Why does G1 Old gen max memory in jmx equal to max heap size?

Question

I start a tomcat instance enabled with jmx, I check G1 old gen memory stat, find max memory = max heap size.

• The environment

# uname -a
Linux bogon 2.6.32-642.15.1.el6.x86_64 #1 SMP Fri Feb 24 14:31:22 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
# cat /etc/redhat-release
CentOS release 6.8 (Final)
# java -version
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)

• jvm option

# ps -ef|grep tomcat
tomcat    3177     1  7 10:23 ?        00:00:06 /usr/java/jdk1.8.0_65//bin/java -Djava.util.logging.config.file=/apps/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -server -Djava.awt.headless=true -Dfile.encoding=UTF-8 -Xms1696m -Xmx1696m -XX:+DisableExplicitGC -XX:MaxGCPauseMillis=200 -XX:+UseStringDeduplication -XX:MetaspaceSize=96m -XX:+UseG1GC -XX:InitiatingHeapOccupancyPercent=45 -XX:MinMetaspaceFreeRatio=50 -XX:MaxMetaspaceFreeRatio=80 -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -XX:+PrintClassHistogram -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -Xloggc:/var/log/tomcat/gc.log -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=2M -XX:+HeapDumpOnOutOfMemoryError -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=8415 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.rmi.port=8415 -classpath /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar -Dcatalina.base=/apps/tomcat -Dcatalina.home=/usr/local/tomcat -Djava.io.tmpdir=/apps/tomcat/temp org.apache.catalina.startup.Bootstrap -Dprocessname=tomcat start

• jmap result

# jmap -heap 3177
Attaching to process ID 3177, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.65-b01

using thread-local object allocation.
Garbage-First (G1) GC with 1 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 1778384896 (1696.0MB)
   NewSize                  = 1363144 (1.2999954223632812MB)
   MaxNewSize               = 1066401792 (1017.0MB)
   OldSize                  = 5452592 (5.1999969482421875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 100663296 (96.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
   regions  = 1696
   capacity = 1778384896 (1696.0MB)
   used     = 84934656 (81.0MB)
   free     = 1693450240 (1615.0MB)
   4.775943396226415% used
G1 Young Generation:
Eden Space:
   regions  = 81
   capacity = 93323264 (89.0MB)
   used     = 84934656 (81.0MB)
   free     = 8388608 (8.0MB)
   91.01123595505618% used
Survivor Space:
   regions  = 0
   capacity = 0 (0.0MB)
   used     = 0 (0.0MB)
   free     = 0 (0.0MB)
   0.0% used
G1 Old Generation:
   regions  = 0
   capacity = 1685061632 (1607.0MB)
   used     = 0 (0.0MB)
   free     = 1685061632 (1607.0MB)
   0.0% used

12869 interned Strings occupying 1777832 bytes.

• Question

The jmx item java.lang:type=MemoryPool,name=G1 Old Gen,Usage.max is 1778384896, it equals to the max heap size.

If Old gen memory is not enough, all heap memory will be used for Old gen memory?

Answer 1

I really faced the same issue, that the max memory of old gen is as the same as the value of -Xmx, while the max memory of eden and survivor are both -1 (unlimited) in JMX interface. Just 5 months after your question, there was an issue ticket for openjdk "Revamp G1 JMX MemoryPool and GarbageCollector MXBean definitions". I'm afraid we can only be waiting for it being resolved.