
+            return true;
+
+        lock();
+        try
+        {
+            return hasUnqueued() || tasks > 0;
+        }
+        finally
+        {
+            unlock();
+        }
+    }
+
+    final void updateWaitingToRunExclusive()
+    {
+        drainUnqueuedExclusive();
+        super.updateWaitingToRunExclusive();
+    }
+
+    final void drainUnqueuedExclusive()
+    {
+        Task cur = Task.reverse(acquireUnqueuedExclusive());
+        while (cur != null)
+            cur = enqueueOneExclusive(cur);
+    }
+
+    final Task acquireUnqueuedExclusive()
+    {
+        return unqueuedUpdater.getAndSet(this, null);
+    }
+
+    final Task enqueueOneExclusive(Task cur)
+    {
+        Invariants.require(cur != null);
+        Task next = cur.next;
+        cur.next = null;
+        if (cur.isReadyToCleanup()) completeTaskExclusive(cur);
+        else cur.submitExclusive(this);
+        return next;
+    }
+
+    final Task enqueueOneCleanup(Task cur)
+    {
+        Invariants.require(cur != null);
+        Task next = cur.next;
+        cur.next = null;
+        completeTaskExclusive(cur);
+        return next;
+    }
+
+    final Task enqueueOneSubmit(Task cur)
+    {
+        Invariants.require(cur != null);
+        Task next = cur.next;
+        cur.next = null;
+        cur.submitExclusive(this);
+        return next;
+    }
+
+    @Override
+    public Stream<? extends DebuggableTaskRunner> active()
+    {
+        return loops().active();
+    }
+
+    @Override
+    public Object shutdownNow()
+    {
+        shutdown();
+        return null;
+    }
+}

src/java/org/apache/cassandra/service/accord/AccordExecutorAbstractSemiSyncSubmit.java

@@ -33,9 +33,9 @@ abstract class AccordExecutorAbstractSemiSyncSubmit extends AccordExecutorAbstra
 
     abstract void awaitExclusive() throws InterruptedException;
 
-    <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
     {
-        if (submitted.push(async.apply(p1a, p2, p3, p4)) && !isInLoop())
+        if (push(async.apply(p1a, p2, p3, p4)) == null && !isInLoop())
             notifyWork();
     }
 }

src/java/org/apache/cassandra/service/accord/AccordExecutorAsyncSubmit.java

@@ -26,7 +26,7 @@ import accord.api.Agent;
 import org.apache.cassandra.utils.concurrent.LockWithAsyncSignal;
 
 // WARNING: experimental - needs more testing
-class AccordExecutorAsyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
+public class AccordExecutorAsyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
 {
     private final AccordExecutorLoops loops;
     private final LockWithAsyncSignal lock;
@@ -47,7 +47,7 @@ class AccordExecutorAsyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
     void awaitExclusive() throws InterruptedException
     {
         lock.clearSignal();
-        if (submitted.isEmpty())
+        if (!hasUnqueued())
             lock.await();
     }
 

src/java/org/apache/cassandra/service/accord/AccordExecutorLoops.java

@@ -20,13 +20,13 @@ package org.apache.cassandra.service.accord;
 
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
-import java.util.function.BiFunction;
 import java.util.function.IntFunction;
 import java.util.stream.Stream;
 
 import org.agrona.collections.Long2ObjectHashMap;
 
 import accord.utils.Invariants;
+import accord.utils.TriFunction;
 
 import org.apache.cassandra.concurrent.DebuggableTask.DebuggableTaskRunner;
 import org.apache.cassandra.service.accord.AccordExecutor.Mode;
@@ -54,7 +54,7 @@ class AccordExecutorLoops
     private final AtomicInteger running = new AtomicInteger();
     private final Condition terminated = Condition.newOneTimeCondition();
 
-    public AccordExecutorLoops(Mode mode, int threads, IntFunction<String> loopName, BiFunction<String, Mode, LoopTask> loopFactory)
+    public AccordExecutorLoops(Mode mode, int threads, IntFunction<String> loopName, TriFunction<Integer, String, Mode, LoopTask> loopFactory)
     {
         Invariants.require(mode == RUN_WITH_LOCK ? threads == 1 : threads >= 1);
         running.addAndGet(threads);
@@ -63,7 +63,7 @@ class AccordExecutorLoops
         for (int i = 0; i < threads; ++i)
         {
             String name = loopName.apply(i);
-            LoopTask task = loopFactory.apply(name, mode);
+            LoopTask task = loopFactory.apply(i, name, mode);
             Thread thread = executorFactory().startThread(name, wrap(task), NON_DAEMON, INFINITE_LOOP);
             Thread conflict = loops.putIfAbsent(thread.getId(), thread);
             Invariants.require(conflict == null || !conflict.isAlive(), "Allocated two threads with the same threadId!");

src/java/org/apache/cassandra/service/accord/AccordExecutorSemiSyncSubmit.java

@@ -26,7 +26,7 @@ import java.util.function.IntFunction;
 import accord.api.Agent;
 
 // WARNING: experimental - needs more testing
-class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
+public class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
 {
     private final AccordExecutorLoops loops;
     private final ReentrantLock lock;
@@ -61,7 +61,7 @@ class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
     @Override
     void awaitExclusive() throws InterruptedException
     {
-        if (submitted.isEmpty())
+        if (!hasUnqueued())
             awaitWork();
     }
 

src/java/org/apache/cassandra/service/accord/AccordExecutorSignalLoop.java

@@ -0,0 +1,472 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.service.accord;
+
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.TimeUnit;
+import java.util.function.IntFunction;
+
+import javax.annotation.Nullable;
+
+import accord.api.Agent;
+import accord.utils.Invariants;
+import accord.utils.QuadFunction;
+import accord.utils.QuintConsumer;
+
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugTask;
+import org.apache.cassandra.utils.concurrent.SignalLock;
+
+import static accord.utils.Invariants.nonNull;
+import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
+
+public class AccordExecutorSignalLoop extends AccordExecutorAbstractLoop
+{
+    private static class ShutdownException extends RuntimeException {}
+
+    private final SignalLock lock;
+    private final AccordExecutorLoops loops;
+    private int readyToRunTarget = 1;
+    private final int readyToRunLimit;
+    // TODO (desired): intrusive queue using Task.next, but a little challenging because we reuse SequentialQueueTask so have ABA problem
+    private final ConcurrentLinkedQueue<Task> readyToRun = new ConcurrentLinkedQueue<>();
+
+    private Task pendingSequentialHead, pendingSequentialTail;
+    private Task pendingCleanupHead, pendingCleanupTail;
+    private Task pendingNewHead, pendingNewTail;
+    private int pendingCount;
+
+    private boolean shutdown;
+
+    public AccordExecutorSignalLoop(int executorId, Mode mode, int threads, long spinInterval, long stopCheckInterval, TimeUnit units, IntFunction<String> name, Agent agent)
+    {
+        this(new SignalLock(threads, spinInterval, stopCheckInterval, units), executorId, mode, threads, name, agent);
+    }
+
+    public AccordExecutorSignalLoop(SignalLock lock, int executorId, Mode mode, int threads, IntFunction<String> name, Agent agent)
+    {
+        super(lock, executorId, agent);
+        Invariants.require(threads < SignalLock.MAX_THREADS);
+        this.lock = lock;
+        this.loops = new AccordExecutorLoops(mode, threads, name, this::task);
+        this.readyToRunLimit = Math.min(threads * 4, SignalLock.MAX_SIGNAL_COUNT);
+    }
+
+    <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    {
+        Task next = async.apply(p1a, p2, p3, p4);
+        Task prev = push(next);
+        if (prev == null)
+            lock.signalLockWork();
+    }
+
+    @Override
+    final void beforeUnlockExternal()
+    {
+    }
+
+    private Task pollReadyToRun()
+    {
+        return readyToRun.poll();
+    }
+
+    private void addReadyToRun(Task task)
+    {
+        readyToRun.add(task);
+    }
+
+    private boolean hasReadyToRun()
+    {
+        return !readyToRun.isEmpty();
+    }
+
+    private LoopTask task(int index, String id, Mode mode)
+    {
+        return new LoopTask(index, id);
+    }
+
+    class LoopTask extends AccordExecutorLoops.LoopTask
+    {
+        final int index;
+
+        LoopTask(int index, String id)
+        {
+            super(id);
+            this.index = index;
+        }
+
+        private Task awaitWork()
+        {
+            while (true)
+            {
+                if (lock.awaitAsyncOrLock(index))
+                {
+                    try
+                    {
+                        if (DEBUG_EXECUTION) debug.onEnterLock();
+                        fetchWorkExclusive();
+                    }
+                    catch (Throwable t)
+                    {
+                        unlock();
+                        throw t;
+                    }
+
+                    if (!unlockAndAcquire())
+                        continue;
+                }
+
+                if (shutdown)
+                    throw new ShutdownException();
+
+                return nonNull(pollReadyToRun());
+            }
+        }
+
+        private Task cleanupAndMaybeGetWork(@Nullable Task cleanup)
+        {
+            if (cleanup == null)
+                return null;
+
+            if (lock.tryAcquireAsyncWork())
+            {
+                if (shutdown)
+                    throw new ShutdownException();
+
+                return pushCleanupAndReturn(cleanup, nonNull(pollReadyToRun()));
+            }
+
+            if (!tryLock())
+                return pushCleanupAndReturn(cleanup, null);
+
+            try
+            {
+                completeTaskExclusive(cleanup);
+                clearRunning();
+                fetchWorkExclusive();
+            }
+            catch (Throwable t)
+            {
+                unlock();
+                throw t;
+            }
+
+            if (unlockAndAcquire())
+            {
+                if (shutdown)
+                    throw new ShutdownException();
+
+                return nonNull(pollReadyToRun());
+            }
+            return null;
+        }
+
+        private Task pushCleanupAndReturn(Task cleanup, Task result)
+        {
+            clearRunning();
+            cleanup.setReadyToCleanup();
+            if (push(cleanup) == null)
+                lock.signalLockWork();
+            return result;
+        }
+
+        final boolean tryLock()
+        {
+            return onTryLock(lock.tryLock(index));
+        }
+
+        final boolean unlockAndAcquire()
+        {
+            if (Invariants.isParanoid()) paranoidUnlockExclusive();
+            if (DEBUG_EXECUTION) debug.onExitLock();
+            return lock.unlockAndAcquireAsyncWork();
+        }
+
+        private boolean enqueueOnePending()
+        {
+            if (pendingCount == 0)
+                return false;
+
+            if (pendingSequentialHead != null)
+            {
+                pendingSequentialHead = enqueueOneCleanup(pendingSequentialHead);
+                if (pendingSequentialHead == null)
+                    pendingSequentialTail = null;
+            }
+            else if (pendingCleanupHead != null)
+            {
+                pendingCleanupHead = enqueueOneCleanup(pendingCleanupHead);
+                if (pendingCleanupHead == null)
+                    pendingCleanupTail = null;
+            }
+            else
+            {
+                pendingNewHead = enqueueOneSubmit(pendingNewHead);
+                if (pendingNewHead == null)
+                    pendingNewTail = null;
+            }
+            --pendingCount;
+            return true;
+        }
+
+        private void fetchWorkExclusive()
+        {
+            boolean hasReadyToRun = hasReadyToRun();
+            boolean hadWaitingToRun = hasAlreadyWaitingToRun();
+            {
+                int prevPendingUnqueued = pendingCount;
+                updateAndEnqueuePendingUntilHasWaitingToRun(prevPendingUnqueued / 2);
+            }
+
+            if (hadWaitingToRun && hasReadyToRun)
+            {
+                lock.addAndGetEnabledThreadCount(1);
+            }
+            else if (hadWaitingToRun)
+            {
+                readyToRunTarget = Math.min(readyToRunLimit, readyToRunTarget + (1+readyToRunTarget)/2);
+            }
+            else if (hasReadyToRun && readyToRunTarget > 1)
+            {
+                --readyToRunTarget;
+            }
+
+            boolean hasDrainedSignal = false;
+            while (true)
+            {
+                long state = lock.state();
+                int signals = SignalLock.asyncSignalCount(state);
+                int waiters = SignalLock.waitingEnabledThreadCount(state);
+                if (signals >= readyToRunTarget)
+                {
+                    if (enqueueOnePending() || (updatePendingUnqueued() && enqueueOnePending())) continue;
+                    else if (hasDrainedSignal)
+                        lock.signalLockWorkExclusive();
+                    return;
+                }
+                else if (waiters > 0 && signals > 1 && SignalLock.activeEnabledThreadCount(state) == 1)
+                {
+                    // ensure at least one other thread is running if there's enough work for it; it will spin up other threads if necessary
+                    lock.propagateAsyncWorkSignals(1);
+                }
+
+                Task task = pollAlreadyWaitingToRunExclusive();
+                if (task == null)
+                {
+                    if (updateAndEnqueuePendingUntilHasWaitingToRun(0))
+                        continue;
+
+                    lock.clearLockWork();
+                    hasDrainedSignal = true;
+                    if (!updateAndEnqueuePendingUntilHasWaitingToRun(0))
+                    {
+                        if (tasks == 0)
+                            notifyQuiescentExclusive();
+                        return;
+                    }
+                }
+                else
+                {
+                    try { task.preRunExclusive(); }
+                    catch (Throwable t)
+                    {
+                        try { task.fail(t); }
+                        catch (Throwable t2) { try { t.addSuppressed(t2); } catch (Throwable t3) {} }
+                        try { completeTaskExclusive(task); }
+                        catch (Throwable t2) { try { t.addSuppressed(t2); } catch (Throwable t3) {} }
+                        continue;
+                    }
+                    if (DEBUG_EXECUTION) DebugTask.get(task).onPreRun();
+
+                    addReadyToRun(task);
+                    boolean incremented = lock.incrementAsyncWork(false);
+                    Invariants.require(incremented);
+                }
+            }
+        }
+
+        private boolean updatePendingUnqueued()
+        {
+            if (!hasUnqueued())
+                return false;
+
+            int count = 0;
+            Task addSequentialHead = null, addSequentialTail = null;
+            Task addCleanupHead = null, addCleanupTail = null;
+            Task addNewHead = null, addNewTail = null;
+            {
+                Task cur = Task.reverse(acquireUnqueuedExclusive());
+                while (cur != null)
+                {
+                    Task next = cur.next;
+                    if (!cur.isReadyToCleanup())
+                    {
+                        if (addNewHead == null) addNewHead = addNewTail = setNextNull(cur);
+                        else addNewHead = reverseOne(addNewHead, cur);
+                    }
+                    else if (cur instanceof SequentialQueueTask)
+                    {
+                        if (addSequentialHead == null) addSequentialHead = addSequentialTail = setNextNull(cur);
+                        else addSequentialHead = reverseOne(addSequentialHead, cur);
+                    }
+                    else
+                    {
+                        if (addCleanupHead == null) addCleanupHead = addCleanupTail = setNextNull(cur);
+                        else addCleanupHead = reverseOne(addCleanupHead, cur);
+                    }
+                    ++count;
+                    cur = next;
+                }
+            }
+
+            pendingCount += count;
+            if (addSequentialHead != null)
+            {
+                if (pendingSequentialHead == null) pendingSequentialHead = addSequentialHead;
+                else pendingSequentialTail.next = addSequentialHead;
+                pendingSequentialTail = addSequentialTail;
+            }
+            if (addCleanupHead != null)
+            {
+                if (pendingCleanupHead == null) pendingCleanupHead = addCleanupHead;
+                else pendingCleanupTail.next = addCleanupHead;
+                pendingCleanupTail = addCleanupTail;
+            }
+            if (addNewHead != null)
+            {
+                if (pendingNewHead == null) pendingNewHead = addNewHead;
+                else pendingNewTail.next = addNewHead;
+                pendingNewTail = addNewTail;
+            }
+            return true;
+        }
+
+        private Task reverseOne(Task prev, Task cur)
+        {
+            cur.next = prev;
+            return cur;
+        }
+
+        private Task setNextNull(Task cur)
+        {
+            cur.next = null;
+            return cur;
+        }
+
+        private boolean updateAndEnqueuePendingUntilHasWaitingToRun(int processAtLeast)
+        {
+            updatePendingUnqueued();
+            int count = 0;
+            while (enqueueOnePending())
+            {
+                if (++count >= processAtLeast && hasAlreadyWaitingToRun())
+                    return true;
+            }
+            return hasAlreadyWaitingToRun();
+        }
+
+        @Override
+        public void run()
+        {
+            lock.register(index, Thread.currentThread());
+            Task task = null;
+            while (true)
+            {
+                try
+                {
+                    try { task = cleanupAndMaybeGetWork(task); }
+                    catch (Throwable t) { task = null; throw t; }
+                    if (task == null)
+                        task = awaitWork();
+
+                    try
+                    {
+                        setRunning(task);
+                        task.runInternal();
+                    }
+                    catch (Throwable t)
+                    {
+                        try { task.fail(t); }
+                        catch (Throwable t2)
+                        {
+                            try
+                            {
+                                t2.addSuppressed(t);
+                                agent.onException(t2);
+                            }
+                            catch (Throwable t3) { /* empty to ensure we definitely loop so we cleanup the task */ }
+                        }
+                    }
+                }
+                catch (ShutdownException ignore)
+                {
+                    break;
+                }
+                catch (Throwable t)
+                {
+                    agent.onException(t);
+                }
+            }
+        }
+    }
+
+    @Override
+    public void shutdown()
+    {
+        lock.lock();
+        try
+        {
+            shutdown = true;
+            lock.signalAllRegistered();
+        }
+        finally
+        {
+            lock.unlock();
+        }
+    }
+
+    @Override
+    AccordExecutorLoops loops()
+    {
+        return loops;
+    }
+
+    @Override
+    boolean isInLoop()
+    {
+        return loops.isInLoop();
+    }
+
+    @Override
+    boolean isOwningThread()
+    {
+        return lock.isOwner();
+    }
+
+    @Override
+    public boolean isTerminated()
+    {
+        return loops.isTerminated();
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
+    {
+        return loops.awaitTermination(timeout, unit);
+    }
+}

