RFR: 8307348 - Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v4]

[Aleksey Shipilev]

On Wed, 3 May 2023 12:16:13 GMT, olivergillespie <duke at openjdk.org> wrote:

>> ObjectCount(AfterGC) event does a full single-threaded heap scan at a safepoint. After https://bugs.openjdk.org/browse/JDK-8215624, it is trivial to use the parallel version of the heap scan, reducing the time spent at the safepoint, and thus reducing the overhead of this event.
>> 
>> The performance improvement is obvious, but just for confirmation, on my 16-core host, at around 1GB occupancy:
>> 
>> 
>> Before: 770ms ( [3.059s][debug][gc,phases         ] GC(13) Report Object Count 770.317ms )
>>  After:  92ms ( [2.335s][debug][gc,phases         ] GC(13) Report Object Count  91.742ms )
>> 
>> 
>> Question 1: Should this be the default behaviour for populate_table (use the number active workers as the parallelism, if nothing else specified)?
>> 
>> Question 2: Is active_workers the correct value to use here? Or is max_workers more appropriate?
>
> olivergillespie has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use ParallelGCThreads instead of active_workers

I looked if there might be a better option, like passing the `WorkerThreads*` from callers to actually figure out the number of active workers from the GC itself, but all of this cuts rather deep. I would say that should be done in a separate PR. `ParallelGCThreads` should work well meanwhile.

@albertnetymk, @tschatzl might have an opinion here.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13774#pullrequestreview-1413089971