RFR: Mixed evacuation [v8]

[Kelvin Nilsen]

On Sat, 24 Apr 2021 16:03:16 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This code adds to generational Shenandoah the ability to perform concurrent garbage collection of young-gen and old-gen memory. Following completion of an old-gen concurrent marking effort, we select certain old-gen heap regions to serve as candidates for future collection sets. All dead objects within the old-gen heap regions that are not part of this candidate set are coalesced and filled so that remembered-set scanning of these old-gen heap regions will not be confused by "zombie objects" (objects that old-gen has decided are dead, which reside in regions that have not yet been collected). After concurrently coalescing and filling these dead objects, each subsequent young-gen evacuation pass includes a subset of the old-gen candidates until all candidates have been collected. This code passes TIER1 and hotspot-gc-shenandoah jtreg tests without regressions. A new jtreg test has been added to exercise concurrent old/young GC.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Additional changes requested by reviewers
>   
>   This code builds and tests TIER1 and hotspot_gc_shenandoah with no regressions.  It also runs a generational GC stress test.

Thanks

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PR: https://git.openjdk.java.net/shenandoah/pull/29