Withdrawn: 8256265 G1: Improve parallelism in regions that failed evacuation

[duke]

On Wed, 1 Dec 2021 07:57:51 GMT, Hamlin Li <mli at openjdk.org> wrote:

> Summary
> -------
> 
> 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.
> Try to improve the parallelsim when walking over the regions by 
> 
>  - first, split a region into tasks;
>  - then, process these task in parallel and load balance among GC threads;
>  - last, necessary cleanup
> 
> NOTE: load balance part of code is almost same as G1ParScanThreadState, if necessary and feasible, consider to refactor this part into a shared code base.
> 
> Performance Test
> -------
> 
> The perf test based on lastest implementation + JDK-8277736 shows that:
> 
>  - when `ParallelGCThreads`=32, when `G1EvacuationFailureALotCSetPercent` <= 50, the parallelism bring more benefit than regression;
> - when `ParallelGCThreads`=128, whatever `G1EvacuationFailureALotCSetPercent` is, the parallelism bring more benefit than regression;
> 
> other related evac failure vm options:
>  - `G1EvacuationFailureALotInterval`=1
>  - `G1EvacuationFailureALotCount`=1
> 
> For detailed perf test result, please check:
> 
>  - https://bugs.openjdk.java.net/secure/attachment/97227/parallel.evac.failure-threads.32.png
>  - https://bugs.openjdk.java.net/secure/attachment/97228/parallel.evac.failure-threads.128.png
>  
> For the situation like G1EvacuationFailureALotCSetPercent > 50 and ParallelGCThreads=32 , we could fall back to current implmentation, or further optimize the thread sizing at this phase if necessary.
> 
> NOTE: I don't include perf data for `Remove Self Forwards`, because the comparison of pause time in this phase does not well show the improvement of this implementation, I think the reason is that the original implementation is not load balanced, and the new implementation is. But as `Remove Self Forwards` is part of `Post Evacuate Cleanup 1`, so only `Post Evacuate Cleanup 1` well show the improvement of the new implementation.
> It could be a potential improvement to refine the Pause time data in `Remove Self Forwards` phase.

This pull request has been closed without being integrated.

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PR: https://git.openjdk.java.net/jdk/pull/6627