RFR: 8307348 - Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v7]

[olivergillespie]

On Fri, 5 May 2023 15:54:47 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> olivergillespie has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix use of WithActiveWorkers
>>   
>>   My scope was incorrect, thanks @albertnetymk
>>   Also fix ResourceMark usage, thanks Aleksey.
>
> src/hotspot/share/gc/shared/gcVMOperations.cpp line 175:
> 
>> 173:     // Can't run with more threads than provided by the WorkerThreads.
>> 174:     const uint capped_parallel_thread_num = MIN2(_parallel_thread_num, workers->max_workers());
>> 175:     WithActiveWorkers with_active_workers(workers, capped_parallel_thread_num);
> 
> I would *almost* rely on the workgang's 'active_workers()' here because all collectors set this proportional to heap size (people tend to start 128M VMs on hundreds-of-thread machines... ). The problem is that one heap inspection VM operation. Maybe put the `WithActiveWorkers` there and use `workers->active_workers()` here?
> 
> This is just a suggestion.

Sorry, I don't think I understand the suggestion. Are you suggesting to use WithActiveWorkers only for the heap inspection VM op, and workers->active_workers() everywhere else (for the collector use-case)?

> The problem is that one heap inspection VM operation. Maybe put the WithActiveWorkers there

This area *is* that one heap inspection VM operation, isn't it?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13774#discussion_r1186278692