RFR(S): JDK-8019845 NPG: Memory leak during class redefinition

[Mikael Gerdin]

Fred,

On 07/25/2013 05:15 PM, frederic parain wrote:
> Greetings,
>
> Please review this small fix:
>
> open webrev: http://cr.openjdk.java.net/~fparain/8019845/webrev.00/
> bug link: http://bugs.sun.com/view_bug.do?bug_id=8019845

Looks good to me.

>
> The bug title is "NPG: Memory leak during class redefinition" but
> the leak is in the metaspace code.
>
> To store metadata, the VM allocates memory chunks using
> SpaceManager::allocate(). In this method, a "raw_word_size"
> is computed from the MetaspaceObject size and some
> alignment constraints to determine the size of the memory
> chunk needed. A look up is performed in the free list to
> find a chunk of this size, if none is found, a new chunk
> is allocated.
>
> When a memory chunk is not used anymore, for instance when
> a class is redefined and the old metadata are discarded, it
> is returned to the free list.

Chunk has a specific meaning in the context of Metaspace, as in a 
Metachunk. I realize that the term is very general but it's easy to get 
the terms confused.


>
> The problem is that the memory chunk is returned to the free
> list using its MetaspaceObject size without considering alignment
> constraints. If the "raw_word_size" computed in
> SpaceManager::allocate() is different from the MetaspaceObject
> size, look ups in the free list cannot find returned chunks
> because of the size difference, causing the leak.

I suspect that in *debug builds the "leak" would be even larger because 
raw_word_size adds the size of the Metablock header which is used for 
accounting in *debug builds.

/Mikael

>
> The fix is to return memory chunks using the same size
> computation as the one used in SpaceManager::allocate().
>
> Tested with:
>    RedefineBigClass.sh output (checking the memory leak
>    doesn't show up anymore)
>    JPRT
>    vm.quick.testlist
>    vm.runtime.testlist
>    vm.parallel_class_loading.testlist
>
> Thanks,
>
> Fred
>