RFR: 8360046: Scalability issue when submitting virtual threads with almost empty tasks [v23]

[Doug Lea]

> This set of updates reduces contention-based performance loss under heavy over-subscription, while also improving perfomance more generally.

Doug Lea has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 35 additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8360046
 - Cleanup
 - For testing
 - Merge branch 'openjdk:master' into JDK-8360046
 - restore positioning of threadlocal clears
 - Add comment
 - Merge branch 'openjdk:master' into JDK-8360046
 - Signal on setParallelism
 - Drop added Contended region; simplify accordingly
 - Tweaks
 - ... and 25 more: https://git.openjdk.org/jdk/compare/64b74bd0...654c1dc6

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26479/files
  - new: https://git.openjdk.org/jdk/pull/26479/files/0db798b5..654c1dc6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26479&range=22
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26479&range=21-22

  Stats: 9533 lines in 293 files changed: 6653 ins; 1426 del; 1454 mod
  Patch: https://git.openjdk.org/jdk/pull/26479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26479/head:pull/26479

PR: https://git.openjdk.org/jdk/pull/26479