jdk 8u40 - MaxMetaspaceSize and CompressedClassSpaceSize

[Vitaly Davidovich]

Hi guys,

It seems that putting a cap on MaxMetaspaceSize has no impact on amount of
memory reserved *unless* CompressedClassSpaceSize is also set.  For example,

java -Xmx64m -XX:MaxMetaspaceSize=16m Test

yields the following:

 Metaspace       used 2421K, capacity 4486K, committed 4864K, reserved
1056768K
  class space    used 261K, capacity 386K, committed 512K, reserved 1048576K

Adding -XX:CompressedClassSpaceSize=16m yields:

Metaspace       used 2421K, capacity 4486K, committed 4864K, reserved 24576K
  class space    used 261K, capacity 386K, committed 512K, reserved 16384K

According to
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/considerations.html
:

>
> If UseCompressedOops is turned on and UseCompressedClassesPointers is
> used, then two logically different areas of native memory are used for
> class metadata. UseCompressedClassPointers uses a 32-bit offset to
> represent the class pointer in a 64-bit process as does UseCompressedOops for
> Java object references. A region is allocated for these compressed class
> pointers (the 32-bit offsets). The size of the region can be set with
> CompressedClassSpaceSize and is 1 gigabyte (GB) by default. The space for
> the compressed class pointers is reserved as space allocated by mmap at
> initialization and committed as needed. The MaxMetaspaceSize applies to
> the sum of the committed compressed class space and the space for the other
> class metadata.


Unless I'm misunderstanding, the last sentence there seems to imply that
MaxMetaspaceSize would put a cap on the compressed class space as well.  Or
is the "committed" part of the last sentence the key? There's no way to cap
the *reserved* amount with 1 flag? Basically, what's the java 8 equivalent
to MaxPermSize=X in java 7 and earlier?

The reason this came up is because java 8 seems to have a much higher
virtual mem reserve charge than java 7, even for an app with a simple main
that does nothing, and metaspace/compressed class space reservation appears
to be the biggest contributor.

Thanks