RFR: 8307348 - Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v3]

[olivergillespie]

> 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 compile error
  
  ```
   === Output from failing command(s) repeated here ===
  * For target hotspot_variant-server_libjvm_objs_gcTrace.o:
  /home/runner/work/jdk/jdk/src/hotspot/share/gc/shared/gcTrace.cpp: In member function 'void GCTracer::report_object_count_after_gc(BoolObjectClosure*)':
  /home/runner/work/jdk/jdk/src/hotspot/share/gc/shared/gcTrace.cpp:114:48: error: invalid use of incomplete type 'class CollectedHeap'
    114 |       WorkerThreads* workers = Universe::heap()->safepoint_workers();
        |                                                ^~
  In file included from /home/runner/work/jdk/jdk/src/hotspot/share/gc/shared/gcTrace.cpp:35:
  /home/runner/work/jdk/jdk/src/hotspot/share/memory/universe.hpp:42:7: note: forward declaration of 'class CollectedHeap'
     42 | class CollectedHeap;
        |       ^~~~~~~~~~~~~
  
  * All command lines available in /home/runner/work/jdk/jdk/build/linux-x64/make-support/failure-logs.
  === End of repeated output ===
  ```

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13774/files
  - new: https://git.openjdk.org/jdk/pull/13774/files/88eb1ede..22b9b6d5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13774&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13774&range=01-02

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13774.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13774/head:pull/13774

PR: https://git.openjdk.org/jdk/pull/13774