RFR: Mixed evacuation [v2]

[Zhengyu Gu]

On Thu, 8 Apr 2021 15:32:13 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:
> 
>   Chasing bug during prepare_for_old_collection
>   
>   One bug fix, a few cosmetic improvements, a change in loop structure to possibly avoid a gcc
>   optimization error.

Changes requested by zgu (Reviewer).

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp line 439:

> 437:       // Instead, skip to the following marked object, as indicated by the first following mark bit in
> 438:       // marking_context.
> 439:       int size = obj->size();

I am not sure that it is safe to walk dead oops, e.g. their classes might have been unloaded. I suggest to use marking_context->get_next_marked_addr() instead.

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp line 510:

> 508:         // Instead, skip to the following marked object, as indicated by the first following mark bit in
> 509:         // marking_context.
> 510:         int size = obj->size();

Same as above

src/hotspot/share/gc/shenandoah/shenandoahOldGC.cpp line 167:

> 165:   //
> 166:   // TraceCollectorStats tcs(heap->monitoring_support()->concurrent_collection_counters());
> 167:   // EventMark em("%s", msg);

Yes, I think we want them.

src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp line 67:

> 65:   f(finish_mark,                                    "  Finish Mark")                   \
> 66:   SHENANDOAH_PAR_PHASE_DO(finish_mark_,             "    FM: ", f)                     \
> 67:   f(coalesce_and_fill,                              "Coalesce and Fill Old Dead")      \

Does not seem to be used anywhere.

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