RFR: Mixed evacuation [v7]

[Zhengyu Gu]

On Wed, 21 Apr 2021 23:48:09 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:
> 
>   Cosmetic improvements requested by review
>   
>   Following these changes, we ran jtreg TIER1 and hotspot_gc_shenandoah tests with no regressions.
>   Also ran a stress Extremem workload.

Changes requested by zgu (Reviewer).

src/hotspot/share/gc/shenandoah/mode/shenandoahMode.cpp line 58:

> 56: ShenandoahOldHeuristics* ShenandoahMode::initialize_old_heuristics(ShenandoahGeneration* generation) const {
> 57: 
> 58:   if (ShenandoahGCHeuristics != NULL) {

Should be assert or guarantee ?

src/hotspot/share/gc/shenandoah/mode/shenandoahPassiveMode.cpp line 65:

> 63: 
> 64: ShenandoahOldHeuristics* ShenandoahPassiveMode::initialize_old_heuristics(ShenandoahGeneration* generation) const {
> 65:   if (ShenandoahGCHeuristics != NULL) {

Looks like you can convert this if stmt to assert or guarantee?

src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp line 130:

> 128:   virtual bool is_concurrent_mark_in_progress() = 0;
> 129: 
> 130:  private:

Style nit: no leading space. The same as "protected" above (pre-exist)

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 496:

> 494:   CollectedHeap(),
> 495:   _gc_generation(NULL),
> 496:   _old_heuristics(NULL),

Minor style nit: jdk17 started to use nullptr to replace NULL

src/hotspot/share/gc/shenandoah/shenandoahOldGC.cpp line 88:

> 86:   entry_coalesce_and_fill();
> 87: 
> 88:   old_heuristics->start_old_evacuations();

Could you encapsulate in entry_old_evacuations() with proper timing/logging?  it really helps future debugging the crashes with hs_err file.

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