src/java/org/apache/cassandra/service/accord/AccordExecutorSimple.java

@@ -35,7 +35,6 @@ class AccordExecutorSimple extends AccordExecutor
 {
     final ExecutorPlus executor;
     final ReentrantLock lock;
-    private Task active;
 
     public AccordExecutorSimple(int executorId, String name, Agent agent)
     {
@@ -87,30 +86,25 @@ class AccordExecutorSimple extends AccordExecutor
         lock.lock();
         try
         {
-            runningThreads = 1;
             while (true)
             {
                 Task task = pollWaitingToRunExclusive();
-                active = task;
                 if (task == null)
                 {
-                    runningThreads = 0;
                     notifyQuiescentExclusive();
                     return;
                 }
 
-                try { task.preRunExclusive(self); task.runInternal(); }
+                try { task.preRunExclusive(); task.runInternal(); }
                 catch (Throwable t) { task.fail(t); }
                 finally
                 {
                     completeTaskExclusive(task);
-                    active = null;
                 }
             }
         }
         finally
         {
-            runningThreads = 0;
             if (hasWaitingToRun())
                 executor.execute(this::run);
             lock.unlock();
@@ -118,7 +112,7 @@ class AccordExecutorSimple extends AccordExecutor
     }
 
     @Override
-    <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
     {
         lock.lock();
         try

src/java/org/apache/cassandra/service/accord/AccordExecutorSyncSubmit.java

@@ -27,7 +27,7 @@ import accord.api.Agent;
 import accord.utils.QuadFunction;
 import accord.utils.QuintConsumer;
 
-class AccordExecutorSyncSubmit extends AccordExecutorAbstractLockLoop
+public class AccordExecutorSyncSubmit extends AccordExecutorAbstractLockLoop
 {
     private final AccordExecutorLoops loops;
     private final ReentrantLock lock;
@@ -89,16 +89,16 @@ class AccordExecutorSyncSubmit extends AccordExecutorAbstractLockLoop
         hasWork.signal();
     }
 
-    <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Task> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
     {
-        lock.lock();
+        lock();
         try
         {
             submitExternalExclusive(sync, p1s, p2, p3, p4);
         }
         finally
         {
-            lock.unlock();
+            unlock();
         }
     }
 

src/java/org/apache/cassandra/service/accord/AccordObjectSizes.java

@@ -22,7 +22,6 @@ import java.util.UUID;
 import java.util.function.ToLongFunction;
 
 import accord.api.Key;
-import accord.api.Result;
 import accord.api.RoutingKey;
 import accord.local.Command;
 import accord.local.CommandBuilder;
@@ -39,6 +38,7 @@ import accord.primitives.FullKeyRoute;
 import accord.primitives.FullRangeRoute;
 import accord.primitives.KeyDeps;
 import accord.primitives.Keys;
+import accord.primitives.Known;
 import accord.primitives.PartialKeyRoute;
 import accord.primitives.PartialRangeRoute;
 import accord.primitives.PartialTxn;
@@ -50,6 +50,7 @@ import accord.primitives.RoutingKeys;
 import accord.primitives.SaveStatus;
 import accord.primitives.Seekable;
 import accord.primitives.Seekables;
+import accord.primitives.Status;
 import accord.primitives.Timestamp;
 import accord.primitives.Txn.Kind;
 import accord.primitives.TxnId;
@@ -67,10 +68,8 @@ import org.apache.cassandra.service.accord.api.TokenKey;
 import org.apache.cassandra.service.accord.serializers.ResultSerializers;
 import org.apache.cassandra.service.accord.serializers.TableMetadatasAndKeys;
 import org.apache.cassandra.service.accord.txn.AccordUpdate;
-import org.apache.cassandra.service.accord.txn.TxnData;
 import org.apache.cassandra.service.accord.txn.TxnQuery;
 import org.apache.cassandra.service.accord.txn.TxnRead;
-import org.apache.cassandra.service.accord.txn.TxnResult;
 import org.apache.cassandra.service.accord.txn.TxnWrite;
 import org.apache.cassandra.utils.ByteBufferUtil;
 import org.apache.cassandra.utils.ObjectSizes;
@@ -80,6 +79,7 @@ import static accord.local.cfk.CommandsForKey.InternalStatus.ACCEPTED;
 import static accord.primitives.SaveStatus.Invalidated;
 import static accord.primitives.SaveStatus.NotDefined;
 import static accord.primitives.SaveStatus.PreAccepted;
+import static accord.primitives.SaveStatus.Stable;
 import static accord.primitives.SaveStatus.TruncatedUnapplied;
 import static accord.primitives.Status.Durability.NotDurable;
 import static accord.primitives.TxnId.NO_TXNIDS;
@@ -293,20 +293,13 @@ public class AccordObjectSizes
         return size;
     }
 
-    public static long results(Result result)
-    {
-        if (result == ResultSerializers.APPLIED)
-            return 0;
-        return ((TxnResult) result).estimatedSizeOnHeap();
-    }
-
     private static class CommandEmptySizes
     {
         private final static PartitionKey EMPTY_KEY = new PartitionKey(EMPTY_ID, new BufferDecoratedKey(new Murmur3Partitioner.LongToken(1), ByteBufferUtil.EMPTY_BYTE_BUFFER));
         private final static TokenKey EMPTY_TOKEN_KEY = new TokenKey(EMPTY_ID, new Murmur3Partitioner.LongToken(1));
         private final static TxnId EMPTY_TXNID = new TxnId(42, 42, 0, Kind.Read, Domain.Key, new Node.Id(42));
 
-        private static Command build(SaveStatus saveStatus, boolean hasDeps, boolean hasTxn, boolean executes)
+        private static Command build(SaveStatus saveStatus, boolean executes)
         {
             Keys keys = Keys.of(EMPTY_KEY);
             FullKeyRoute route = new FullKeyRoute(EMPTY_TOKEN_KEY, new RoutingKey[]{ EMPTY_TOKEN_KEY });
@@ -315,30 +308,30 @@ public class AccordObjectSizes
                                        .durability(NotDurable)
                                        .executeAt(EMPTY_TXNID)
                                        .promised(Ballot.ZERO);
-            if (hasDeps)
+            if (saveStatus.known.hasAnyDeps())
                 builder.partialDeps(new Deps(KeyDeps.none(route.toParticipants()), RangeDeps.NONE).intersecting(route));
 
-            if (hasTxn)
+            if (saveStatus.known.isDefinitionKnown())
                 builder.partialTxn(new PartialTxn.InMemory(Kind.Read, keys, TxnRead.empty(Domain.Key), null, null, TableMetadatasAndKeys.none(Domain.Key)));
 
-            if (executes)
-            {
+            if (saveStatus.compareTo(Stable) >= 0 && !saveStatus.hasBeen(Status.Truncated))
                 builder.waitingOn(WaitingOn.empty(Domain.Key));
-                builder.result(new TxnData());
-            }
+
+            if (saveStatus.known.is(Known.Outcome.Apply))
+                builder.result(ResultSerializers.APPLIED);
 
             return builder.build(saveStatus);
         }
 
-        final static long NOT_DEFINED = measure(build(NotDefined, false, false, false));
-        final static long PREACCEPTED = measure(build(PreAccepted, false, true, false));
-        final static long NOTACCEPTED = measure(build(SaveStatus.AcceptedInvalidate, false, false, false));
-        final static long ACCEPTED = measure(build(SaveStatus.AcceptedMedium, true, false, false));
-        final static long COMMITTED = measure(build(SaveStatus.Committed, true, true, false));
-        final static long EXECUTED = measure(build(SaveStatus.Applied, true, true, true));
+        final static long NOT_DEFINED = measure(build(NotDefined, false));
+        final static long PREACCEPTED = measure(build(PreAccepted, false));
+        final static long NOTACCEPTED = measure(build(SaveStatus.AcceptedInvalidate, false));
+        final static long ACCEPTED = measure(build(SaveStatus.AcceptedMedium, false));
+        final static long COMMITTED = measure(build(SaveStatus.Committed, false));
+        final static long EXECUTED = measure(build(SaveStatus.Applied, true));
         // TODO (expected): TruncatedAwaitsOnlyDeps
-        final static long TRUNCATED = measure(build(TruncatedUnapplied, false, false, false).participants());
-        final static long INVALIDATED = measure(build(Invalidated, false, false, false).participants());
+        final static long TRUNCATED = measure(build(TruncatedUnapplied, false).participants());
+        final static long INVALIDATED = measure(build(Invalidated, false).participants());
 
         private static void touch() {}
 
@@ -407,7 +400,6 @@ public class AccordObjectSizes
         size += sizeNullable(command.partialDeps(), AccordObjectSizes::dependencies);
         size += sizeNullable(command.acceptedOrCommitted(), AccordObjectSizes::timestamp);
         size += sizeNullable(command.writes(), AccordObjectSizes::writes);
-        size += sizeNullable(command.result(), AccordObjectSizes::results);
         size += sizeNullable(command.waitingOn(), AccordObjectSizes::waitingOn);
         return size;
     }

src/java/org/apache/cassandra/service/accord/AccordResult.java

@@ -66,8 +66,9 @@ public class AccordResult<V> extends AsyncFuture<V> implements BiConsumer<V, Thr
     final RequestBookkeeping bookkeeping;
     final long startedAtNanos, deadlineAtNanos;
     final boolean isTxnRequest;
+    final @Nullable accord.api.Tracing tracing;
 
-    public AccordResult(@Nullable TxnId txnId, Seekables<?, ?> keysOrRanges, RequestBookkeeping bookkeeping, long startedAtNanos, long deadlineAtNanos, boolean isTxnRequest)
+    public AccordResult(@Nullable TxnId txnId, Seekables<?, ?> keysOrRanges, RequestBookkeeping bookkeeping, long startedAtNanos, long deadlineAtNanos, boolean isTxnRequest, @Nullable accord.api.Tracing tracing)
     {
         this.txnId = txnId;
         this.keysOrRanges = keysOrRanges;
@@ -75,6 +76,7 @@ public class AccordResult<V> extends AsyncFuture<V> implements BiConsumer<V, Thr
         this.startedAtNanos = startedAtNanos;
         this.deadlineAtNanos = deadlineAtNanos;
         this.isTxnRequest = isTxnRequest;
+        this.tracing = tracing;
     }
 
     @Override
@@ -125,6 +127,12 @@ public class AccordResult<V> extends AsyncFuture<V> implements BiConsumer<V, Thr
         if (isDone())
             return false;
 
