JVM Heap Memory Allocation : G1 vs CMS

People misunderstand about how G1 GC collector memory allocate at JVM boot up. We normally configure max and min values for heap but for G1 GC collector, no need to configure min value. Let's try out two scenarios and have a look at the observations.

G1 GC Collector

ps -ef | grep java

uranadh   4645 4639 16 19:46 ?        00:01:02 /usr/bin/java -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms5000m -Xmx5000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar -os linux -ws gtk -arch x86_64 -showsplash /home/uranadh/software/eclipse//plugins/org.eclipse.platform_4.6.3.v20170301-0400/splash.bmp -launcher /home/uranadh/software/eclipse/eclipse -name Eclipse --launcher.library /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher.gtk.linux.x86_64_1.1.401.v20161122-1740/eclipse_1618.so -startup /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar --launcher.appendVmargs -exitdata 240005 -product org.eclipse.epp.package.jee.product -vm /usr/bin/java -vmargs -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms5000m -Xmx5000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar

uranadh   4792 4787 33 19:48 ?        00:01:20 /usr/bin/java -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms10000m -Xmx10000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar -os linux -ws gtk -arch x86_64 -showsplash /home/uranadh/software/eclipse//plugins/org.eclipse.platform_4.6.3.v20170301-0400/splash.bmp -launcher /home/uranadh/software/eclipse/eclipse -name Eclipse --launcher.library /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher.gtk.linux.x86_64_1.1.401.v20161122-1740/eclipse_1618.so -startup /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar --launcher.appendVmargs -exitdata 28800f -product org.eclipse.epp.package.jee.product -vm /usr/bin/java -vmargs -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms10000m -Xmx10000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar

uranadh   4938 4932 99 19:51 ?        00:01:21 /usr/bin/java -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms10000m -Xmx10000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar -os linux -ws gtk -arch x86_64 -showsplash /home/uranadh/software/eclipse//plugins/org.eclipse.platform_4.6.3.v20170301-0400/splash.bmp -launcher /home/uranadh/software/eclipse/eclipse -name Eclipse --launcher.library /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher.gtk.linux.x86_64_1.1.401.v20161122-1740/eclipse_1618.so -startup /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar --launcher.appendVmargs -exitdata 2b0012 -product org.eclipse.epp.package.jee.product -vm /usr/bin/java -vmargs -Dosgi.requiredJavaVersion=1.8 -XX:+UseG1GC -XX:+UseStringDeduplication -Dosgi.requiredJavaVersion=1.8 -Xms10000m -Xmx10000m -jar /home/uranadh/software/eclipse//plugins/org.eclipse.equinox.launcher_1.3.201.v20161025-1711.jar

uranadh   5087 2966  0 19:52 pts/19   00:00:00 grep --color=auto java

uranadh@uranadh:~$ free -m

                 total            used        free      shared  buff/cache   available

Mem:          15953        4674        9148     257              2130     10683

Swap:          4767           0            4767

Conclusion :

Even though we have bootup three JVM instances with altogether 25GB memory allocation it had been only used < 4674 M .

The reason is when we use G1 GC option with JVM, it doesn’t matter initial heap size (Xms) value and only matter is maximum heap size ( Xmx) . Which means when JVM bootup it doesn’t allocate Xms size of memory.

