RFR: JDK-8315479: GenShen: Expand old-gen while selecting collection set during GLOBAL GC

[Kelvin Nilsen]

On Fri, 1 Sep 2023 16:49:35 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> With GenShen, the size of old generation is as small as possible in order to maximize memory available to young gen.  This generally allows less frequent (and therefore more efficient) young-gen collections.
> 
> Under normal operation (concurrent marking of old followed by a series of mixed evacuations), old-gen is expanded at the end of a previous GC cycle in order to accommodate evacuation of mixed-evacuation candidates.
> 
> However, when we do a GLOBAL collection, we need to expand OLD gen during the selection of the collection set, depending on how many old-gen regions are selected to be evacuated during the GC evacuation phase.  This PR makes the change.

Though GHA checks have all passed, our internal CI pipelines have encountered a few problems:
1. stress_dacapo (5 of 5 runs) encounters:

#  Error: Verify init-mark remembered set violation; clean card should be dirty

This issue is being addressed under separate ticket: https://bugs.openjdk.org/browse/JDK-8315044

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PR Comment: https://git.openjdk.org/shenandoah/pull/318#issuecomment-1707070009