diff --git a/CHANGES.txt b/CHANGES.txt
index 5bcf25667c..36934c6454 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 4.2
+ * Use G1GC by default, and update default G1GC settings (CASSANDRA-18027)
  * SimpleSeedProvider can resolve multiple IP addresses per DNS record (CASSANDRA-14361)
  * Remove mocking in InternalNodeProbe spying on StorageServiceMBean (CASSANDRA-18152)
  * Add compaction_properties column to system.compaction_history table and nodetool compactionhistory command (CASSANDRA-18061)
diff --git a/conf/jvm11-server.options b/conf/jvm11-server.options
index 7e78467853..0f73d4cfed 100644
--- a/conf/jvm11-server.options
+++ b/conf/jvm11-server.options
@@ -11,39 +11,41 @@
 
 
 ### CMS Settings
--XX:+UseConcMarkSweepGC
--XX:+CMSParallelRemarkEnabled
--XX:SurvivorRatio=8
--XX:MaxTenuringThreshold=1
--XX:CMSInitiatingOccupancyFraction=75
--XX:+UseCMSInitiatingOccupancyOnly
--XX:CMSWaitDuration=10000
--XX:+CMSParallelInitialMarkEnabled
--XX:+CMSEdenChunksRecordAlways
+##-XX:+UseConcMarkSweepGC
+##-XX:+CMSParallelRemarkEnabled
+##-XX:SurvivorRatio=8
+##-XX:MaxTenuringThreshold=1
+##-XX:CMSInitiatingOccupancyFraction=75
+##-XX:+UseCMSInitiatingOccupancyOnly
+##-XX:CMSWaitDuration=10000
+##-XX:+CMSParallelInitialMarkEnabled
+##-XX:+CMSEdenChunksRecordAlways
 ## some JVMs will fill up their heap when accessed via JMX, see CASSANDRA-6541
--XX:+CMSClassUnloadingEnabled
+##-XX:+CMSClassUnloadingEnabled
 
 
 
 ### G1 Settings
 ## Use the Hotspot garbage-first collector.
-#-XX:+UseG1GC
-#-XX:+ParallelRefProcEnabled
+-XX:+UseG1GC
+-XX:+ParallelRefProcEnabled
+-XX:MaxTenuringThreshold=1
+-XX:G1HeapRegionSize=16m
 
 #
 ## Have the JVM do less remembered set work during STW, instead
 ## preferring concurrent GC. Reduces p99.9 latency.
-#-XX:G1RSetUpdatingPauseTimePercent=5
+-XX:G1RSetUpdatingPauseTimePercent=5
 #
 ## Main G1GC tunable: lowering the pause target will lower throughput and vise versa.
 ## 200ms is the JVM default and lowest viable setting
 ## 1000ms increases throughput. Keep it smaller than the timeouts in cassandra.yaml.
-#-XX:MaxGCPauseMillis=500
+-XX:MaxGCPauseMillis=300
 
 ## Optional G1 Settings
 # Save CPU time on large (>= 16GB) heaps by delaying region scanning
 # until the heap is 70% full. The default in Hotspot 8u40 is 40%.
-#-XX:InitiatingHeapOccupancyPercent=70
+-XX:InitiatingHeapOccupancyPercent=70
 
 # For systems with > 8 cores, the default ParallelGCThreads is 5/8 the number of logical cores.
 # Otherwise equal to the number of cores when 8 or less.
diff --git a/conf/jvm8-server.options b/conf/jvm8-server.options
index 6214669eab..4c08439b0d 100644
--- a/conf/jvm8-server.options
+++ b/conf/jvm8-server.options
@@ -18,38 +18,40 @@
 #################
 
 ### CMS Settings
--XX:+UseParNewGC
--XX:+UseConcMarkSweepGC
--XX:+CMSParallelRemarkEnabled
--XX:SurvivorRatio=8
--XX:MaxTenuringThreshold=1
--XX:CMSInitiatingOccupancyFraction=75
--XX:+UseCMSInitiatingOccupancyOnly
--XX:CMSWaitDuration=10000
--XX:+CMSParallelInitialMarkEnabled
--XX:+CMSEdenChunksRecordAlways
+##-XX:+UseParNewGC
+##-XX:+UseConcMarkSweepGC
+##-XX:+CMSParallelRemarkEnabled
+##-XX:SurvivorRatio=8
+##-XX:MaxTenuringThreshold=1
+##-XX:CMSInitiatingOccupancyFraction=75
+##-XX:+UseCMSInitiatingOccupancyOnly
+##-XX:CMSWaitDuration=10000
+##-XX:+CMSParallelInitialMarkEnabled
+##-XX:+CMSEdenChunksRecordAlways
 ## some JVMs will fill up their heap when accessed via JMX, see CASSANDRA-6541
--XX:+CMSClassUnloadingEnabled
+##-XX:+CMSClassUnloadingEnabled
 
 ### G1 Settings
 ## Use the Hotspot garbage-first collector.
-#-XX:+UseG1GC
-#-XX:+ParallelRefProcEnabled
+-XX:+UseG1GC
+-XX:+ParallelRefProcEnabled
+-XX:MaxTenuringThreshold=1
+-XX:G1HeapRegionSize=16m
+
 
-#
 ## Have the JVM do less remembered set work during STW, instead
 ## preferring concurrent GC. Reduces p99.9 latency.
-#-XX:G1RSetUpdatingPauseTimePercent=5
-#
+-XX:G1RSetUpdatingPauseTimePercent=5
+
 ## Main G1GC tunable: lowering the pause target will lower throughput and vise versa.
 ## 200ms is the JVM default and lowest viable setting
 ## 1000ms increases throughput. Keep it smaller than the timeouts in cassandra.yaml.
-#-XX:MaxGCPauseMillis=500
+-XX:MaxGCPauseMillis=300
 
 ## Optional G1 Settings
 # Save CPU time on large (>= 16GB) heaps by delaying region scanning
 # until the heap is 70% full. The default in Hotspot 8u40 is 40%.
-#-XX:InitiatingHeapOccupancyPercent=70
+-XX:InitiatingHeapOccupancyPercent=70
 
 # For systems with > 8 cores, the default ParallelGCThreads is 5/8 the number of logical cores.
 # Otherwise equal to the number of cores when 8 or less.