RFR: 8339097: Parallel: Compact GC to split array early for task stealing

[Zhengyu Gu]

On Thu, 29 Aug 2024 19:07:01 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Parallel Compact GC splits a large array in stripes during marking, so that other workers can steal the work.
>> 
>> Currently, it only splits a large array into two tasks, retains and pushes remaining into a task queue for task stealing, depends on next worker to further split the array if possible, that creates artificial dependency.
>> 
>> I would like purpose to have the first worker breaking up the array, to eliminate the dependency.
>
> src/hotspot/share/gc/parallel/psCompactionManager.inline.hpp line 129:
> 
>> 127:     while (end_index < len) {
>> 128:       cm->push_objarray(obj, end_index);
>> 129:       end_index += (size_t)ObjArrayMarkingStride;
> 
> Don't make this change.  PSCM should instead be changed to use PartialArrayTaskStepper and
> PartialArrayState, with associated elimination of the separate ObjArrayTask queue.

Okay, eliminating `ObjArrayTask` queue sounds great. I wonder how hard to backport to 17u, because I am really targeting the change to 17u.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20745#discussion_r1737039810