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