RFR: Expand old on demand [v42]

[Kelvin Nilsen]

On Wed, 10 May 2023 23:41:39 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix whitespace
>
> src/hotspot/share/gc/shenandoah/heuristics/shenandoahHeuristics.cpp line 342:
> 
>> 340:     }
>> 341:   }
>> 342:   heap->reserve_promotable_humongous_regions(humongous_regions_promoted);
> 
> would be nice to eliminate this, and do all usage accounting at time regions are re-affiliated (promoted in place).

We may be able to simplify or remove these four reserve_promotable functions in the future.  But we need them for now.  They identify what work needs to be done during evacuation, and may feed into decisions about how to implement abbreviated gc cycles.  When I originally proposed to eliminate these functions, I was thinking we only needed this for updating generation sizes at the end of the GC cycle, but I was wrong.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/248#discussion_r1205913853