RFR: 8165443: Free Collection Set serial phase takes very long on large heaps

[Kim Barrett]

> On Dec 5, 2019, at 1:27 PM, Stefan Johansson <stefan.johansson at oracle.com> wrote:
> Thanks, here are updated webrevs:
> Full: http://cr.openjdk.java.net/~sjohanss/8165443/02/
> Inc: http://cr.openjdk.java.net/~sjohanss/8165443/01-02/

Looks good.

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src/hotspot/share/gc/g1/g1CollectedHeap.cpp
4207   class G1FreeCollectionSetClosure : public HeapRegionClosure {

Since it's a nested class, it could be called FreeCSetClosure, to be
similar to FreeCSetStats.  Just a suggestion.

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src/hotspot/share/gc/g1/heapRegionManager.cpp
 680   uint const num_workers = clamp(max_length(), 1u, workers->active_workers());

Maybe this was already discussed, but it looks to me that the cost per
region for the rebuild task is pretty low, so that it might be that
this should use one thread per N regions, N > 1.  So use "max_length()
/ N".  I don't know what N should be.  For a similar case in reference
processing we added the experimental option ReferencesPerThread with a
default value of 1000.  (Note that I'm not encouraging the addition of
an option here.)

This should probably be deferred for further experimentation and measurement.

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