Why is Tomcat/JVM using 4.5Gb virtual memory on a server with only 4Gb RAM and no swap?

March 28, 2019 813 views
Server Optimization Java Ubuntu 16.04

I’ve changed /etc/default/tomcat7 to have Xms and Xmx of 2048m on a 4Gb server:

JAVA_OPTS=“-Djava.security.egd=file:/dev/./urandom -Djava.awt.headless=true -Xms2048m -Xmx2048m -XX:MaxPermSize=256m -XX:+UseConcMarkSweepGC -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/usr/share/tomcat7”

However I see tomcat7 with a VIRT of 4575652 and a RES of 1.150g in “top”.

  • Why is Tomcat/JVM using 4.5Gb virtual memory on a server with only 4Gb?
  • Why does Tomcat/JVM not “page in” the Xms of 2048m?

And I thought I understood virtual memory and the JVM :-(

Any help would be greatly appreciated. Thanks.

2 Answers

So I would recommend you to use the pmap PID :)

  1. After that you will see the more loaded files.
  2. Because you have heap, meta space. stack https://dzone.com/articles/understanding-the-java-memory-model-and-the-garbag

Thanks gonchik,

I actually found out that along with the stack and static/code segments the main culprit was glibc’s use of “arenas” for memory allocation.

The default is to allocate 8 x 64Kb spaces per CPU. So in my case that was a whole gigabyte of unexpected RAM use.

This can be tuned by setting the environment variable MALLOC_ARENA_MAX within the startup scripts of Tomcat and the JVM.

The more “arenas” the better the memory allocation concurrency but I have found little difference in my application from the default 8 to a value of 1. I guess it depends how often the JVM needs memory.

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