RFR (S): 7066063: CMS: "Conservation Principle" assert failed

[Srinivas Ramakrishna]

Hi Bengt --

Thanks for the nice description! The change looks right to me. (I seem
to recall having spent a bunch of time on this many moons ago, and
still not managed to get rid of the last occurrence of this assert;
looks like you may have found the "last" case that was confounding the
assert! :-)

Reviewed!
-- ramki

On Mon, May 20, 2013 at 1:43 PM, Bengt Rutisson
<bengt.rutisson at oracle.com> wrote:
>
> Hi everyone,
>
> Could I have a couple of reviews for this small change?
> http://cr.openjdk.java.net/~brutisso/7066063/webrev.00/
>
> Background :
>
> We hit this assert in the allocation statistics for CMS:
>
> assert(prevSweep() + splitBirths() + coalBirths() >= splitDeaths() +
> coalDeaths() + (ssize_t)count) failed: Conservation Principle
>
> The problem is in CompactibleFreeListSpace::reset(). That method calls
> FreeList<Chunk>::return_chunk_at_head(), which calls increment_count().
>
> All other call sites that increment the count also updates either the split
> birth or coal birth statistics, but CompactibleFreeListSpace::reset() does
> not do that.
>
> My reproducer hits the assert every time I run it. If I add a call to
> coalBirth() in CompactibleFreeListSpace::reset() it never hits the assert.
>
> Here are the other calls to return_chunk_at_head and how they update the
> statistics:
>
> CompactibleFreeListSpace:: par_get_chunk_of_blocks()
>  -> calls set_split_births()
>
> CompactibleFreeListSpace::returnChunkToFreeList()
> -> coalBirth() in
> CompactibleFreeListSpace::addChunkToFreeListsAtEndRecordingStats()
> -> coalBirth() in SweepClosure::flush_cur_free_chunk()
> -> split_birth() in CompactibleFreeListSpace::getChunkFromLinearAllocBlock()
>
> CompactibleFreeListSpace::splitChunkAndReturnRemainder()
> -> split_birth() in CompactibleFreeListSpace::split()
>
>
> So, all other calls than CompactibleFreeListSpace::reset() either increment
> the split birth or coal birth counters. When we set up the heap we
> eventually end up in the
> CompactibleFreeListSpace::addChunkToFreeListsAtEndRecordingStats() which
> increments the coal birth.
>
> Since CompactibleFreeListSpace::reset() is called from
> CompactibleFreeListSpace::reset_after_compaction() when the freed up memory
> from the compaction is added to the free list it seems reasonable to update
> the same counter as when we first set up the heap. Thus, incrementing the
> coal birth seem more correct than incrementing the split birth count.
>
> Thanks,
> Bengt