RFR: Mixed evacuation

[Roman Kennke]

On Wed, 7 Apr 2021 21:21:30 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> 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.

Please add this text in the PR description above. Klick '...' -> 'Edit' and enter the text there.

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