RFR: 8307348 - Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v6]

[Aleksey Shipilev]

On Fri, 5 May 2023 11:55:14 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:
> 
>   Fix imports

This looks better to me. Some comments:

src/hotspot/share/memory/heapInspection.cpp line 569:

> 567: uintx HeapInspection::populate_table(KlassInfoTable* cit, BoolObjectClosure *filter, WorkerThreads* workers) {
> 568:   // Try parallel first.
> 569:   ResourceMark rm;

I think this `ResourceMark` should remain at both blocks. So that on failed exit from the parallel block, we rollback resource allocations and start over with serial walk.

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PR Review: https://git.openjdk.org/jdk/pull/13774#pullrequestreview-1414674820
PR Review Comment: https://git.openjdk.org/jdk/pull/13774#discussion_r1186021364