RFR: 8256265: G1: Improve parallelism in regions that failed evacuation [v12]

[Hamlin Li]

On Fri, 11 Mar 2022 14:28:22 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Currently G1 assigns a thread per failed evacuated region. This can in effect serialize the whole process as often (particularly with region pinning) there is only one region to fix up.
>> 
>> This patch tries to improve parallelism when walking over the regions in chunks
>> 
>> Latest implementation scans regions in chunks to bring parallelism, it's based on JDK-8278917 which changes to uses prev bitmap to mark evacuation failure objs.
>> 
>> Here's the summary of performance data based on latest implementation, basically, it brings better and stable performance than baseline at "Post Evacuate Cleanup 1/remove self forwardee" phase. (Although some regression is spotted when calculate the results in geomean, becuase one pause time from baseline is far too small than others.)
>> 
>> The performance benefit trend is:
>>  - pause time (Post Evacuate Cleanup 1) is decreased from 76.79% to 2.28% for average time, from 71.61% to 3.04% for geomean, when G1EvacuationFailureALotCSetPercent is changed from 2 to 90 (-XX:ParallelGCThreads=8)
>>  - pause time (Post Evacuate Cleanup 1) is decreased from 63.84% to 15.16% for average time, from 55.41% to 12.45% for geomean, when G1EvacuationFailureALotCSetPercent is changed from 2 to 90 (-XX:ParallelGCThreads=<default=123>)
>> ( Other common Evacuation Failure configurations are:
>> -XX:+G1EvacuationFailureALot -XX:G1EvacuationFailureALotInterval=0 -XX:G1EvacuationFailureALotCount=0 )
>> 
>> For more detailed performance data, please check the related bug.
>
> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Albert review

Thanks for the detailed reviews. :)

I'm not sure if it's feasible to move `prepare_regions` into post_evacuate_cleanup_1 as a G1BatchedTask.
My concern is that in `prepare_regions()->PrepareEvacFailureRegionTask::prepare_region()->HeapRegion::note_self_forwarding_removal_start()`, nTAMS is set, and nTAMS is used *cleanup 1* phase at RemoveSelfForwardPtrObjClosure to set next bitmap. If move `prepare_regions` into post_evacuate_cleanup_1 as a G1BatchedTask, there is no guarantee that preparation will be done before the usage of nTAMS in RemoveSelfForwardPtrObjClosure.
How do you think about it?

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PR: https://git.openjdk.java.net/jdk/pull/7047