+        if (tracing != null)
+        {
+            tracing.trace(null, "Failed: " + fail);
+            tracing.done();
+        }
+
         RequestExecutionException report;
         CoordinationFailed coordinationFailed = findCoordinationFailed(fail);
         TxnId txnId = this.txnId;
@@ -215,8 +223,18 @@ public class AccordResult<V> extends AsyncFuture<V> implements BiConsumer<V, Thr
         if (success == RetryWithNewProtocolResult.instance)
         {
             bookkeeping.markRetryDifferentSystem();
+            if (tracing != null) tracing.trace(null, "Got retry different system error from Accord, will retry");
+            // TODO (expected): proxy this through the accord Tracing system to avoid duplicate tracing
             Tracing.trace("Got retry different system error from Accord, will retry");
         }
+        else
+        {
+            if (tracing != null)
+            {
+                tracing.trace(null, "Success");
+                tracing.done();
+            }
+        }
         return true;
     }
 

src/java/org/apache/cassandra/service/accord/AccordService.java

@@ -46,6 +46,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import accord.api.ProtocolModifiers;
+import accord.api.Tracing;
 import accord.coordinate.CoordinateMaxConflict;
 import accord.coordinate.CoordinateTransaction;
 import accord.coordinate.KeyBarriers;
@@ -162,6 +163,7 @@ import org.apache.cassandra.utils.concurrent.UncheckedInterruptedException;
 
 import static accord.api.Journal.TopologyUpdate;
 import static accord.api.ProtocolModifiers.FastExecution.MAY_BYPASS_SAFESTORE;
+import static accord.coordinate.Coordination.CoordinationKind.Client;
 import static accord.impl.progresslog.DefaultProgressLog.ModeFlag.CATCH_UP;
 import static accord.local.durability.DurabilityService.SyncLocal.Self;
 import static accord.local.durability.DurabilityService.SyncRemote.All;
@@ -180,7 +182,6 @@ import static org.apache.cassandra.config.AccordConfig.CatchupMode.FALLBACK_TO_H
 import static org.apache.cassandra.config.AccordConfig.CatchupMode.HARD;
 import static org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode.RESET;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccord;
-import static org.apache.cassandra.config.DatabaseDescriptor.getAccordCommandStoreShardCount;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordGlobalDurabilityCycle;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordShardDurabilityCycle;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordShardDurabilityMaxSplits;
@@ -473,7 +474,7 @@ public class AccordService implements IAccordService, Shutdownable
                              topologyService,
                              time, new AtomicUniqueTimeWithStaleReservation(time),
                              () -> dataStore,
-                             new KeyspaceSplitter(new EvenSplit<>(getAccordCommandStoreShardCount(), getPartitioner().accordSplitter())),
+                             new KeyspaceSplitter(new EvenSplit<>(getAccord().commandStoreShardCount(), getPartitioner().accordSplitter())),
                              agent,
                              new DefaultRandom(),
                              scheduler,
@@ -1025,7 +1026,7 @@ public class AccordService implements IAccordService, Shutdownable
 
     public static <V> V getBlocking(AsyncChain<V> async, @Nullable TxnId txnId, Seekables<?, ?> keysOrRanges, RequestBookkeeping bookkeeping, long startedAt, long deadline, boolean isTxnRequest)
     {
-        AccordResult<V> result = new AccordResult<>(txnId, keysOrRanges, bookkeeping, startedAt, deadline, isTxnRequest);
+        AccordResult<V> result = new AccordResult<>(txnId, keysOrRanges, bookkeeping, startedAt, deadline, isTxnRequest, null);
         async.begin(result);
         return result.awaitAndGet();
     }
@@ -1037,7 +1038,7 @@ public class AccordService implements IAccordService, Shutdownable
 
     public static <V> V getBlocking(AsyncResult<V> async, @Nullable TxnId txnId, Seekables<?, ?> keysOrRanges, RequestBookkeeping bookkeeping, long startedAt, long deadline, boolean isTxnRequest)
     {
-        AccordResult<V> result = new AccordResult<>(txnId, keysOrRanges, bookkeeping, startedAt, deadline, isTxnRequest);
+        AccordResult<V> result = new AccordResult<>(txnId, keysOrRanges, bookkeeping, startedAt, deadline, isTxnRequest, null);
         async.invoke(result);
         return result.awaitAndGet();
     }
@@ -1142,7 +1143,8 @@ public class AccordService implements IAccordService, Shutdownable
         ClientRequestBookkeeping bookkeeping = txn.isWrite() ? accordWriteBookkeeping : accordReadBookkeeping;
         bookkeeping.metrics.keySize.update(txn.keys().size());
         long deadlineNanos = requestTime.computeDeadline(timeout);
-        AccordResult<TxnResult> result = new AccordResult<>(txnId, txn.keys(), bookkeeping, requestTime.startedAtNanos(), deadlineNanos, true);
+        Tracing tracing = agent().tracing().trace(txnId, txn.keys(), Client);
+        AccordResult<TxnResult> result = new AccordResult<>(txnId, txn.keys(), bookkeeping, requestTime.startedAtNanos(), deadlineNanos, true, tracing);
         node.coordinate(txnId, txn, minEpoch, deadlineNanos).begin((BiConsumer) result);
         return result;
     }

src/java/org/apache/cassandra/service/accord/AccordTask.java

@@ -70,10 +70,11 @@ import org.apache.cassandra.concurrent.DebuggableTask;
 import org.apache.cassandra.metrics.LogLinearDecayingHistograms;
 import org.apache.cassandra.service.accord.AccordCacheEntry.Status;
 import org.apache.cassandra.service.accord.AccordCommandStore.Caches;
-import org.apache.cassandra.service.accord.AccordExecutor.SubmittableTask;
+import org.apache.cassandra.service.accord.AccordExecutor.Task;
 import org.apache.cassandra.service.accord.AccordExecutor.TaskQueue;
 import org.apache.cassandra.service.accord.AccordKeyspace.CommandsForKeyAccessor;
 import org.apache.cassandra.service.accord.api.TokenKey;
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugTask;
 import org.apache.cassandra.service.accord.serializers.CommandSerializers;
 import org.apache.cassandra.tracing.Tracing;
 import org.apache.cassandra.utils.Clock;
@@ -100,9 +101,8 @@ import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_RU
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_SCAN_RANGES;
 import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
 import static org.apache.cassandra.service.accord.debug.DebugExecution.DebugTask.SANITY_CHECK;
-import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
-public abstract class AccordTask<R> extends SubmittableTask implements Function<SafeCommandStore, R>, Cancellable, DebuggableTask
+public abstract class AccordTask<R> extends Task implements Function<SafeCommandStore, R>, Cancellable, DebuggableTask
 {
     private static final Logger logger = LoggerFactory.getLogger(AccordTask.class);
     private static final NoSpamLogger noSpamLogger = NoSpamLogger.getLogger(logger, 1, TimeUnit.MINUTES);
@@ -629,6 +629,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     {
         if (SANITY_CHECK)
         {
+            DebugTask debug = DebugTask.get(this);
             if (debug.sanityCheck == null)
                 debug.sanityCheck = new ArrayList<>(commands.size());
             debug.sanityCheck.add(safeCommand.current());
@@ -637,13 +638,13 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
 
     private void save(List<Journal.CommandUpdate> diffs, Runnable onFlush)
     {
-        if (SANITY_CHECK && debug.sanityCheck != null)
+        if (SANITY_CHECK && DebugTask.get(this).sanityCheck != null)
         {
             Condition condition = Condition.newOneTimeCondition();
             this.commandStore.appendCommands(diffs, condition::signal);
             condition.awaitUninterruptibly();
 
-            for (Command check : debug.sanityCheck)
+            for (Command check : DebugTask.get(this).sanityCheck)
                 this.commandStore.sanityCheckCommand(commandStore.unsafeGetRedundantBefore(), check);
 
             if (onFlush != null) onFlush.run();
@@ -655,7 +656,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     }
 
     @Override
-    protected void preRunExclusive(Thread assigned)
+    protected void preRunExclusive()
     {
         state(ASSIGNED);
         queued = null;
@@ -673,10 +674,10 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     @Override
     public void runInternal()
     {
-        runningAt = nanoTime();
+        onRunning();
         logger.trace("Running {} with state {}", this, state);
         AccordSafeCommandStore safeStore = null;
-        try (Closeable close = locals.get())
+        try (Closeable close = resources.get())
         {
             if (Tracing.isTracing())
                 Tracing.trace(preLoadContext.describe());
@@ -717,7 +718,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
 
             commandStore.complete(safeStore);
             safeStore = null;
-            if (DEBUG_EXECUTION) debug.onRunComplete();
+            onRunComplete();
             if (!flush)
                 finish(result, null);
         }
@@ -757,9 +758,9 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     @Override
     protected void cleanupExclusive(AccordExecutor executor)
     {
-        super.cleanupExclusive(executor);
         Invariants.expect(state.isExecuted());
         releaseResources(commandStore.cachesExclusive());
+        super.cleanupExclusive(executor);
         executor.keys.increment(commandsForKey == null ? 0 : commandsForKey.size(), runningAt);
         if (histogramBuffer != null)
         {
@@ -825,18 +826,21 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
             {
                 rangeScanner.cleanup(caches);
                 rangeScanner = null;
+                if (DEBUG_EXECUTION) DebugTask.get(this).onReleasedRangeScanner();
             }
             if (commands != null)
             {
                 commands.forEach((k, v) -> caches.commands().release(v, this));
                 commands.clear();
                 commands = null;
+                if (DEBUG_EXECUTION) DebugTask.get(this).onReleasedCommands();
             }
             if (commandsForKey != null)
             {
                 commandsForKey.forEach((k, v) -> caches.commandsForKeys().release(v, this));
                 commandsForKey.clear();
                 commandsForKey = null;
+                if (DEBUG_EXECUTION) DebugTask.get(this).onReleasedCommandsForKeys();
             }
             if (waitingToLoad != null)
             {

src/java/org/apache/cassandra/service/accord/debug/DebugExecution.java

@@ -29,7 +29,9 @@ import accord.local.Command;
 
 import org.apache.cassandra.config.CassandraRelevantProperties;
 import org.apache.cassandra.metrics.LogLinearHistogram;
-import org.apache.cassandra.service.accord.AccordExecutor;
+import org.apache.cassandra.service.accord.AccordExecutor.Task;
+import org.apache.cassandra.utils.Closeable;
+import org.apache.cassandra.utils.WithResources;
 
 import static org.apache.cassandra.config.CassandraRelevantProperties.DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED;
 import static org.apache.cassandra.utils.Clock.Global.nanoTime;
@@ -38,9 +40,10 @@ public class DebugExecution
 {
     private static final Logger logger = LoggerFactory.getLogger(DebugExecution.class);
     public static final boolean DEBUG_EXECUTION = CassandraRelevantProperties.ACCORD_DEBUG_EXECUTION.getBoolean(false);
-    private static final long REPORT_MIN_LATENCY_MICROS = 10_000;
+    private static final long REPORT_MIN_LATENCY_MICROS = 20_000;
     private static final long REPORT_CPU_RATIO = 2;
     private static final long REPORT_MAX_LATENCY_MICROS = 50_000;
+    private static final long REPORT_CPU_MICROS = 10_000;
 
     // TODO (expected): use sharded histogram so we can report global stats
     public static class DebugExecutor
@@ -54,8 +57,8 @@ public class DebugExecution
         final LogLinearHistogram sequentialExecutorWaitingToRunLatency = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
         final LogLinearHistogram sequentialExecutorSetHeadToRunLatency = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
         final LogLinearHistogram pollToRun = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
-        final LogLinearHistogram applying = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
         final LogLinearHistogram running = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram runToCleanup = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
         final LogLinearHistogram cleanup = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
         final LogLinearHistogram taskTotal = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
 
@@ -84,6 +87,7 @@ public class DebugExecution
 
         public void onExitLock()
         {
+            // TODO (expected): specialise this for despatch loop, which can reasonably handle longer lock hold periods
             unlockedAt = nanoTime();
             unlockedAtCpu = nowCpu();
             long lockedForMicros = (unlockedAt - lockedAt)/1000;
@@ -138,7 +142,7 @@ public class DebugExecution
         final int commandStoreId;
 
         long setTaskAt, waitingAt;
-        AccordExecutor.Task prev;
+        Task prev;
 
         public DebugSequentialExecutor(DebugExecutor owner, int commandStoreId)
         {
@@ -146,13 +150,13 @@ public class DebugExecution
             this.commandStoreId = commandStoreId;
         }
 
-        public void onSetTask(AccordExecutor.Task next)
+        public void onSetTask(Task next)
         {
             if (next == null) setTaskAt = 0;
             else setTaskAt = nanoTime();
         }
 
-        public void onComplete(AccordExecutor.Task completed)
+        public void onComplete(Task completed)
         {
             long readyAt = setTaskAt;
             if (waitingAt > setTaskAt)
@@ -178,55 +182,102 @@ public class DebugExecution
         }
     }
 
-    public static class DebugTask
+    public static final class DebugTask implements WithResources
     {
         public static final boolean SANITY_CHECK = DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED.getBoolean();
         private static final boolean DEBUG = DEBUG_EXECUTION || SANITY_CHECK;
-        public static DebugTask maybeDebug() { return DEBUG ? new DebugTask() : null; }
+
+        public static WithResources maybeDebug(WithResources resources, Task task) { return DEBUG ? new DebugTask(resources, task) : resources; }
+        public static DebugTask get(Task task) { return (DebugTask)task.unwrap().resources; }
+
+        final WithResources resources;
+        final Task task;
+        public DebugTask(WithResources resources, Task task)
+        {
+            this.resources = resources;
+            this.task = task;
+        }
 
         public List<Command> sanityCheck; // for AccordTask only
-        long polledAt, appliedAt, completedAt;
-        long polledAtCpu, completedAtCpu;
+        long polledAt, preRunAt, runCompleteAt, completedAt;
+        long releasedRangeScannerAt, releasedCommandsAt, releasedCommandsForKeyAt;
+        long runningAtCpu, runCompleteAtCpu;
+        Thread thread;
 
         public void onPolled()
         {
             polledAt = nanoTime();
-            polledAtCpu = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
+        }
+
+        public void onPreRun()
+        {
+            preRunAt = nanoTime();
+        }
+
+        public void onRunning()
+        {
+            thread = Thread.currentThread();
+            runningAtCpu = nowCpu();
         }
 
         public void onRunComplete()
         {
-            appliedAt = nanoTime();
+            runCompleteAtCpu = nowCpu();
+            runCompleteAt = nanoTime();
         }
 
