Will Java 8 always consume at least 1GB of RAM by default when running inside a container?

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We are running a Java application in a Kubernetes cluster. The application itself doesn't have high demand for RAM, but I've noticed that it always consumes 1GB.

kubectl top pods
NAME                                                              CPU(cores)   MEMORY(bytes)
my-application-c0ffee                                             100m           1127Mi
my-application-c0ffee                                             100m           1109Mi

When I checked jcmd <pid> GC.heap_info inside the container, I got the following:

def new generation   total 89216K, used 12090K [0x00000000bc200000, 0x00000000c22c0000, 0x00000000d2c00000)
  ...
tenured generation   total 197620K, used 151615K [0x00000000d2c00000, 0x00000000decfd000, 0x0000000100000000)
  ...
Metaspace       used 146466K, capacity 152184K, committed 152576K, reserved 1183744K
  class space    used 18171K, capacity 19099K, committed 19200K, reserved 1048576K

As I understood, by default Java reserves 1GB of virtual memory size for storing Class information (in order to reference it in a compressed manner using 32 bit references this memory block should be reserved beforehand). When running outside of a container that's not a big deal, because this memory is not actually committed. It's only an address space that is reserved.

But it seems to be completely different situation in case of running inside container, where reserved memory becomes committed.

Does this mean that Java running in a container will by default consume at least 1GB of RAM? And is there any other way to deal with that besides explicitly setting -XX:CompressedClassSpaceSize?

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1 Answers

in case of running inside container, where reserved memory becomes committed

No, reserved memory does not "become committed". The Virtual Size and the Resident Set Size are different metrics, whether in a container or not. What sits in the physical memory is RSS.

kubectl top does not show you the RSS, but rather so-called "working set", which does not always match the real memory usage.

Does this mean that Java running in a container will by default consume at least 1GB of RAM?

No.

is there any other way to deal with that

It depends on your goals. If you'd like to see the actual container memory statistics, look at /sys/fs/cgroup/memory/.../memory.stats and memory.usage_in_bytes. Or if you use docker, run docker stats.

If you'd like to decrease the process' virtual memory instead, turn off -XX:-UseCompressedClassPointers.

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