RFR: 8345732: Provide helpers for using PartialArrayState [v2]

[Thomas Schatzl]

On Fri, 13 Dec 2024 01:29:13 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change that introduces two new helper classes to simplify
>> the usage of PartialArrayStates to manage splitting the processing of large
>> object arrays into parallelizable chunks.  G1 and Parallel young GCs are
>> changed to use this new mechanism.
>> 
>> PartialArrayTaskStats is used to collect and report statistics related to
>> array splitting.  It replaces the direct implementation in PSPromotionManager,
>> and is now also used by G1 young GCs.
>> 
>> PartialArraySplitter packages up most of the work involved in splitting and
>> processing tasks. It provides task allocation and release, enqueuing, chunk
>> claiming, and statistics tracking. It does this by encapsulating existing
>> objects and functionality. Using array splitting is mostly reduced to calling
>> the splitter's start function and then calling it's step function to process
>> partial states. This substantially reduces the amount of code for each client
>> to perform this work.
>> 
>> Testing: mach5 tier1-5
>> 
>> Manually ran some test programs with each of G1 and Parallel, with taskqueue
>> stats logging enabled, and checked that the logged statistics looked okay.
>
> Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into pa-splitter
>  - parallel uses PartialArraySplitter
>  - g1 uses PartialArraySplitter
>  - add PartialArraySplitter
>  - add PartialArrayTaskStats

Apart from these comments not being in the right place, seems good.

src/hotspot/share/gc/shared/partialArraySplitter.inline.hpp line 45:

> 43:   PartialArrayTaskStepper::Step step = _stepper.start(length);
> 44:   // Push any needed partial scan tasks.  Pushed before processing the initial
> 45:   // chunk to allow other workers to steal while we're processing.

This comment (last two lines) now imo better belongs to where this method is called. Same with similar comment in `step()`.

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PR Review: https://git.openjdk.org/jdk/pull/22622#pullrequestreview-2501736071
PR Review Comment: https://git.openjdk.org/jdk/pull/22622#discussion_r1883567632