-        public void onCompleted(AccordExecutor.Task task, DebugExecutor owner)
+        public void onReleasedRangeScanner()
+        {
+            releasedRangeScannerAt = nanoTime();
+        }
+
+        public void onReleasedCommands()
+        {
+            releasedCommandsAt = nanoTime();
+        }
+
+        public void onReleasedCommandsForKeys()
+        {
+            releasedCommandsForKeyAt = nanoTime();
+        }
+
+        public void onCompleted(DebugExecutor owner)
         {
             completedAt = nanoTime();
-            completedAtCpu = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
             if (task.runningAt > 0 && polledAt > 0)
             {
                 long pollToRunMicros = (task.runningAt - polledAt)/1000;
                 owner.pollToRun.increment(pollToRunMicros);
-                long applyingMicros = -1;
-                if (appliedAt > 0)
+                long runningMicros = -1;
+                if (runCompleteAt > 0)
                 {
-                    applyingMicros = (appliedAt - task.runningAt)/1000;
-                    owner.applying.increment(applyingMicros);
-                }
-                long runningMicros = (task.cleanupAt - task.runningAt)/1000;
+                    runningMicros = (runCompleteAt - task.runningAt) / 1000;
                     owner.running.increment(runningMicros);
+                }
+                long runToCleanMicros = (task.cleanupAt - runCompleteAt)/1000;
+                owner.runToCleanup.increment(runToCleanMicros);
                 long cleanupMicros = (completedAt - task.cleanupAt)/1000;
                 owner.cleanup.increment(cleanupMicros);
                 long totalMicros = (completedAt - polledAt)/1000;
                 owner.taskTotal.increment(totalMicros);
-                long totalCpu = (completedAtCpu - polledAtCpu)/1000;
-                if (totalMicros > REPORT_MAX_LATENCY_MICROS || (totalMicros > REPORT_MIN_LATENCY_MICROS && (totalMicros/totalCpu) >= REPORT_CPU_RATIO))
+                long totalCpu = (runCompleteAtCpu - runningAtCpu)/1000;
+                if (totalMicros > REPORT_MAX_LATENCY_MICROS || totalCpu > REPORT_CPU_MICROS || (totalMicros > REPORT_MIN_LATENCY_MICROS && (totalCpu == 0 || totalMicros/totalCpu >= REPORT_CPU_RATIO)))
                 {
-                    report("{}: total {}us {}cpu, running {}us{}, cleanup {}us, pollToRun {}us", task, totalMicros, totalCpu,
-                           runningMicros, (applyingMicros >= 0 ? ", applying " + applyingMicros + "us" : ""), cleanupMicros, pollToRunMicros);
+                    String reason = "";
+                    if (totalMicros > REPORT_MAX_LATENCY_MICROS) reason += "LONG TIME ";
+                    if (totalCpu > REPORT_CPU_MICROS) reason += "HIGH CPU ";
+                    if ((totalMicros > REPORT_MIN_LATENCY_MICROS && (totalMicros/totalCpu) >= REPORT_CPU_RATIO)) reason += "LOW RATIO ";
+                    report("{}{}: total {}us cpu:{}us ({}), pollToRun {}us, running {}us, runToClean {}us, cleanup {}us",
+                           reason, task, totalMicros, totalCpu, thread, pollToRunMicros, runningMicros, runToCleanMicros, cleanupMicros);
                 }
             }
         }
+
+        @Override
+        public Closeable get()
+        {
+            return resources.get();
+        }
     }
 
     private static void report(String message, Object ... params)

src/java/org/apache/cassandra/service/accord/interop/AccordInteropApply.java

@@ -23,7 +23,7 @@ import javax.annotation.Nullable;
 import org.agrona.collections.Int2ObjectHashMap;
 
 import accord.api.LocalListeners;
-import accord.api.Result;
+import accord.api.Result.PersistableResult;
 import accord.coordinate.ExecuteFlag.ExecuteFlags;
 import accord.local.Command;
 import accord.local.Node.Id;
@@ -70,7 +70,7 @@ public class AccordInteropApply extends Apply implements LocalListeners.ComplexL
     public static final Apply.Factory FACTORY = new Apply.Factory()
     {
         @Override
-        public Apply create(Kind kind, Id to, Topologies participates, TxnId txnId, Ballot ballot, Route<?> route, Txn txn, Timestamp executeAt, Deps deps, Writes writes, Result result, FullRoute<?> fullRoute, ExecuteFlags flags)
+        public Apply create(Kind kind, Id to, Topologies participates, TxnId txnId, Ballot ballot, Route<?> route, Txn txn, Timestamp executeAt, Deps deps, Writes writes, PersistableResult result, FullRoute<?> fullRoute, ExecuteFlags flags)
         {
             checkArgument(kind != Kind.Maximal || to.equals(AccordService.nodeId()), "Shouldn't need to send a maximal commit with interop support");
             ConsistencyLevel commitCL = txn.update() instanceof AccordUpdate ? ((AccordUpdate) txn.update()).cassandraCommitCL() : null;
@@ -84,7 +84,7 @@ public class AccordInteropApply extends Apply implements LocalListeners.ComplexL
     public static final IVersionedSerializer<AccordInteropApply> serializer = new ApplySerializer<AccordInteropApply>()
     {
         @Override
-        protected AccordInteropApply deserializeApply(TxnId txnId, Ballot ballot, Route<?> scope, long minEpoch, long waitForEpoch, long maxEpoch, Apply.Kind kind, Timestamp executeAt, PartialDeps deps, PartialTxn txn, @Nullable FullRoute<?> fullRoute, Writes writes, Result result, ExecuteFlags flags)
+        protected AccordInteropApply deserializeApply(TxnId txnId, Ballot ballot, Route<?> scope, long minEpoch, long waitForEpoch, long maxEpoch, Apply.Kind kind, Timestamp executeAt, PartialDeps deps, PartialTxn txn, @Nullable FullRoute<?> fullRoute, Writes writes, PersistableResult result, ExecuteFlags flags)
         {
             return new AccordInteropApply(kind, txnId, ballot, scope, minEpoch, waitForEpoch, maxEpoch, executeAt, deps, txn, fullRoute, writes, result, flags);
         }
@@ -94,12 +94,12 @@ public class AccordInteropApply extends Apply implements LocalListeners.ComplexL
     transient Int2ObjectHashMap<LocalListeners.Registered> listeners;
     boolean failed;
 
-    private AccordInteropApply(Kind kind, TxnId txnId, Ballot ballot, Route<?> route, long minEpoch, long waitForEpoch, long maxEpoch, Timestamp executeAt, PartialDeps deps, @Nullable PartialTxn txn, @Nullable FullRoute<?> fullRoute, Writes writes, Result result, ExecuteFlags flags)
+    private AccordInteropApply(Kind kind, TxnId txnId, Ballot ballot, Route<?> route, long minEpoch, long waitForEpoch, long maxEpoch, Timestamp executeAt, PartialDeps deps, @Nullable PartialTxn txn, @Nullable FullRoute<?> fullRoute, Writes writes, PersistableResult result, ExecuteFlags flags)
     {
         super(kind, txnId, ballot, route, minEpoch, waitForEpoch, maxEpoch, executeAt, deps, txn, fullRoute, writes, result, flags);
     }
 
-    private AccordInteropApply(Kind kind, Id to, Topologies participates, TxnId txnId, Ballot ballot, Route<?> route, Txn txn, Timestamp executeAt, Deps deps, Writes writes, Result result, FullRoute<?> fullRoute, ExecuteFlags flags)
+    private AccordInteropApply(Kind kind, Id to, Topologies participates, TxnId txnId, Ballot ballot, Route<?> route, Txn txn, Timestamp executeAt, Deps deps, Writes writes, PersistableResult result, FullRoute<?> fullRoute, ExecuteFlags flags)
     {
         super(kind, to, participates, txnId, ballot, route, txn, executeAt, deps, writes, result, fullRoute, flags);
     }

src/java/org/apache/cassandra/service/accord/interop/AccordInteropPersist.java

@@ -55,7 +55,7 @@ public class AccordInteropPersist extends Persist
 {
     public AccordInteropPersist(Node node, SequentialAsyncExecutor executor, Topologies topologies, TxnId txnId, Route<?> sendTo, Ballot ballot, Txn txn, Timestamp executeAt, Deps deps, Writes writes, Result result, FullRoute<?> fullRoute, ConsistencyLevel consistencyLevel, ExecuteFlag.CoordinationFlags flags, boolean informDurableOnDone, Apply.Kind applyKind, BiConsumer<? super Result, Throwable> callback)
     {
-        super(node, executor, topologies, txnId, ballot, sendTo, txn, executeAt, deps, writes, result, fullRoute, flags, informDurableOnDone, AccordInteropApply.FACTORY, applyKind, consistencyLevel == ALL ? AllTracker::new : QuorumTracker::new, (ignore, fail) -> {
+        super(node, executor, topologies, txnId, ballot, sendTo, txn, executeAt, deps, writes, result.toPersistable(), fullRoute, flags, informDurableOnDone, AccordInteropApply.FACTORY, applyKind, consistencyLevel == ALL ? AllTracker::new : QuorumTracker::new, (ignore, fail) -> {
             if (fail != null) callback.accept(null, fail);
             else callback.accept(result, null);
         });

src/java/org/apache/cassandra/service/accord/serializers/ApplySerializers.java

@@ -20,7 +20,7 @@ package org.apache.cassandra.service.accord.serializers;
 
 import java.io.IOException;
 
-import accord.api.Result;
+import accord.api.Result.PersistableResult;
 import accord.coordinate.ExecuteFlag.ExecuteFlags;
 import accord.messages.Apply;
 import accord.messages.Apply.ApplyReply;
@@ -64,7 +64,7 @@ public class ApplySerializers
         }
 
         protected abstract A deserializeApply(TxnId txnId, Ballot ballot, Route<?> scope, long minEpoch, long waitForEpoch, long maxEpoch, Apply.Kind kind,
-                                              Timestamp executeAt, PartialDeps deps, PartialTxn txn, FullRoute<?> fullRoute, Writes writes, Result result, ExecuteFlags flags);
+                                              Timestamp executeAt, PartialDeps deps, PartialTxn txn, FullRoute<?> fullRoute, Writes writes, PersistableResult result, ExecuteFlags flags);
 
         @Override
         public A deserializeBody(DataInputPlus in, Version version, TxnId txnId, Route<?> scope, long waitForEpoch) throws IOException
@@ -104,7 +104,7 @@ public class ApplySerializers
     {
         @Override
         protected Apply deserializeApply(TxnId txnId, Ballot ballot, Route<?> scope, long minEpoch, long waitForEpoch, long maxEpoch, Apply.Kind kind,
-                                         Timestamp executeAt, PartialDeps deps, PartialTxn txn, FullRoute<?> fullRoute, Writes writes, Result result, ExecuteFlags flags)
+                                         Timestamp executeAt, PartialDeps deps, PartialTxn txn, FullRoute<?> fullRoute, Writes writes, PersistableResult result, ExecuteFlags flags)
         {
             return Apply.SerializationSupport.create(txnId, ballot, scope, minEpoch, waitForEpoch, maxEpoch, kind, executeAt, deps, txn, fullRoute, writes, result, flags);
         }

src/java/org/apache/cassandra/service/accord/serializers/CheckStatusSerializers.java

@@ -21,7 +21,7 @@ package org.apache.cassandra.service.accord.serializers;
 import java.io.IOException;
 import java.util.Objects;
 
-import accord.api.Result;
+import accord.api.Result.PersistableResult;
 import accord.api.RoutingKey;
 import accord.coordinate.Infer;
 import accord.messages.CheckStatus;
@@ -228,7 +228,7 @@ public class CheckStatusSerializers
                     PartialDeps committedDeps = DepsSerializers.nullablePartialDeps.deserialize(in);
                     Writes writes = CommandSerializers.nullableWrites.deserialize(in, version);
 
-                    Result result = null;
+                    PersistableResult result = null;
                     if (maxKnowledgeStatus.known.outcome().isOrWasApply())
                         result = ResultSerializers.APPLIED;
 

src/java/org/apache/cassandra/service/accord/serializers/ResultSerializers.java

@@ -18,7 +18,7 @@
 
 package org.apache.cassandra.service.accord.serializers;
 
-import accord.api.Result;
+import accord.api.Result.PersistableResult;
 
 import org.apache.cassandra.io.UnversionedSerializer;
 import org.apache.cassandra.io.util.DataInputPlus;
@@ -27,17 +27,17 @@ import org.apache.cassandra.io.util.DataOutputPlus;
 public class ResultSerializers
 {
     // TODO (desired): this is meant to encode e.g. whether the transaction's condition met or not for clients to later query
-    public static final Result APPLIED = new Result(){};
+    public static final PersistableResult APPLIED = new PersistableResult(){};
 
-    public static final UnversionedSerializer<Result> result = new UnversionedSerializer<>()
+    public static final UnversionedSerializer<PersistableResult> result = new UnversionedSerializer<>()
     {
-        public void serialize(Result t, DataOutputPlus out) { }
-        public Result deserialize(DataInputPlus in)
+        public void serialize(PersistableResult t, DataOutputPlus out) { }
+        public PersistableResult deserialize(DataInputPlus in)
         {
             return APPLIED;
         }
 
-        public long serializedSize(Result t)
+        public long serializedSize(PersistableResult t)
         {
             return 0;
         }

src/java/org/apache/cassandra/service/accord/txn/TxnData.java

@@ -116,7 +116,7 @@ public class TxnData extends Int2ObjectHashMap<TxnDataValue> implements TxnResul
 
     private TxnData(int size)
     {
-        super(size, 0.65f);
+        super(size, 0.65f, false);
     }
 
     public static TxnData of(int key, TxnDataValue value)

src/java/org/apache/cassandra/service/accord/txn/TxnResult.java

@@ -20,6 +20,8 @@ package org.apache.cassandra.service.accord.txn;
 
 import accord.api.Result;
 
