RFR: Generation resizing [v4]

[William Kemper]

On Thu, 8 Dec 2022 23:32:14 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:
>> 
>>  - Merge branch 'shenandoah-master' into mmu-instrumentation
>>  - Remove vestigial lock, do not enroll periodic task while holding threads_lock
>>  - Remove unnecessary logging, clean up imports
>>  - Merge from shenandoah/master
>>  - Document the class responsible for adjusting generation sizes
>>  - Revert unnecessary change
>>  - Remove unused time between cycle tracking
>>  - Remove vestigial mmu tracker instance
>>  - Clamp adjustments to min/max when increment is too large
>>  - Adjust generation sizes from safepoint
>>  - ... and 7 more: https://git.openjdk.org/shenandoah/compare/25469283...50896e31
>
> src/hotspot/share/gc/shenandoah/mode/shenandoahGenerationalMode.cpp line 39:
> 
>> 37:   }
>> 38: 
>> 39:   SHENANDOAH_ERGO_OVERRIDE_DEFAULT(GCTimeRatio, 70);
> 
> Does this translate to a GC overhead of 1/71*100% = 1.4%?

I think it is a confusingly named parameter, but I'm interpreting it based on the description: 
> "Adaptive size policy application time to GC time ratio"

Any time the average MMU drops below this number, it attempts to resize the generations.

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PR: https://git.openjdk.org/shenandoah/pull/177