RFR: 8254739: G1: Optimize evacuation failure for regions with few failed objects [v7]

[Hamlin Li]

On Thu, 26 Aug 2021 10:04:54 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> This is a try to optimize evcuation failure for regions.
>> I record every evacuation failure object per region (by G1EvacuationFailureObjsInHR), and then iterate (which indeed includes compact/sort/iteration) these objects directly in RemoveSelfForwardPtrHRClosure.
>> 
>> I have tested it with following parameters, 
>> 
>> -   -XX:+ParallelGCThreads=1/32/64
>> -   -XX:G1EvacuationFailureALotInterval=1
>> -   -XX:G1EvacuationFailureALotCount=2/10/100/1000/10000/100000
>> 
>> It saves "Remove Self Forwards" time all the time ,and in most condition it saves "Evacuate Collection Set" time. 
>> 
>> It brings some performance degradation when -XX:G1EvacuationFailureALotCount is low, such as *2*. To improve this a little, we can record the number evacuation failure object per region, and not record these objects when the number hit some limit. But I'm not sure if it's necessary to do so, as I think such condition is so extreme to be met in real environment, although I'm not quite sure.
>
> Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix compilation error on windows

The test based on new version (segmented array) shows that most (more than 90%) of iteration time is spent on "iterate_internal", compact cost almost no time, and less than 10% time is spent on sort.

And I also attach the perf data on the JBS bug, for "end to end" time/"pause young" time/"Evacuate Collection Set” time/"Post Evacuate Collection Set" time/"Remove Self Forwards" time. Generally I think the new implemention works well for G1EvacuationFailureALotCount == 1/2/..., not just for G1EvacuationFailureALotCount >= 10.

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PR: https://git.openjdk.java.net/jdk/pull/5181