jdk 8u40 - MaxMetaspaceSize and CompressedClassSpaceSize

[Srinivas Ramakrishna]

Hi Vitaly --

Because reserved but unused virtual address space is, for all practical
purposes today, irrelevant for 64 bit processes,
a question to ask is what you see when you do this for 32-bit processes
(where of course the oop compression
comments wouldn't of course apply, but the amount of VA space used would be
much more relevant and parsimony
-- or lack thereof -- in VA space reservation would visibly affect the
bottom-line in a 32-bit VA space).

-- ramki

On Tue, Apr 7, 2015 at 12:27 PM, Vitaly Davidovich <vitalyd at gmail.com>
wrote:

> 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
>