+import static org.apache.cassandra.service.accord.serializers.ResultSerializers.APPLIED;
+
 public interface TxnResult extends Result
 {
     enum Kind
@@ -40,4 +42,10 @@ public interface TxnResult extends Result
     Kind kind();
 
     long estimatedSizeOnHeap();
+
+    default PersistableResult toPersistable()
+    {
+        // TODO (required): should we persist the Kind?
+        return APPLIED;
+    }
 }

src/java/org/apache/cassandra/utils/concurrent/SignalLock.java

@@ -0,0 +1,798 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.utils.concurrent;
+
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicLongArray;
+import java.util.concurrent.atomic.AtomicLongFieldUpdater;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.LockSupport;
+
+import accord.utils.Invariants;
+
+import org.apache.cassandra.utils.Nemesis;
+
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
+
+/**
+ * A lock that supports semi-asynchronous operation; a thread may take the lock synchronously,
+ * or it may await a signal that it should acquire the lock (to perform synchronous work),
+ * or a signal that work has been made available for it outside the lock.
+ *
+ * In some systems it may be that LockSupport.unpark() often results in the calling thread becoming the callee,
+ * which can be harmful if the caller owns the lock. To avoid this problem, we support making available permits
+ * that can be acquired asynchronously, and only on releasing the lock do we attempt to actually directly wake
+ * any threads. In this mode of operation it is up to the user to unlock when there is sufficient work to signal.
+ *
+ * TODO (expected): once we support a high enough JDK, port to compareAndExchange/weakCompareAndSet as appropriate
+ */
+public final class SignalLock implements Lock
+{
+    private static final AtomicLongFieldUpdater<SignalLock> stateUpdater = AtomicLongFieldUpdater.newUpdater(SignalLock.class, "state");
+
+    private static final long HAS_LOCK_WORK = 0x8000000000000000L;
+    // if there is a LOCK_SIGNAL, to avoid unnecessary wakeups we pick a waiting thread to unpark and store it in LOCK_SIGNALLED_MASK bits
+    private static final int LOCK_THREAD_SHIFT = 64 - 7;
+    private static final int LOCK_OWNED_SHIFT = LOCK_THREAD_SHIFT - 1;
+    private static final int ASYNC_SIGNAL_COUNT_SHIFT = LOCK_OWNED_SHIFT - 6;
+    private static final int ENABLED_THREAD_COUNT_SHIFT = ASYNC_SIGNAL_COUNT_SHIFT - 6;
+    public static final int MAX_THREADS = ENABLED_THREAD_COUNT_SHIFT;
+    public static final int THREAD_ID_MASK = 0x3f;
+    public static final int MAX_SIGNAL_COUNT = THREAD_ID_MASK;
+    private static final long LOCK_THREAD_MASK = (long) THREAD_ID_MASK << LOCK_THREAD_SHIFT;
+    private static final long LOCK_OWNED = 1L << LOCK_OWNED_SHIFT;
+    private static final long LOCK_SIGNALLED = 0L;
+    private static final long ASYNC_SIGNAL_INCREMENT = 1L << ASYNC_SIGNAL_COUNT_SHIFT;
+    private static final long ENABLED_THREAD_COUNT_INCREMENT = 1L << ENABLED_THREAD_COUNT_SHIFT;
+    private static final long ENABLED_THREAD_COUNT_MASK = (long)THREAD_ID_MASK << ENABLED_THREAD_COUNT_SHIFT;
+    private static final long WAITING_THREADS_MASK = -1L >>> (64 - MAX_THREADS);
+
+    private final boolean allowStealing;
+    private final Thread[] registered;
+    @Nemesis private volatile long state;
+    private Thread owner;
+    private final ConcurrentLinkedQueue<Thread> locking = new ConcurrentLinkedQueue<>();
+    private final AtomicLongArray stopLog;
+    private final AtomicLong stopCheck;
+    private final long stopCheckIntervalNanos;
+    private final long spinIntervalNanos;
+    private boolean preferRegistered;
+    private boolean autoPreferRegistered = true;
+
+    private int depth;
+
+    public SignalLock(int threadCount)
+    {
+        this(true, threadCount, 0, 0, null);
+    }
+
+    public SignalLock(int threadCount, long spinInterval, long stopCheckInterval, TimeUnit units)
+    {
+        this(true, threadCount, spinInterval, stopCheckInterval, units);
+    }
+
+    public SignalLock(boolean allowStealing, int threadCount, long spinInterval, long stopCheckInterval, TimeUnit units)
+    {
+        Invariants.requireArgument(threadCount <= MAX_THREADS, "Supports at most " + MAX_THREADS + " registered threads");
+        this.allowStealing = allowStealing;
+        this.registered = new Thread[threadCount];
+        this.state = (long)threadCount << ENABLED_THREAD_COUNT_SHIFT;
+        this.spinIntervalNanos = spinInterval <= 0 ? 0 : units.toNanos(spinInterval);
+        if (stopCheckInterval > 0)
+        {
+            this.stopLog = new AtomicLongArray(threadCount);
+            this.stopCheck = new AtomicLong();
+            this.stopCheckIntervalNanos = units.toNanos(stopCheckInterval);
+        }
+        else
+        {
+            this.stopLog = null;
+            this.stopCheck = null;
+            this.stopCheckIntervalNanos = -1;
+        }
+    }
+
+    public void register(int index, Thread thread)
+    {
+        registered[index] = thread;
+    }
+
+    public Thread registeredThread(int index)
+    {
+        return registered[index];
+    }
+
+    public void lock()
+    {
+        Thread self = Thread.currentThread();
+        if (tryLock(self))
+            return;
+
+        locking.add(self);
+        while (true)
+        {
+            long cur = state;
+            if (isLockAvailable(cur, ANONYMOUS_OWNER, self))
+            {
+                long upd = setLockThread(clearLockThread(cur), ANONYMOUS_OWNER, LOCK_OWNED);
+                if (!stateUpdater.compareAndSet(this, cur, upd))
+                    continue;
+
+                Invariants.require(depth == 0);
+                Invariants.require(owner == null);
+                depth = 1;
+                owner = self;
+
+                Invariants.require(locking.peek() == self);
+                locking.poll();
+                return;
+            }
+
+            LockSupport.park();
+        }
+    }
+
+    @Override
+    public boolean tryLock()
+    {
+        return tryLock(Thread.currentThread());
+    }
+
+    private boolean tryLock(Thread self)
+    {
+        return tryLock(ANONYMOUS_OWNER, self);
+    }
+
+    public boolean tryLock(int thread)
+    {
+        Invariants.require(thread >= 0 && thread < registered.length);
+        Thread self = registeredThread(thread);
+        return tryLock(thread, self);
+    }
+
+    private boolean tryLock(int threadOrAnonymous, Thread self)
+    {
+        if (owner != self)
+        {
+            while (true)
+            {
+                long cur = state;
+                if (!isLockAvailable(cur, threadOrAnonymous, null))
+                    return false;
+
+                long upd = setLockThread(clearLockThread(cur), threadOrAnonymous, LOCK_OWNED);
+                if (!stateUpdater.compareAndSet(this, cur, upd))
+                    continue;
+
+                Invariants.require(depth == 0);
+                Invariants.require(owner == null);
+                owner = self;
+                break;
+            }
+        }
+
+        ++depth;
+        return true;
+    }
+
+    /**
+     * Wait for the lock to be signalled and acquire it, or wait for additional work to be signalled.
+     * Return true if we acquired the lock.
+     */
+    public boolean awaitAsyncOrLock(int thread)
+    {
+        return awaitAsyncOrLock(thread, 1);
+    }
+
+    public boolean awaitAsyncOrLock(int thread, int burstSignalOnAcquire)
+    {
+        long threadBit = threadBit(thread);
+        Thread self = registered[thread];
+        Invariants.require(self == Thread.currentThread());
+        Invariants.require(owner != self);
+        Invariants.require((state & threadBit) == 0);
+
+        boolean hasPaused = false;
+        if (spinIntervalNanos > 0)
+        {
+            while (true)
+            {
+                long cur = state;
+                if (hasLockWork(cur) && isLockAvailable(cur, thread, null))
+                {
+                    if (tryTakeLockInAwaitLoop(cur, thread, self, hasPaused)) return true;
+                    else continue;
+                }
+                else if (tryAcquireAsyncInAwaitLoop(cur, thread, 0L, hasPaused, burstSignalOnAcquire))
+                {
+                    return false;
+                }
+
+                int enabledCount = enabledThreadCount(cur);
+                if (thread >= enabledCount || enabledCount <= 1)
+                    break; // if we're a disabled thread, or the last thread, move to blocking loop
+
+                hasPaused = pause(thread, hasPaused);
+                int waitingEnabledThreadCount = waitingEnabledThreadCount(cur);
+                int multiplier = 1 + waitingEnabledThreadCount == 0 ? 0 : ThreadLocalRandom.current().nextInt(2 * waitingEnabledThreadCount(cur));
+                LockSupport.parkNanos(spinIntervalNanos * multiplier);
+            }
+        }
+
+        long cur = stateUpdater.addAndGet(this, threadBit);
+        while (true)
+        {
+            do   // dummy do-while-false loop, to ensure we refresh cur
+            {
+                boolean isSignalled = (cur & threadBit) == 0;
+                if (isSignalled)
+                {
+                    Invariants.require(lockThread(cur) != thread);
+                    unpause(thread, hasPaused);
+                    propagateAsyncWorkSignals(cur, burstSignalOnAcquire);
+                    return false;
+                }
+                else if (hasLockWork(cur) && isLockAvailable(cur, thread, null))
+                {
+                    if (tryTakeLockInAwaitLoop(cur, thread, self, hasPaused)) return true;
+                    else continue;
+                }
+                else if (tryAcquireAsyncInAwaitLoop(cur, thread, threadBit, hasPaused, burstSignalOnAcquire))
+                {
+                    return false;
+                }
+
+                hasPaused = pause(thread, hasPaused);
+                LockSupport.park();
+            }
+            while (false);
+
+            cur = state;
+        }
+    }
+
+    private boolean tryTakeLockInAwaitLoop(long cur, int thread, Thread self, boolean hasPaused)
+    {
+        long upd = setLockThread(clearLockThread(cur), thread, LOCK_OWNED) ^ threadBit(thread);
+        if (!stateUpdater.compareAndSet(this, cur, upd))
+            return false;
+
+        Invariants.require(depth == 0);
+        Invariants.require(owner == null);
+        owner = self;
+        depth = 1;
+        unpause(thread, hasPaused);
+        return true;
+    }
+
+    private boolean pause(int thread, boolean hasPaused)
+    {
+        if (!hasPaused && stopLog != null)
+            stopLog.set(thread, nanoTime());
+        return true;
+    }
+
+    private void unpause(int thread, boolean hasPaused)
+    {
+        if (hasPaused && stopLog != null)
+        {
+            long start = stopLog.getAndSet(thread, 0);
+            long stop = nanoTime();
+            updateStopCheck(stop - start, stop);
+        }
+    }
+
+    private void updateStopCheck(long stoppedFor, long now)
+    {
+        long current = stopCheck.addAndGet(stoppedFor);
+        if (current > now && stopCheck.compareAndSet(current, now - stopCheckIntervalNanos))
+            decrementEnabledThreadCount();
+        else if (current < now - 2 * stopCheckIntervalNanos)
+            stopCheck.compareAndSet(current, now - stopCheckIntervalNanos);
+    }
+
+    @Override
+    public void unlock()
+    {
+        unlock(1, false);
+    }
+
+    public void unlock(int burstSignal)
+    {
+        unlock(burstSignal, false);
+    }
+
+    public boolean unlockAndAcquireAsyncWork()
+    {
+        return unlockAndAcquireAsyncWork(1);
+    }
+
+    public boolean unlockAndAcquireAsyncWork(int burstSignal)
+    {
+        return unlock(burstSignal, true);
+    }
+
+    private boolean unlock(int burstSignal, boolean acquire)
+    {
+        Thread self = owner;
+        Invariants.require(self == Thread.currentThread());
+        Invariants.require(!acquire || depth == 1, "Cannot acquire async work with reentrancy (depth " + depth + ')');
+        return --depth == 0 && releaseAndSignalExclusive(burstSignal, acquire);
+    }
+
+    private boolean releaseAndSignalExclusive(int burstSignal, boolean acquire)
+    {
+        boolean acquired = acquire && tryAcquireAsyncWork();
+        owner = null;
+        Thread next = locking.peek();
+        boolean preferRegistered = this.preferRegistered;
+        while (true)
+        {
+            long cur = state;
+            boolean wakeupLockWork = hasLockWork(cur) && hasMoreWaitersThanSignals(cur);
+            if (next == null && !wakeupLockWork)
+            {
+                if (stateUpdater.compareAndSet(this, cur, clearLockThread(cur)))
+                {
+                    next = locking.peek();
+                    if (next != null)
+                        LockSupport.unpark(next);
+                    break;
+                }
+                continue;
+            }
+
+            int thread = ANONYMOUS_OWNER;
+            if (wakeupLockWork)
+            {
+                if (preferRegistered || next == null)
+                    thread = pickThreadToSignalForLock(cur);
+                if (next != null && autoPreferRegistered)
+                    this.preferRegistered = !preferRegistered;
+            }
+
+            long upd = setLockThread(clearLockThread(cur), thread, LOCK_SIGNALLED);
+            if (stateUpdater.compareAndSet(this, cur, upd))
+            {
+                if (thread >= 0) unpark(thread);
+                else LockSupport.unpark(next);
+                break;
+            }
+
+            // cancel the flip of prefer registered (if any)
+            this.preferRegistered = preferRegistered;
+        }
+
+        propagateAsyncWorkSignals(burstSignal);
+        return acquired;
+    }
+
+    private boolean isLockAvailable(long cur, int thread, Thread waiting)
+    {
+        if (isLockOwned(cur))
+            return thread >= 0 && lockThread(cur) == thread;
+
+        if (thread >= 0 || waiting == null)
+            return allowStealing || !isLockOwnedOrSignalled(cur);
+
+        return locking.peek() == waiting && lockThread(cur) < 0;
+    }
+
+    private boolean tryAcquireAsyncInAwaitLoop(long cur, int thread, long threadBitIfWaiting, boolean hasPaused, int burstSignalOnAcquire)
+    {
+        while (true)
+        {
+            int signals = asyncSignalCount(cur);
+            if (signals == 0)
+                return false;
+
+            if ((cur & threadBitIfWaiting) != 0)
+                return false;
+
+            long upd = (cur - ASYNC_SIGNAL_INCREMENT) ^ threadBitIfWaiting;
+            if (!stateUpdater.compareAndSet(this, cur, upd))
+            {
+                cur = state;
+                continue;
+            }
+
+            propagateAsyncWorkSignals(upd, burstSignalOnAcquire);
+            unpause(thread, hasPaused);
+            return true;
+        }
+    }
+
+    public boolean incrementAsyncWork(boolean signalIfWaiting)
+    {
+        while (true)
+        {
+            long cur = state;
+            int count = asyncSignalCount(cur);
+            if (count == MAX_THREADS)
+                return false;
+
+            if (signalIfWaiting)
+            {
+                int thread = pickThreadToSignalAsync(cur);
+                if (thread != NO_THREAD)
+                {
+                    long upd = cur ^ threadBit(thread);
+                    if (stateUpdater.compareAndSet(this, cur, upd))
+                        return unpark(thread);
+                    continue;
+                }
+            }
+
+            long upd = cur + ASYNC_SIGNAL_INCREMENT;
+            if (stateUpdater.compareAndSet(this, cur, upd))
+                return true;
+        }
+    }
+
+    public boolean tryAcquireAsyncWork()
+    {
+        while (true)
+        {
+            long cur = state;
+            int count = asyncSignalCount(cur);
+            if (count == 0)
+                return false;
+
+            long upd = cur - ASYNC_SIGNAL_INCREMENT;
+            if (stateUpdater.compareAndSet(this, cur, upd))
+                return true;
+        }
+    }
+
+    public void propagateAsyncWorkSignals()
+    {
+        propagateAsyncWorkSignals(Integer.MAX_VALUE);
+    }
+
+    public void propagateAsyncWorkSignals(int burstSignal)
+    {
+        propagateAsyncWorkSignals(state, burstSignal);
+    }
+
+    private void propagateAsyncWorkSignals(long cur, int burstSignal)
+    {
+        while (burstSignal > 0)
+        {
+            int signals = asyncSignalCount(cur);
+            int thread = pickThreadToSignalAsync(cur);
+            if (signals == 0 || thread == NO_THREAD)
+                return;
+
+            long threadBit = threadBit(thread);
+            Invariants.require((threadBit & cur) != 0);
+            long upd = (cur - ASYNC_SIGNAL_INCREMENT) ^ threadBit;
+            if (!stateUpdater.compareAndSet(this, cur, upd)) cur = state;
+            else
+            {
+                unpark(thread);
+                --burstSignal;
+                cur = upd;
+            }
+        }
+    }
+
+    /**
+     * Signal a waiting registered thread to try and acquire the lock.
+     * Return true if the lock is held, or the signal has been propagated
+     */
+    private void wakeupForLockWork()
+    {
+        while (true)
+        {
+            long cur = state;
+
+            if (!hasLockWork(cur) || isLockOwnedOrSignalled(cur))
+                return;
+
+            // signal lock owners from end backwards, and async work from front forwards
+            int thread = pickThreadToSignalForLock(cur);
+            if (thread < 0)
+                return;
+
+            Invariants.require(!isLockOwned(cur));
+            long upd = setLockThread(cur, thread, LOCK_SIGNALLED);
+            if (stateUpdater.compareAndSet(this, cur, upd))
+            {
+                unpark(thread);
+                return;
+            }
+        }
+    }
+
+    private boolean unpark(int thread)
+    {
+        LockSupport.unpark(registered[thread]);
+        return true;
+    }
+
+    /**
+     * Signal that there is work to do on the lock
+     * If the lock is free and there are any waiting threads, wake one
+     */
+    public void signalLockWork()
+    {
+        if (setHasLockWork())
+            wakeupForLockWork();
+    }
+
+    /**
+     * Signal that there is work to do on the lock
+     * If the lock is free and there are any waiting threads, wake one
+     */
+    public void signalLockWorkExclusive()
+    {
+        setHasLockWork();
+    }
+
+    private boolean setHasLockWork()
+    {
+        return 0 == (stateUpdater.getAndUpdate(this, v -> v | HAS_LOCK_WORK) & HAS_LOCK_WORK);
+    }
+
+    public void signalAllRegistered()
+    {
+        stateUpdater.updateAndGet(this, v -> v & ~WAITING_THREADS_MASK);
+        for (Thread thread : registered)
+        {
+            if (thread != null)
+                LockSupport.unpark(thread);
+        }
+    }
+
+    /**
+     * Work that requires the lock has been finished.
+     * This must be called prior to draining all work that has been asynchronously submitted for processing with the lock.
+     * Otherwise, the caller must check whether new work has been submitted prior to calling unlock,
+     * and if so first call signalLockExclusive().
+     */
+    public void clearLockWork()
+    {
+        stateUpdater.updateAndGet(this, v -> v & ~HAS_LOCK_WORK);
+    }
+
+    public long state()
+    {
+        return state;
+    }
+
+    public int asyncSignalCount()
+    {
+        return asyncSignalCount(state);
+    }
+
+    public int waitingThreadCount()
+    {
+        return waitingThreadCount(state);
+    }
+
+    public int enabledThreadCount()
+    {
+        return enabledThreadCount(state);
+    }
+
+    // if enabledThreadCount() == 1 return false; otherwise reduce by one
+    private boolean decrementEnabledThreadCount()
+    {
+        while (true)
+        {
+            long cur = state;
+            if (enabledThreadCount(cur) == 1)
+                return false;
+
+            if (stateUpdater.compareAndSet(this, cur, cur - ENABLED_THREAD_COUNT_INCREMENT))
+                return true;
+        }
+    }
+
+    public int addAndGetEnabledThreadCount(int delta)
+    {
+        Invariants.require(delta > 0 && delta < threadCount());
+        while (true)
+        {
+            long cur = state;
+            int count = enabledThreadCount(cur);
+            int newCount = Math.min(threadCount(), count + delta);
+            if (count == newCount)
+                return 0;
+
+            if (stateUpdater.compareAndSet(this, cur, setEnabledThreadCount(cur, newCount)))
+            {
+                propagateAsyncWorkSignals(1);
+                return newCount - count;
+            }
+        }
+    }
+
+    public void setEnabledThreadCount(int count)
+    {
+        Invariants.require(count > 0 && count <= threadCount());
+        stateUpdater.accumulateAndGet(this, count, SignalLock::setEnabledThreadCount);
+        propagateAsyncWorkSignals(1);
+    }
+
+    public int threadCount()
+    {
+        return registered.length;
+    }
+
+    public int waitingEnabledThreadCount()
+    {
+        return waitingEnabledThreadCount(state);
+    }
+
+    public int activeThreadCount()
+    {
+        long cur = state;
+        int active = registered.length - waitingThreadCount(cur);
+        int lockThread = lockThread(cur);
+        if (lockThread >= 0 && (cur & threadBit(lockThread)) != 0)
+            ++active;
+        return active;
+    }
+
+    public boolean hasLockWork()
+    {
+        return hasLockWork(state);
+    }
+
+    public boolean hasOwner()
+    {
+        return isLockOwned(state);
+    }
+
+    public boolean isOwner()
+    {
+        return Thread.currentThread() == owner;
+    }
+
+    public void setAutoPreferRegistered(boolean autoPreferRegistered)
+    {
+        this.autoPreferRegistered = autoPreferRegistered;
+    }
+
+    public void setPreferRegistered(boolean preferRegistered)
+    {
+        this.preferRegistered = preferRegistered;
+    }
+
+    private static final int NO_THREAD = 64;
+    private static final int ANONYMOUS_OWNER = -1;
+    private static int pickThreadToSignalAsync(long state)
+    {
+        int lockThread = lockThread(state);
+        if (lockThread >= 0)
+            state &= ~threadBit(lockThread);
+        return Long.numberOfTrailingZeros(state & threadMask(enabledThreadCount(state)));
+    }
+
+    private static int pickThreadToSignalForLock(long state)
+    {
+        return 63 - Long.numberOfLeadingZeros(state & threadMask(enabledThreadCount(state)));
+    }
+
+    private static int lockThread(long state)
+    {
+        return (int) ((state & LOCK_THREAD_MASK) >>> LOCK_THREAD_SHIFT) - 2;
+    }
+
+    private static boolean isLockOwnedOrSignalled(long state)
+    {
+        return 0 != (state & LOCK_THREAD_MASK);
+    }
+
+    private static boolean isLockOwned(long state)
+    {
+        return 0 != (state & LOCK_OWNED);
+    }
+
+    private static long setLockThread(long state, int thread, long owned)
+    {
+        Invariants.require(!isLockOwnedOrSignalled(state));
+        state |= (long) (thread + 2) << LOCK_THREAD_SHIFT;
+        return state | owned;
+    }
+
+    private static long clearLockThread(long state)
+    {
+        state &= ~(LOCK_THREAD_MASK | LOCK_OWNED);
+        return state;
+    }
+
+    public static int asyncSignalCount(long state)
+    {
+        return (int) ((state >>> ASYNC_SIGNAL_COUNT_SHIFT) & THREAD_ID_MASK);
+    }
+
+    public static int waitingThreadCount(long state)
+    {
+        return Long.bitCount(state & WAITING_THREADS_MASK);
+    }
+
+    public static int enabledThreadCount(long state)
+    {
+        return (int) ((state >>> ENABLED_THREAD_COUNT_SHIFT) & THREAD_ID_MASK);
+    }
+
+    private static long setEnabledThreadCount(long state, long count)
+    {
+        Invariants.require(count <= MAX_THREADS);
+        return (state & ~ENABLED_THREAD_COUNT_MASK) | ((long)count << ENABLED_THREAD_COUNT_SHIFT);
+    }
+
+    public static int activeEnabledThreadCount(long state)
+    {
+        return activeThreadCount(enabledThreadCount(state), state);
+    }
+
+    public static int activeThreadCount(int threadCount, long state)
+    {
+        return threadCount - Long.bitCount(state & threadMask(threadCount));
+    }
+
+    private static long threadMask(int threadCount)
+    {
+        return (1L << threadCount) - 1;
+    }
+
+    public static int waitingEnabledThreadCount(long state)
+    {
+        return Long.bitCount(state & threadMask(enabledThreadCount(state)));
+    }
+
+    public static boolean hasMoreWaitersThanSignals(long state)
+    {
+        return waitingEnabledThreadCount(state) > asyncSignalCount(state);
+    }
+
+    private static long threadBit(int threadIndex)
+    {
+        return 1L << threadIndex;
+    }
+
+    private static boolean hasLockWork(long state)
+    {
+        return (state & HAS_LOCK_WORK) != 0;
+    }
+
+    public void lockInterruptibly()
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException
+    {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public Condition newCondition()
+    {
+        throw new UnsupportedOperationException();
+    }
+}

