RFR: 8349763: Expose more ForkJoinPool parameters for virtual thread scheduler to reduce startup jitter and improve stability

[Jiawei Tang]

Since the parameters `-Djdk.virtualThreadScheduler.parallelism=N` , `-Djdk.virtualThreadScheduler.maxPoolSize=M`, `-Djdk.virtualThreadScheduler.minimumRunnable=Y` have already been made available, it would be worth considering opening up additional ForkJoinPool-related parameters: `-Djdk.virtualThreadScheduler.corePoolSize=X`, `-Djdk.virtualThreadScheduler.keepAliveTime=Z` and `-Djdk.virtualThreadScheduler.timeUnit`.

In particular, configuring corePoolSize can help reduce jitter caused by thread ramp-up during application startup, while keepAliveTime and timeUnit ensures more threads are available within the time expected by users. Opening these parameters would be highly meaningful for optimizing virtual thread scheduling.

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Commit messages:
 - 8349763: Expose more ForkJoinPool parameters for virtual thread scheduler to reduce startup jitter and improve stability

Changes: https://git.openjdk.org/jdk/pull/23549/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23549&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349763
  Stats: 27 lines in 1 file changed: 25 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23549.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23549/head:pull/23549

PR: https://git.openjdk.org/jdk/pull/23549