RFR: Mixed evacuation

[Kelvin Nilsen]

On Wed, 7 Apr 2021 21:03:24 GMT, earthling-amzn <github.com+71722661+earthling-amzn at openjdk.org> wrote:

>> Mixed evacuation
>
> Think it wants a description of the changes in the body of the PR.

I'm not sure how to address the "empty pull request body" problem.  The purpose of this pull request is described in the commit comments:
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 exercises concurrent old/young GC.

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