sample_eden()

[Jon Masamitsu]

On 7/29/13 2:29 PM, Srinivas Ramakrishna wrote:
> ah, makes sense! :-)
>
> So, if the scavenge failed, leaving stuff in Eden and the survivor
> spaces, the chunks should still be valid if a CMS collection could
> happen.
I expect that would be the case although I'd have to at the promotion
failure handling code again.
> (I wonder after asking that question, though, how CMS would deal with
> such a situation -- two active survivor spaces, i think it deals OK
> with it, but not sure if both scans today parallelized or not, or if
> the question is moot because the failed scavenge causes a bail out to
> a stop-world gc... perhaps the latter?) Probably an academic question,
> but still... :-)

Worth looking into.

Jon

>
> - ramki
>
> On Mon, Jul 29, 2013 at 1:46 PM, Jon Masamitsu <jon.masamitsu at oracle.com> wrote:
>> 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
>>>>
>>>>
>>>>