RFR: 8348595: GenShen: Fix generational free-memory no-progress check [v2]

[Kelvin Nilsen]

On Fri, 31 Jan 2025 01:15:01 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

>> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Respond to reviewer feedback
>>   
>>   In testing suggested refinements, I discovered a bug in original
>>   implementation.  ShenandoahFreeSet::capacity() does not represent the
>>   size of young generation.  It represents the total size of the young
>>   regions that had available memory at the time we most recently rebuilt
>>   the ShenandoahFreeSet.
>>   
>>   I am rerunning the performance tests following this suggested change.
>
> src/hotspot/share/gc/shenandoah/shenandoahMetrics.cpp line 52:
> 
>> 50:   size_t free_actual   = free_set->available();
>> 51:   // The sum of free_set->capacity() and ->reserved represents capacity of young in generational, heap in non-generational.
>> 52:   size_t free_expected = ((free_set->capacity() + free_set->reserved()) / 100) * ShenandoahCriticalFreeThreshold;
> 
> We may pass ShenandoahGeneration as parameter to `is_good_progress` to simplify the calculation of free_expected, it should be like:
> `
> generation->max_capacity() / 100 * ShenandoahCriticalFreeThreshold
> `
> Good part is, free_expected might be more accurate in Full GC/Degen for global cycle, e.g. Full GC collects memory for global, `free_expected` should be calculated using the metrics from global generation. But either way, `free_expected` is not clearly defined in generational mode now, current code also works.

Thanks for this suggestion.  I've made change.  It turns out there was actually a bug in the original implementation, so I am retesting the performance results.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23306#discussion_r1947334711