test/distributed/org/apache/cassandra/distributed/test/accord/AccordLoadTest.java

@@ -23,8 +23,10 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.BitSet;
 import java.util.Comparator;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.PriorityQueue;
 import java.util.Random;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
@@ -53,6 +55,11 @@ import org.junit.Test;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import accord.local.CommandStore;
+import accord.local.PreLoadContext;
+import accord.local.SafeCommand;
+import accord.primitives.PartialDeps;
+import accord.primitives.TxnId;
 import accord.utils.Functions;
 
 import org.apache.cassandra.config.CassandraRelevantProperties;
@@ -81,13 +88,17 @@ import org.apache.cassandra.service.accord.debug.CoordinationKinds;
 import org.apache.cassandra.service.accord.debug.TxnKindsAndDomains;
 import org.apache.cassandra.utils.EstimatedHistogram;
 import org.apache.cassandra.utils.concurrent.UncheckedInterruptedException;
+import org.apache.cassandra.utils.concurrent.WaitQueue;
 
+import static accord.coordinate.Coordination.CoordinationKind.Client;
+import static accord.coordinate.Coordination.CoordinationKind.Execute;
 import static java.lang.System.currentTimeMillis;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.concurrent.TimeUnit.NANOSECONDS;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.apache.cassandra.db.ColumnFamilyStore.FlushReason.UNIT_TESTS;
 import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.LEAKY;
+import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.RING;
 import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.SLOWEST;
 
 public class AccordLoadTest extends AccordTestBase
@@ -104,10 +115,11 @@ public class AccordLoadTest extends AccordTestBase
                   .set("accord.shard_durability_target_splits", "8")
                   .set("accord.shard_durability_max_splits", "16")
                   .set("accord.shard_durability_cycle", "1m")
-                  .set("accord.queue_submission_model", "SEMI_SYNC")
+                  .set("accord.queue_submission_model", "SIGNAL")
+//                  .set("accord.queue_submission_model", "SEMI_SYNC")
                   .set("accord.command_store_shard_count", "8")
-                  .set("concurrent_accord_operations", "8")
-                  .set("accord.queue_shard_count", "2")
+                  .set("accord.queue_thread_count", "4")
+                  .set("accord.queue_shard_count", "1")
                   .set("accord.replica_execution", "ALL")
                   .set("accord.send_stable", "TO_ALL_REPLICA_EXECUTABLE_ELSE_FOR_READS")
                   .set("accord.send_minimal", "false")
@@ -138,6 +150,7 @@ public class AccordLoadTest extends AccordTestBase
         final IntSupplier keySelector;
         final boolean readBeforeWrite;
         final float traceSlowest;
+        final int traceLast;
         final int[][] artificialLatencies;
 
         Settings(SettingsBuilder builder)
@@ -163,6 +176,7 @@ public class AccordLoadTest extends AccordTestBase
             this.readBeforeWrite = builder.readBeforeWrite;
             this.artificialLatencies = builder.artificialLatencies;
             this.traceSlowest = builder.traceSlowest;
+            this.traceLast = builder.traceLast;
         }
     }
 
@@ -189,6 +203,7 @@ public class AccordLoadTest extends AccordTestBase
         IntSupplier keySelector;
         boolean readBeforeWrite;
         float traceSlowest;
+        int traceLast;
         int[][] artificialLatencies;
 
         public SettingsBuilder setRepairInterval(int repairInterval)
@@ -305,6 +320,12 @@ public class AccordLoadTest extends AccordTestBase
             return this;
         }
 
+        public SettingsBuilder setTraceLast(int traceLast)
+        {
+            this.traceLast = traceLast;
+            return this;
+        }
+
         public SettingsBuilder setKeySelector(IntSupplier keySelector)
         {
             this.keySelector = keySelector;
@@ -475,12 +496,29 @@ public class AccordLoadTest extends AccordTestBase
                 }
             }
 
