RFR: Add generational full gc support

[Roman Kennke]

On Fri, 27 Aug 2021 15:29:41 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> In situations when Generational Shenandoah triggers full-gc, we now allow full gc to run.  The implementation of full gc preserves the distinction between young and old memory regions because premature promotion of young objects into old regions may result in inefficient subsequent old-gc efforts.
>
> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp line 221:
> 
>> 219:       // that the PLAB be disabled for all future purposes.  We may want to introduce a new service to make the
>> 220:       // PLABs parsable while still allowing the PLAB to serve future allocation requests that arise during the
>> 221:       // next evacuation pass.
> 
> If I understand correctly, we already have this in ShenandoahHeap::ensure_parsability(bool retire_tlabs)

Oh wait, probably not available for PLABs.

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PR: https://git.openjdk.java.net/shenandoah/pull/61