From d954c19b21bcf53da3f15fae40a0751c6dcc73df Mon Sep 17 00:00:00 2001
From: Stefan Miklosovic <smiklosovic@apache.org>
Date: Fri, 28 Mar 2025 10:46:34 +0100
Subject: [PATCH] CASSANDRA-20296 follow-up: by default, explicitly set
 -XX:MaxDirectMemorySize to half of -Xmx

patch by Stefan Miklosovic; reviewed by Michael Semb Wever for CASSANDRA-20296
---
 NEWS.txt                |  1 +
 conf/cassandra-env.sh   | 14 ++++++++++++++
 conf/jvm-server.options |  6 ++++++
 3 files changed, 21 insertions(+)

diff --git a/NEWS.txt b/NEWS.txt
index 5b5ec9b92a..efbc8d7ac8 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -74,6 +74,7 @@ Upgrading
       storage-attached indexes (SAI) to make migration between the two safer. This behavior can be switched off via
       the flag `sai_options.prioritize_over_legacy_index` (which defaults to `false`) in `cassandra.yaml` or via
       `setPrioritizeSAIOverLegacyIndex(boolean)` in the JMX MBean `org.apache.cassandra.db:type=StorageService`.
+    - Java's -XX:MaxDirectMemorySize is by default explicitly half of max heap size (-Xmx) instead of implicitly equal to it.
 
 5.0.1
 =====
diff --git a/conf/cassandra-env.sh b/conf/cassandra-env.sh
index 2098161979..2d83763b89 100644
--- a/conf/cassandra-env.sh
+++ b/conf/cassandra-env.sh
@@ -56,11 +56,14 @@ calculate_heap_sizes()
         heap_limit="31744"
     fi
     half_system_memory_in_mb=`expr $system_memory_in_mb / 2`
+    quarter_system_memory_in_mb=`expr $system_memory_in_mb / 4`
     if [ "$half_system_memory_in_mb" -gt "$heap_limit" ] ; then
         CALCULATED_MAX_HEAP_SIZE="${heap_limit}M"
+        CALCULATED_MAX_DIRECT_MEMORY_SIZE="`expr $heap_limit / 2`M"
         CALCULATED_CMS_HEAP_NEWSIZE="8G"
     else
         CALCULATED_MAX_HEAP_SIZE="${half_system_memory_in_mb}M"
+        CALCULATED_MAX_DIRECT_MEMORY_SIZE="${quarter_system_memory_in_mb}M"
         CALCULATED_CMS_HEAP_NEWSIZE="`expr $half_system_memory_in_mb / 4`M"
     fi
 }
@@ -87,6 +90,8 @@ echo $JVM_OPTS | grep -q Xmx
 DEFINED_XMX=$?
 echo $JVM_OPTS | grep -q Xms
 DEFINED_XMS=$?
+echo $JVM_OPTS | grep -q MaxDirectMemorySize
+DEFINED_MAX_DIRECT_MEMORY_SIZE=$?
 echo $JVM_OPTS | grep -q ParallelGCThreads
 DEFINED_PARALLEL_GC_THREADS=$?
 echo $JVM_OPTS | grep -q ConcGCThreads
@@ -112,6 +117,7 @@ calculate_heap_sizes
 
 #MAX_HEAP_SIZE="20G"
 #HEAP_NEWSIZE="10G"
+#MAX_DIRECT_MEMORY_SIZE="10G"
 
 # Set this to control the amount of arenas per-thread in glibc
 #export MALLOC_ARENA_MAX=4
@@ -130,6 +136,10 @@ elif [ "x$MAX_HEAP_SIZE" = "x" ] ||  [ "x$HEAP_NEWSIZE" = "x" -a $USING_G1 -ne 0
     exit 1
 fi
 
+if [ "x$MAX_DIRECT_MEMORY_SIZE" = "x" ]; then
+    MAX_DIRECT_MEMORY_SIZE="$CALCULATED_MAX_DIRECT_MEMORY_SIZE"
+fi
+
 if [ "x$MALLOC_ARENA_MAX" = "x" ] ; then
     export MALLOC_ARENA_MAX=4
 fi
@@ -144,6 +154,10 @@ elif [ $DEFINED_XMX -ne 0 ] || [ $DEFINED_XMS -ne 0 ]; then
      exit 1
 fi
 
+if [ $DEFINED_MAX_DIRECT_MEMORY_SIZE -ne 0 ]; then
+     JVM_OPTS="$JVM_OPTS -XX:MaxDirectMemorySize=${MAX_DIRECT_MEMORY_SIZE}"
+fi
+
 # We only set -Xmn flag if it was not defined in jvm-server.options file
 # and CMS is being used.  If defined, both Xmn and Xmx must be defined together.
 if [ $DEFINED_XMN -eq 0 ] && [ $DEFINED_XMX -ne 0 ]; then
diff --git a/conf/jvm-server.options b/conf/jvm-server.options
index 547a06dce3..f68d875a51 100644
--- a/conf/jvm-server.options
+++ b/conf/jvm-server.options
@@ -166,6 +166,12 @@
 # For production use you may wish to adjust this for your environment.
 # If that's the case, see MAX_HEAP_SIZE (and HEAP_NEWSIZE for CMS) in cassandra-env.sh
 
+#####################
+# OFF-HEAP SETTINGS #
+#####################
+
+# By default, this setting is half of max heap size
+#-XX:MaxDirectMemorySize=
 
 ###################################
 # EXPIRATION DATE OVERFLOW POLICY #