Request for Review (M) - 8150994: UseParallelGC fails with UseDynamicNumberOfGCThreads with specjbb2005

[Kim Barrett]

> On Apr 12, 2016, at 11:50 PM, Jon Masamitsu <jon.masamitsu at oracle.com> wrote:
> 
> The ParallelGC collector compacts by moving live objects from source
> regions to destination regions.  The destination regions are regions that have
> been emptied of live objects and the source regions are regions that hold
> the original locations of the live objects.  When a source region is emptied,
> it becomes a destination region.  At the start of the compaction the
> destination regions are distributed to per thread lists processed by the GC
> workers.   The original implementation had tasks that drain the initial
> destination lists and tasks that steal destination regions from other
> GC workers.  The draining tasks and stealing tasks shared the per thread
> lists of destination regions.  The bug was that the sharing was not
> synchronized correctly.
> 
> See [1] for more details.
> 
> The fix is to combine the draining tasks and stealing tasks so that
> sharing of the destination lists is not needed.
> 
> [1] https://bugs.openjdk.java.net/browse/JDK-8150994
> 
> [2] http://cr.openjdk.java.net/~jmasa/8150994/webrev.00/
> 
> Thanks.
> 
> Jon

Looks good.

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src/share/vm/gc/parallel/psParallelCompact.cpp
2248         if (++which == parallel_gc_threads) {
2249           assert(which <= parallel_gc_threads,

The comparison on 2248 was changed from task_count (which no longer
exists) to parallel_gc_threads, thereby guaranteeing the asserted
condition.

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