[jdk16] RFR: 8258985: Parallel WeakProcessor may use too few threads

[Kim Barrett]

Please review this fix to the parallel WeakProcessor's computation of the
number of worker threads to use.  It was previously limited by the current
value of active_workers(), whatever that happens to be.  It should be
limited by total_workers(), just as with the parallel ReferenceProcessor.
(Both are subject to ReferencesPerThread.)

Testing
mach5 tier1
Some hand testing (Linux-x64) to verify the expected number of threads are
being used.

Note: That hand testing suggests some further tuning of ReferencesPerThread
might be in order. With the current default of 1000, I often saw in testing
that some threads were started late enough that no work was left for them.
I'll file a separate RFE for that.

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Commit messages:
 - Use total workers rather than active

Changes: https://git.openjdk.java.net/jdk16/pull/75/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=75&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258985
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/75.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/75/head:pull/75

PR: https://git.openjdk.java.net/jdk16/pull/75