Do java flags Xms and Xmx overwrite flag XX:+UseCGroupMemoryLimitForHeap?

Issue

I'm running a containerized java application in Kubernetes.

In order to make the jvm reserve memory according to the container specifications, the flags -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap must be set.

If both those flags are set along with Xms and Xmx flags, what would the behavior of the jvm be? Do one flag overwrite the other?

For example, if we had java -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap -Xms -Xms2500M -Xmx2500M -jar myjar.jar in a pod with container limits 4Gi for requests and 4Gi for responses, in a host machine that has 128Gi memory, how much memory would the JVM reserve?

Solution

The -Xmx flag overwrites the -XX:+UseCGroupMemoryLimitForHeap flag.

The flag -XX:+UseCGroupMemoryLimitForHeap lets the JVM detect what the max heap size in a container should be.

The -Xmx flag set the max heap size to a fixed size.

To answer your example, the JVM would reserve 2500M heap space. There will be some additional memory usage for non-heap and jvm stuff.

To further tune your memory usage in a container you could use the -XX:MaxRAMFraction flag. See this article: https://blog.csanchez.org/2017/05/31/running-a-jvm-in-a-container-without-getting-killed/

Answered By - Chris