+            if (settings.traceLast > 0)
+            {
+                int traceLast = settings.traceLast;
+                for (int i = 0 ; i < cluster.size() ; ++i)
+                {
+                    cluster.get(i + 1).runOnInstance(() -> {
+                        AccordTracing tracing = ((AccordAgent) AccordService.unsafeInstance().agent()).tracing();
+                        tracing.setPattern(2, pattern -> pattern.withKinds(TxnKindsAndDomains.parse("{KW}"))
+                                                                    .withTraceNew(CoordinationKinds.ALL),
+                                           RING, -1, traceLast, LEAKY, 10, 1, CoordinationKinds.ALL);
+                    });
+                }
+            }
+
             final AtomicBoolean stop = new AtomicBoolean();
+            final AtomicBoolean pauseOrStop = new AtomicBoolean();
+            final WaitQueue waitQueue = WaitQueue.newWaitQueue();
             Random random = new Random();
             Semaphore completed = new Semaphore(0);
             AtomicIntegerArray coordinatorIndexes = new AtomicIntegerArray(clientCount);
             final List<java.util.concurrent.Future<?>> clients = new ArrayList<>();
             final AtomicReferenceArray<RateLimiter> rateLimiters = new AtomicReferenceArray<>(clientCount);
+            final ConcurrentHashMap<TxnId, Boolean> debugLatency = new ConcurrentHashMap<>();
             final AtomicReference<EstimatedHistogram> readHistogram = new AtomicReference<>(new EstimatedHistogram(200));
             final AtomicReference<EstimatedHistogram> writeHistogram = new AtomicReference<>(new EstimatedHistogram(200));
             if (settings.clients >= cluster.size())
@@ -496,8 +534,18 @@ public class AccordLoadTest extends AccordTestBase
                 coordinatorIndexes.set(client, client + 1);
                 clients.add(clientExecutor.submit(() -> {
                     final Semaphore inFlight = new Semaphore(settings.clientConcurrency);
-                    while (!stop.get())
+                    while (true)
                     {
+                        while (pauseOrStop.get())
+                        {
+                            if (stop.get())
+                                break;
+
+                            WaitQueue.Signal signal = waitQueue.register();
+                            if (pauseOrStop.get()) signal.awaitThrowUncheckedOnInterrupt();
+                            else signal.cancel();
+                        }
+
                         int coordinatorIdx = coordinatorIndexes.get(clientIndex);
                         ICoordinator coordinator = cluster.coordinator(coordinatorIdx);
                         try
@@ -519,7 +567,8 @@ public class AccordLoadTest extends AccordTestBase
                                     completed.release();
                                     if (fail == null)
                                     {
-                                        writeHistogram.get().add(NANOSECONDS.toMicros(System.nanoTime() - commandStart));
+                                        long elapsed = System.nanoTime() - commandStart;
+                                        writeHistogram.get().add(NANOSECONDS.toMicros(elapsed));
                                         synchronized (initialised)
                                         {
                                             keys.forEachInt(initialised::set);
@@ -549,7 +598,10 @@ public class AccordLoadTest extends AccordTestBase
                                     inFlight.release();
                                     completed.release();
                                     if (fail == null)
-                                        writeHistogram.get().add(NANOSECONDS.toMicros(System.nanoTime() - commandStart));
+                                    {
+                                        long elapsed = System.nanoTime() - commandStart;
+                                        writeHistogram.get().add(NANOSECONDS.toMicros(elapsed));
+                                    }
                                     else
                                         logger.error("{}", fail.toString());
                                 }, "BEGIN TRANSACTION\n" +
@@ -713,8 +765,7 @@ public class AccordLoadTest extends AccordTestBase
                 Long nowMillis = System.currentTimeMillis();
                 EstimatedHistogram reads = readHistogram.getAndSet(new EstimatedHistogram(200));
                 EstimatedHistogram writes = writeHistogram.getAndSet(new EstimatedHistogram(200));
-                float traceSlowest = settings.traceSlowest;
-                if (traceSlowest > 0f)
+                if (settings.traceSlowest > 0f)
                 {
                     cluster.forEach(() -> {
                         AccordTracing tracing = ((AccordAgent)AccordService.instance().agent()).tracing();
@@ -734,6 +785,135 @@ public class AccordLoadTest extends AccordTestBase
                         tracing.eraseAll();
                     });
                 }
+                if (settings.traceLast > 0)
+                {
+                    pauseOrStop.set(true);
+                    Map<String, List<List<String>>> print = new HashMap<>();
+                    for (int i = 1 ; i <= cluster.size() ; ++i)
+                    {
+                        cluster.get(i).acceptOnInstance(out -> {
+                            AccordService service = (AccordService)AccordService.instance();
+                            AccordTracing tracing = ((AccordAgent)AccordService.instance().agent()).tracing();
+                            PriorityQueue<SortedByElapsed> candidates = new PriorityQueue<>(Comparator.comparingLong(c -> -c.elapsedMicros));
+                            tracing.forEach(Functions.alwaysTrue(), (txnId, events) -> {
+                                events.forEach(event -> {
+                                    if (event.kind == Client)
+                                    {
+                                        long doneAtMicros = event.doneAtMicros();
+                                        long elapsedMicros = doneAtMicros - event.txnId().hlc();
+                                        if (elapsedMicros > 350000 && elapsedMicros < 390000)
+                                            candidates.add(new SortedByElapsed(txnId, elapsedMicros));
+                                    }
+                                });
+                            });
+
+                            AtomicInteger storeId = new AtomicInteger();
+                            while (!candidates.isEmpty())
+                            {
+                                SortedByElapsed sortedCandidate = candidates.poll();
+                                if (sortedCandidate.elapsedMicros < 300000)
+                                    return;
+
+                                TxnId candidate = sortedCandidate.txnId;
+                                storeId.lazySet(-1);
+                                tracing.forEach(candidate, events -> {
+                                    events.forEach(event -> {
+                                        if (storeId.get() >= 0)
+                                            return;
+                                        for (Message message : event.messages())
+                                        {
+                                            if (message.nodeId < 0 && message.commandStoreId >= 0)
+                                            {
+                                                storeId.set(message.commandStoreId);
+                                                break;
+                                            }
+                                        }
+                                    });
+                                });
+
+                                if (storeId.get() >= 0)
+                                {
+                                    CommandStore commandStore = service.node().commandStores().forId(storeId.get());
+                                    List<List<String>> result = AccordService.getBlocking(commandStore.submit(PreLoadContext.contextFor(candidate, "LoadTest"), safeStore -> {
+                                        SafeCommand safeCommand = safeStore.unsafeGet(candidate);
+                                        PartialDeps deps = safeCommand.current().partialDeps();
+                                        if (deps == null)
+                                            return null;
+                                        List<List<String>> infos = new ArrayList<>();
+                                        for (TxnId txnId : deps.txnIds())
+                                        {
+                                            List<String> info = new ArrayList<>();
+                                            info.add(txnId.toString());
+                                            infos.add(info);
+                                        }
+                                        List<String> info = new ArrayList<>();
+                                        info.add(candidate.toString());
+                                        infos.add(info);
+                                        return infos;
+                                    }));
+
+                                    if (result != null)
+                                    {
+                                        for (List<String> info : result)
+                                        {
+                                            TxnId txnId = TxnId.parse(info.get(0));
+                                            AccordService.getBlocking(commandStore.execute(PreLoadContext.contextFor(txnId, "LoadTest"), safeStore -> {
+                                                SafeCommand safeCommand = safeStore.unsafeGet(txnId);
+                                                if (safeCommand.current().executeAt != null)
+                                                    info.add(safeCommand.current().executeAt.toString());
+                                            }));
+                                        }
+
+                                        out.put(candidate.toString(), result);
+                                        return;
+                                    }
+                                }
+                            }
+                        }, print);
+                    }
+
+                    for (int i = 1 ; i <= cluster.size() ; ++i)
+                    {
+                        cluster.get(i).acceptOnInstance(out -> {
+                            AccordTracing tracing = ((AccordAgent)AccordService.instance().agent()).tracing();
+                            for (Map.Entry<String, List<List<String>>> e : out.entrySet())
+                            {
+                                TxnId parentId = TxnId.parse(e.getKey());
+                                for (List<String> infos : e.getValue())
+                                {
+                                    TxnId depId = TxnId.parse(infos.get(0));
+                                    tracing.forEach(depId, events -> {
+                                        events.forEach(event -> {
+                                            infos.add(event.kind + ": [" + (event.idMicros - parentId.hlc()) + "..." + (event.doneAtMicros() - parentId.hlc()) + "][" + (event.idMicros - depId.hlc()) + "..." + (event.doneAtMicros() - depId.hlc()) + "]");
+                                            if (event.kind == Execute)
+                                            {
+                                                for (Message message : event.messages())
+                                                {
+                                                    if (message.nodeId == parentId.node.id)
+                                                    {
+                                                        long atMicros = (event.idMicros + (message.atNanos - event.atNanos)/1000) - parentId.hlc();
+                                                        infos.add(atMicros + ": " + message.message);
+                                                    }
+                                                }
+                                            }
+                                        });
+                                    });
+                                }
+                            }
+                        }, print);
+                    }
+
+                    for (Map.Entry<String, List<List<String>>> e : print.entrySet())
+                    {
+                        System.out.println("======" + e.getKey() + "======");
+                        for (List<String> infos : e.getValue())
+                            System.out.println(infos);
+                    }
+                    if (!print.isEmpty())
+                        System.out.println();
+                    pauseOrStop.set(false);
+                    waitQueue.signalAll();
+                }
                 cluster.forEach(() -> {
                     refresh(AccordExecutorMetrics.INSTANCE.elapsedRunning);
                     refresh(AccordExecutorMetrics.INSTANCE.elapsed);
@@ -879,16 +1059,17 @@ public class AccordLoadTest extends AccordTestBase
             ws[i] = iw;
         }
         System.out.println(Arrays.toString(ws));
+        Arrays.fill(ws, 0);
         for (int i = 0 ; i < qs.length ; ++i)
         {
-            int wj = i == 0 ? 1 : 0;
-            for (int j = 1 ; j < qs.length ; ++j)
+            for (int j = 0 ; j < qs.length ; ++j)
             {
                 if (j == i) continue;
-                if (qs[j] > qs[wj])
-                    wj = j;
+                if (qs[j] > 2*qs[i]) continue;
+                int w = qs[i] + 4*qs[j] + LATENCIES[i][j];
+                if (w > ws[i])
+                    ws[i] = w;
             }
-            ws[i] = qs[i] + 4*qs[wj] + LATENCIES[i][wj];
         }
         System.out.println(Arrays.toString(ws));
     }
@@ -910,11 +1091,33 @@ public class AccordLoadTest extends AccordTestBase
         DistributedTestBase.beforeClass();
         AccordLoadTest.setUp();
         AccordLoadTest test = new AccordLoadTest();
+        try
+        {
             test.setup();
             test.testLoad(withArtificialLatencies(ycsbA(new SettingsBuilder(), 100_000)
+//                                                  .setRatePerSecond(400).setMinRatePerSecond(200)
+//                                                  .setRatePerSecond(800).setMinRatePerSecond(200)
                                                   .setRatePerSecond(1600).setMinRatePerSecond(200)
                                                   .setIncreaseRatePerSecondInterval(5000)
-//                                              .setTraceSlowest(0.5f)
+//                                                  .setTraceLast(5000)
             ).build());
         }
+        finally
+        {
+            test.tearDown();
+        }
+    }
+
+    static class SortedByElapsed
+    {
+        final TxnId txnId;
+        final long elapsedMicros;
+
+        SortedByElapsed(TxnId txnId, long elapsedMicros)
+        {
+            this.txnId = txnId;
+            this.elapsedMicros = elapsedMicros;
+        }
+    }
+
 }

test/distributed/org/apache/cassandra/fuzz/topology/AccordBootstrapTest.java

@@ -65,7 +65,7 @@ public class AccordBootstrapTest extends FuzzTestBase
                                         .withConfig((config) -> config.with(Feature.NETWORK, Feature.GOSSIP)
                                                                       .set("write_request_timeout", "2s")
                                                                       .set("request_timeout", "5s")
-                                                                      .set("concurrent_accord_operations", 2)
+                                                                      .set("accord.queue_thread_count", 2)
                                                                       .set("accord.shard_durability_target_splits", "1")
                                                                       .set("accord.shard_durability_max_splits", "4")
                                                                       .set("accord.catchup_on_start_fail_latency", "60s")

test/distributed/org/apache/cassandra/fuzz/topology/AccordTopologyMixupTest.java

@@ -276,7 +276,7 @@ public class AccordTopologyMixupTest extends TopologyMixupTestBase<AccordTopolog
             c.set("accord.command_store_shard_count", 1)
              .set("accord.queue_shard_count", 1)
              .set("accord.shard_durability_target_splits", 4)
-             .set("concurrent_accord_operations", 1)
+             .set("accord.queue_thread_count", 1)
              .set("paxos_variant", Config.PaxosVariant.v2.name());
         }
 

test/distributed/org/apache/cassandra/fuzz/topology/HarryOnAccordTopologyMixupTest.java

@@ -77,7 +77,7 @@ public class HarryOnAccordTopologyMixupTest extends HarryTopologyMixupTest
             config.set("accord.command_store_shard_count", 1)
                   .set("accord.queue_shard_count", 1)
                   .set("accord.shard_durability_target_splits", 4)
-                  .set("concurrent_accord_operations", 1);
+                  .set("accord.queue_thread_count", 1);
         }
 
