RFR: 8231672: Simplify the reference processing parallelization framework

[Leo Korinth]

On Thu, 4 Mar 2021 09:31:50 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> With the change of parallel gc to use the same workgang implementation that other gcs uses, it is possible to remove the abstraction used to hide different worker thread management implementations (TaskExecutor).
>> 
>> At the same time as removing the TaskExecutor, also remove part of the special casing (mostly code duplication), for single threaded execution. 
>> 
>> I consider the new method `prepare_run_task` that modifies the state of the context a step backwards from what was. However, I think removing the Executor with its proxy-tasks and removing separate code paths for serial execution makes the change worth this step back. We also removes ~270 lines of code.
>> 
>> Some comments:
>> 1) I use references in some places where they can replace pointers. I could go much further here, but I did not want to bloat the pull request, maybe later change all closures to be references as well? Should I create enhancements for this?
>> 
>> 2) I added an enum class ThreadModel instead of using a boolean, this could also be used in many more places. I dislike sending lots of bools with a comment like `true /* _is_mt */`. It also adds type safety if a method takes many bools. However, with this limited change, and not many hard-coded bools, it feels a bit overkill and I am willing to remove the enum, what do you think?
>> 
>> Testing:
>> hotspot_gc  and tier 1-3 has passed earlier versions before minor cleanups. I intend to re-run tests after review changes.
>
> src/hotspot/share/gc/parallel/psParallelCompact.cpp line 2098:
> 
>> 2096: };
>> 2097: 
>> 2098: class PCRefProcClosureContext : public AbstractClosureContext {
> 
> The abbreviation used for Parallel Compact is `ParallelCompact` or `ParCompact[ion]` or `PSParallelCompact` otherwise afaics. Please do not add even more of those (yeah, there is already `PCAddThreadRootsMarkingTaskClosure`)...

I will rename PC -> ParallelCompact. I will ignore the historical roots of the "mirror" class names PCRefProcTask, RefProcTaskExecutor.

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PR: https://git.openjdk.java.net/jdk/pull/2782