JDK12 memory behavior in docker container

Question

It was my understanding that as of JDK12 the old experimental flag -XX:+UseCGroupMemoryLimitForHeap was removed but going to be baseline e.g. all JDK12 apps would read cgroup memory and set that as -Xms and -Xmx internally by default. However when I ran -XX:+PrintFlagsFinal with no -Xmx specified it showed I had only 512MB of memory for MaxHeap in a container with a limit of 2GB of memory allocated.

Did I miss something and the community decided to remove the -XX:+UseCGroupMemoryLimitForHeap flag with no intention of supporting behavior?

Karol Dowbecki · Accepted Answer · 2020-01-16T17:55:39.170

3

This is correct. Java 12 respects cgroup RAM limit (2GB) and ignores the total RAM available on the host (which is larger than 2GB). Since -Xmx wasn't specified the default JVM behavior is to use 25% of available RAM and you end up with 512 MB heap. In practice -Xmx is a specific value which overrides -XX:+UseCGroupMemoryLimitForHeap and other flags governing dynamic heap sizing.

Assuming you don't want to specify -Xmx directly you most likely want to use -XX:MaxRAMFraction=1 which will make the JVM use 100% of the available RAM. Do note that under load it may kill the container.

edited Jan 16 '20 at 17:55

answered Jan 16 '20 at 17:31

Karol Dowbecki

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thank you that last part is what I didn't understand about it setting it to 25%. I'll add the MaxRAMFraction to something more reasonable. – Jordan Shaw Jan 16 '20 at 20:59
Also check [`-XX:+UseContainerSupport`](https://stackoverflow.com/a/54517077/1602555). – Karol Dowbecki Jan 17 '20 at 10:58

JDK12 memory behavior in docker container

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