sample_eden()

[Jon Masamitsu]

Ramki,

This has gotten interesting.  When

1) a System.gc() is called

and

2) UseCMSCompactAtFullCollection is set to false

the CMS generation tells GenCollectedHeap that CMS
does not collect the young gen.  That's right but I hadn't
appreciated that in that circumstance a young gen GC
was done before the CMS gen is collected.  Makes sense
but the GC epilogue code is called after all the generations
have been collected by GenCollectedHeap so the
_eden_chunk_index is not reset between the young GC
and the CMS GC.

I think the right fix is to use the occupancy of eden to
decide what work we need to do instead of _eden_chunk_index.

Jon

On 7/26/13 6:41 PM, Srinivas Ramakrishna wrote:
> Don't have the code in front of me to check, but yes that would seem to be the thing to do. I thought it was reset in the young gen gc epilogue ...
>
> ysr1729
>
> On Jul 26, 2013, at 14:46, Jon Masamitsu <jon.masamitsu at oracle.com> wrote:
>
>> Hiroshi,
>>
>> I'm looking at an assertion failure with CMSParallelInitialMarkEnabled
>> and CMSEdenChunksRecordAlways both enabled.  The assertion
>> failure is in do_young_space_rescan()
>>
>>   5506      assert(mr.is_empty() || space->used_region().contains(mr),
>>   5507             "Should be in space");
>>
>> and the failure occurs because _eden_chunk_index is > 0 and
>> eden is empty.
>>
>> A young GC has just occurred and a System.gc() is in progress where the
>> System.gc() is executing the the usual phases of CMS in a stop-the-world
>> fashion.  A rarely seen scenario I think.  That is, the initial mark is being
>> executed.
>>
>> I was looking at the places where _eden_chunk_index is reinitialized to
>> 0.  I don't think you added any in you changes, right?
>>
>> I was thinking that _eden_chunk_index should be reset to 0 after
>> a young GC where we know that eden is empty.  What do you think?
>>
>> Jon
>>
>>
>>