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

Question

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:+

Answer 1

Think about it this way, before that UseCGroupMemoryLimitForHeap was added, you could specify Xmx with a value larger than what your pod had (it would look only on the host memory, not the pod itself), eventually being killed. By default that was 1/4 of the memory if not specified. This happens because heap is calculated like:

heap = memory / MaxRAMFraction

And running:

java -XX:+PrintFlagsFinal | grep MaxRAMFraction

would reveal that MaxRAMFraction = 4.

Now that UseCGroupMemoryLimitForHeap was added, you can tell how much heap from the pod itself will be taken. By default, that is still 1/4; but you can adjust that via MaxRAMFraction, of course.

If you specify both arguments, Xms wins. always. And 1) this is pretty logic (as UseCGroupMemoryLimitForHeap is more specific than Xmx) 2) this is exactly proven by my experiments just now. No matter the order in which they are specified, Xmx always wins.