CMS GC Collector

uranadh@uranadh:~$ ps -ef | grep java

uranadh   9501 6976  1 10:07 pts/4    00:00:26 /home/uranadh/software/jdk1.8.0_131/bin/java -Xloggc:/home/uranadh/software/apache-cassandra-3.11.2/logs/gc.log -ea -XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42 -XX:+HeapDumpOnOutOfMemoryError -Xss256k -XX:StringTableSize=1000003 -XX:+AlwaysPreTouch -XX:-UseBiasedLocking -XX:+UseTLAB -XX:+ResizeTLAB -XX:+UseNUMA -XX:+PerfDisableSharedMem -Djava.net.preferIPv4Stack=true -Xms6G -Xmx6G -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:+CMSClassUnloadingEnabled -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -XX:+PrintPromotionFailure -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10M -Xmn400M -XX:+UseCondCardMark -XX:CompileCommandFile=/home/uranadh/software/apache-cassandra-3.11.2/conf/hotspot_compiler -javaagent:/home/uranadh/software/apache-cassandra-3.11.2/lib/jamm-0.3.0.jar -Dcassandra.jmx.local.port=7199 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.password.file=/etc/cassandra/jmxremote.password -Djava.library.path=/home/uranadh/software/apache-cassandra-3.11.2/lib/sigar-bin -XX:OnOutOfMemoryError=kill -9 %p -Dlogback.configurationFile=logback.xml -Dcassandra.logdir=/home/uranadh/software/apache-cassandra-3.11.2/logs -Dcassandra.storagedir=/home/uranadh/software/apache-cassandra-3.11.2/data -Dcassandra-foreground=yes -cp /home/uranadh/software/apache-cassandra-3.11.2/conf:/home/uranadh/software/apache-cassandra-3.11.2/build/classes/main:/home/uranadh/software/apache-cassandra-3.11.2/build/classes/thrift:/home/uranadh/software/apache-cassandra-3.11.2/lib/HdrHistogram-2.1.9.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/ST4-4.0.8.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/airline-0.6.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/antlr-runtime-3.5.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/apache-cassandra-3.11.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/apache-cassandra-thrift-3.11.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/asm-5.0.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/caffeine-2.2.6.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/cassandra-driver-core-3.0.1-shaded.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/commons-cli-1.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/commons-codec-1.9.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/commons-lang3-3.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/commons-math3-3.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/compress-lzf-0.8.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/concurrent-trees-2.4.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/concurrentlinkedhashmap-lru-1.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/disruptor-3.0.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/ecj-4.4.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/guava-18.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/high-scale-lib-1.0.6.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/hppc-0.5.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jackson-core-asl-1.9.13.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jackson-mapper-asl-1.9.13.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jamm-0.3.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/javax.inject.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jbcrypt-0.3m.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jcl-over-slf4j-1.7.7.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jctools-core-1.2.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jflex-1.6.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jna-4.2.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/joda-time-2.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/json-simple-1.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jstackjunit-0.0.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/libthrift-0.9.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/log4j-over-slf4j-1.7.7.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/logback-classic-1.1.3.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/logback-core-1.1.3.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/lz4-1.3.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/metrics-core-3.1.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/metrics-jvm-3.1.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/metrics-logback-3.1.0.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/netty-all-4.0.44.Final.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/ohc-core-0.4.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/ohc-core-j8-0.4.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/reporter-config-base-3.0.3.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/reporter-config3-3.0.3.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/sigar-1.6.4.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/slf4j-api-1.7.7.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/snakeyaml-1.11.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/snappy-java-1.1.1.7.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/snowball-stemmer-1.3.0.581.1.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/stream-2.5.2.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/thrift-server-0.3.7.jar:/home/uranadh/software/apache-cassandra-3.11.2/lib/jsr223/*/*.jar org.apache.cassandra.service.CassandraDaemon

uranadh  11160 1579  0 10:31 ?     00:00:00 eog /home/uranadh/Pictures/java-heap-CMS.png

uranadh  11315 2966  0 10:36 pts/19   00:00:00 grep --color=auto java

uranadh@uranadh:~$ free -m

                 total            used        free          shared  buff/cache   available

Mem:          15953        9882        2446         335        3624        5362

Swap:          4767           0            4767

After killing Cassandra instance

uranadh@uranadh:~$ free -m

                 total            used        free          shared  buff/cache   available

Mem:          15953        3295        9032         335        3624           11948

Swap:          4767           0            4767

uranadh@uranadh:~$

Conclusion :

We have bootup JVM instance with 6GB memory allocation and when we check OS memory usage JVM instance used 6GB of memory.. But when we monitor JVM memory utilization it only consumed few MBs only

The reason is when we use CMS GC option with JVM, it allocated initial heap size (Xms)  when JVM bootup. OS see this as already utilized memory (6GB) but really it only utilized few MBs .