         @Override

test/simulator/test/org/apache/cassandra/simulator/test/AccordExecutorTest.java

@@ -0,0 +1,164 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.simulator.test;
+
+import java.util.Collection;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Future;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.LockSupport;
+import java.util.function.BooleanSupplier;
+
+import org.junit.Test;
+
+import accord.api.AsyncExecutor;
+import accord.local.SequentialAsyncExecutor;
+
+import org.apache.cassandra.concurrent.ExecutorFactory;
+import org.apache.cassandra.config.DatabaseDescriptor;
+import org.apache.cassandra.distributed.api.IIsolatedExecutor.SerializableSupplier;
+import org.apache.cassandra.service.accord.AccordExecutor;
+import org.apache.cassandra.service.accord.AccordExecutorAsyncSubmit;
+import org.apache.cassandra.service.accord.AccordExecutorSignalLoop;
+import org.apache.cassandra.service.accord.api.AccordAgent;
+
+import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITHOUT_LOCK;
+import static org.apache.cassandra.service.accord.AccordService.toFuture;
+
+// TODO (required): have simulator intercept ReentantLock so we can test SyncSubmit and SemiSyncSubmit
+public class AccordExecutorTest extends SimulationTestBase
+{
+    static final int THREAD_COUNT = 8;
+
+    @Test
+    public void signalLoopTest()
+    {
+        executorTest(() -> new AccordExecutorSignalLoop(1, RUN_WITHOUT_LOCK, THREAD_COUNT, -1, -1, TimeUnit.MICROSECONDS, i ->"Loop" + i, new AccordAgent()));
+    }
+
+    @Test
+    public void signalSpinLoopTest()
+    {
+        executorTest(() -> new AccordExecutorSignalLoop(1, RUN_WITHOUT_LOCK, THREAD_COUNT, 10, 100, TimeUnit.MICROSECONDS, i ->"Loop" + i, new AccordAgent()));
+    }
+
+    @Test
+    public void ayncSubmitTest()
+    {
+        executorTest(() -> new AccordExecutorAsyncSubmit(1, RUN_WITHOUT_LOCK, THREAD_COUNT, i -> "Loop" + i, new AccordAgent()));
+    }
+
+    public void executorTest(SerializableSupplier<AccordExecutor> supplier)
+    {
+        simulate(arr(() -> {
+                     try
+                     {
+                         DatabaseDescriptor.daemonInitialization();
+                         AccordExecutor executor = supplier.get();
+                         Lock lock = executor.unsafeLock();
+                         SequentialAsyncExecutor sequentialExecutor = executor.newSequentialExecutor();
+                         Executor lockExecutor = ExecutorFactory.Global.executorFactory().sequential("lock");
+                         ConcurrentLinkedQueue<Future<?>> await = new ConcurrentLinkedQueue<>();
+
+                         for (float sleepChance : new float[] { 0f, 0.01f, 0.1f })
+                             for (float lockChance : new float[] { 0f, 0.01f, 0.1f })
+                                submitLoop(lock, executor, sequentialExecutor, lockExecutor, 200, 10, await, sleepChance, lockChance);
+                     }
+                     catch (Throwable t)
+                     {
+                         throw new RuntimeException(t);
+                     }
+                 }),
+                 () -> {}, 1L);
+    }
+
+    private static void submitLoop(Lock lock, AccordExecutor executor, SequentialAsyncExecutor sequentialExecutor, Executor lockExecutor, int outerLoop, int innerLoop, ConcurrentLinkedQueue<Future<?>> await, float sleepChance, float lockChance) throws ExecutionException, InterruptedException
+    {
+        while (outerLoop-- > 0)
+        {
+            for (int i = 0; i < innerLoop; ++i)
+                submitRecursive(lock, executor, sequentialExecutor, 1 + i, await, sleepChance, lockChance);
+
+            AtomicBoolean done = new AtomicBoolean();
+            submitUntil(lock, lockExecutor, sleepChance, done::get);
+            while (!await.isEmpty())
+                await.poll().get();
+            done.set(true);
+            System.out.println("Loop " + (1 + outerLoop));
+        }
+    }
+
+    private static void submitRecursive(Lock lock, AccordExecutor executor, SequentialAsyncExecutor sequentialExecutor, int count, Collection<Future<?>> await, float sleepChance, float lockChance)
+    {
+        AsyncExecutor submitTo = ThreadLocalRandom.current().nextBoolean() ? executor : sequentialExecutor;
+        await.add(toFuture(submitTo.chain(() -> {
+            ThreadLocalRandom rnd = ThreadLocalRandom.current();
+            boolean locked = false;
+            if (rnd.nextFloat() < lockChance)
+            {
+                if (rnd.nextBoolean()) locked = lock.tryLock();
+                else { locked = true; lock.lock(); }
+            }
+            try
+            {
+                if (count > 1)
+                    submitRecursive(lock, executor, sequentialExecutor, count -1, await, sleepChance, lockChance);
+                if (rnd.nextFloat() < sleepChance)
+                    LockSupport.parkNanos(rnd.nextInt(10000, 100000));
+            }
+            finally
+            {
+                if (locked)
+                    lock.unlock();
+            }
+        }).beginAsResult()));
+    }
+
+    private static void submitUntil(Lock lock, Executor executor, float sleepChance, BooleanSupplier done)
+    {
+        if (done.getAsBoolean())
+            return;
+
+        executor.execute(() -> {
+
+            ThreadLocalRandom rnd = ThreadLocalRandom.current();
+            boolean tryLock = rnd.nextBoolean();
+            boolean locked = !tryLock;
+            if (tryLock) locked = lock.tryLock();
+            else lock.lock();
+            try
+            {
+                if (rnd.nextFloat() < sleepChance)
+                    LockSupport.parkNanos(rnd.nextInt(10000, 100000));
+
+                submitUntil(lock, executor, sleepChance, done);
+            }
+            finally
+            {
+                if (locked)
+                    lock.unlock();
+            }
+        });
+    }
+}

test/unit/org/apache/cassandra/db/virtual/AccordDebugKeyspaceTest.java

@@ -256,7 +256,7 @@ public class AccordDebugKeyspaceTest extends CQLTester
         Config.setOverrideLoadConfig(() -> {
             Config config = new YamlConfigurationLoader().loadConfig();
             config.accord.queue_shard_count = new OptionaldPositiveInt(1);
-            config.concurrent_accord_operations = 1;
+            config.accord.queue_thread_count = new OptionaldPositiveInt(1);
             config.accord.command_store_shard_count = new OptionaldPositiveInt(1);
             config.accord.enable_virtual_debug_only_keyspace = true;
             config.accord.permit_fast_quorum_medium_path = true;

test/unit/org/apache/cassandra/hints/HintsServiceTest.java

@@ -269,7 +269,7 @@ public class HintsServiceTest
                     {
                         accordTxnCount.incrementAndGet();
                         TxnId txnId = AccordTestUtils.txnId(42, 43, 44);
-                        AccordResult<TxnResult> result = new AccordResult<>(txnId, Keys.EMPTY, new LatencyRequestBookkeeping(null), requestTime.startedAtNanos(), requestTime.startedAtNanos(), true);
+                        AccordResult<TxnResult> result = new AccordResult<>(txnId, Keys.EMPTY, new LatencyRequestBookkeeping(null), requestTime.startedAtNanos(), requestTime.startedAtNanos(), true, null);
                         result.accept(new TxnData(), null);
                         return result;
                     }

test/unit/org/apache/cassandra/service/accord/AccordCommandStoreTest.java

@@ -30,7 +30,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import accord.api.Key;
-import accord.api.Result;
+import accord.api.Result.PersistableResult;
 import accord.local.Command;
 import accord.local.PreLoadContext;
 import accord.local.StoreParticipants;
@@ -134,7 +134,7 @@ public class AccordCommandStoreTest
         LargeBitSet waitingOnApply = new LargeBitSet(3);
         waitingOnApply.set(1);
         Command.WaitingOn waitingOn = new Command.WaitingOn(dependencies.keyDeps.keys(), dependencies.rangeDeps, new ImmutableBitSet(waitingOnApply), new ImmutableBitSet(2));
-        Pair<Writes, Result> result = getBlocking(AccordTestUtils.processTxnResult(commandStore, txnId, txn, executeAt));
+        Pair<Writes, PersistableResult> result = getBlocking(AccordTestUtils.processTxnResult(commandStore, txnId, txn, executeAt));
 
         Command expected = Command.Executed.executed(txnId, SaveStatus.Applied, AllQuorums, StoreParticipants.all(route),
                                                      promised, executeAt, txn, dependencies, accepted,

test/unit/org/apache/cassandra/service/accord/AccordTestUtils.java

@@ -41,6 +41,7 @@ import accord.api.Journal;
 import accord.api.ProgressLog.NoOpProgressLog;
 import accord.api.RemoteListeners.NoOpRemoteListeners;
 import accord.api.Result;
+import accord.api.Result.PersistableResult;
 import accord.api.RoutingKey;
 import accord.api.Timeouts;
 import accord.coordinate.Coordinations;
@@ -240,15 +241,15 @@ public class AccordTestUtils
         return Ballot.fromValues(epoch, hlc, new Node.Id(node));
     }
 
-    public static AsyncChain<Pair<Writes, Result>> processTxnResult(AccordCommandStore commandStore, TxnId txnId, PartialTxn txn, Timestamp executeAt) throws Throwable
+    public static AsyncChain<Pair<Writes, PersistableResult>> processTxnResult(AccordCommandStore commandStore, TxnId txnId, PartialTxn txn, Timestamp executeAt) throws Throwable
     {
-        AtomicReference<AsyncChain<Pair<Writes, Result>>> result = new AtomicReference<>();
+        AtomicReference<AsyncChain<Pair<Writes, PersistableResult>>> result = new AtomicReference<>();
         getBlocking(commandStore.execute((PreLoadContext.Empty)() -> "Test",
                                          safeStore -> result.set(processTxnResultDirect(safeStore, txnId, txn, executeAt))));
         return result.get();
     }
 
-    public static AsyncChain<Pair<Writes, Result>> processTxnResultDirect(SafeCommandStore safeStore, TxnId txnId, PartialTxn txn, Timestamp executeAt)
+    public static AsyncChain<Pair<Writes, PersistableResult>> processTxnResultDirect(SafeCommandStore safeStore, TxnId txnId, PartialTxn txn, Timestamp executeAt)
     {
         TxnRead read = (TxnRead) txn.read();
         return AsyncChains.allOf(read.keys().stream().map(key -> read.read(safeStore, key, executeAt))
@@ -256,7 +257,7 @@ public class AccordTestUtils
                                                .map(list -> {
                                                    Data data = list.stream().reduce(Data::merge).orElse(new TxnData());
                                                    return Pair.create(txnId.is(Write) ? txn.execute(txnId, executeAt, data) : null,
-                                                                      txn.query().compute(txnId, executeAt, txn.keys(), data, txn.read(), txn.update()));
+                                                                      txn.query().compute(txnId, executeAt, txn.keys(), data, txn.read(), txn.update()).toPersistable());
                                                });
     }
 

test/unit/org/apache/cassandra/service/accord/EpochSyncTest.java

@@ -121,7 +121,7 @@ import org.apache.cassandra.utils.Pair;
 
 import static accord.utils.Property.commands;
 import static accord.utils.Property.stateful;
-import static org.apache.cassandra.config.DatabaseDescriptor.getAccordCommandStoreShardCount;
+import static org.apache.cassandra.config.DatabaseDescriptor.getAccord;
 import static org.apache.cassandra.config.DatabaseDescriptor.getPartitioner;
 
 public class EpochSyncTest
@@ -671,7 +671,7 @@ public class EpochSyncTest
                 this.token = token;
                 this.epoch = epoch;
                 MockCluster.Clock clock = new MockCluster.Clock(0);
-                Node node = Utils.createNode(id, ignore -> AsyncResults.settable(), new MessageSink.NoOpSink(), clock, new TestAgent(clock), new TokenKey.KeyspaceSplitter(new ShardDistributor.EvenSplit<>(getAccordCommandStoreShardCount(), getPartitioner().accordSplitter())));
+                Node node = Utils.createNode(id, ignore -> AsyncResults.settable(), new MessageSink.NoOpSink(), clock, new TestAgent(clock), new TokenKey.KeyspaceSplitter(new ShardDistributor.EvenSplit<>(getAccord().commandStoreShardCount(), getPartitioner().accordSplitter())));
                 // TODO (review): Should there be a real scheduler here? Is it possible to adapt the Scheduler interface to scheduler used in this test?
                 TimeService time = TimeService.ofNonMonotonic(globalExecutor::currentTimeMillis, TimeUnit.MILLISECONDS);
                 this.topologyService = new AccordTopologyService(id, mapper, messagingService, scheduler);

test/unit/org/apache/cassandra/service/accord/serializers/CommandsForKeySerializerTest.java

@@ -120,7 +120,6 @@ import org.apache.cassandra.schema.TableId;
 import org.apache.cassandra.service.StorageService;
 import org.apache.cassandra.service.accord.api.AccordAgent;
 import org.apache.cassandra.service.accord.api.TokenKey;
-import org.apache.cassandra.service.accord.txn.TxnData;
 import org.apache.cassandra.service.accord.txn.TxnWrite;
 import org.apache.cassandra.simulator.RandomSource.Choices;
 import org.apache.cassandra.utils.AccordGenerators;
@@ -222,7 +221,7 @@ public class CommandsForKeySerializerTest
             {
                 if (txnId.is(Kind.Write))
                     builder.writes(new Writes(txnId, executeAt, txn.keys(), new TxnWrite(TableMetadatas.none(), Collections.emptyList(), SimpleBitSets.allSet(1))));
-                builder.result(new TxnData());
+                builder.result(ResultSerializers.APPLIED);
             }
             return builder;
         }

test/unit/org/apache/cassandra/utils/AccordGenerators.java

@@ -84,9 +84,9 @@ import org.apache.cassandra.service.accord.AccordTestUtils;
 import org.apache.cassandra.service.accord.TokenRange;
 import org.apache.cassandra.service.accord.api.PartitionKey;
 import org.apache.cassandra.service.accord.api.TokenKey;
+import org.apache.cassandra.service.accord.serializers.ResultSerializers;
 import org.apache.cassandra.service.accord.serializers.TableMetadatas;
 import org.apache.cassandra.service.accord.topology.FetchTopologies;
-import org.apache.cassandra.service.accord.txn.TxnData;
 import org.apache.cassandra.service.accord.txn.TxnWrite;
 
 import static accord.local.CommandStores.RangesForEpoch;
@@ -292,7 +292,7 @@ public class AccordGenerators
             {
                 if (txnId.is(Write))
                     builder.writes(new Writes(txnId, executeAt, keysOrRanges, new TxnWrite(TableMetadatas.none(), Collections.emptyList(), SimpleBitSets.allSet(1))));
-                builder.result(new TxnData());
+                builder.result(ResultSerializers.APPLIED);
             }
             return builder.build(saveStatus);
         }