RFR: Plab fallback to minsize

[William Kemper]

On Fri, 6 Jan 2023 19:45:14 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> If there is not sufficient memory within a region to allocate the desired size of a PLAB, try to allocate a smaller PLAB that is still larger than the minimum PLAB size.  This allows more efficient packing of PLABs into available old-gen heap regions.
> 
> This patch has been exercised by an internal CI regression testing pipeline and has not introduced any regressions.
> 
> Modest reductions in evacuation efforts are seen on benchmarks that do heavy promotion.  This is presumably because there are fewer promotion failures, thus less need to repeatedly copy "old" objects within young-gen spaces.  Of note, but not entirely explained, is the observation that some evacuation effort has shifted from mutator threads to GC worker threads.
> 
> Some regression in jhiccup pause times were observed.  This will be explored further after additional patches are integrated.

The evacuation metrics between mutators and gc workers are fairly unstable - likely because it depends so much on when and which threads get scheduled. I've been thinking of masking them in the reports for this reason.

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PR: https://git.openjdk.org/shenandoah/pull/195