RFR: 8360046: Scalability issue when submitting virtual threads with almost empty tasks [v21]

[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 32 additional commits since the last revision:

 - 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
 - Merge branch 'openjdk:master' into JDK-8360046
 - Seperate repolling policies
 - Faster quiescent termination
 - ... and 22 more: https://git.openjdk.org/jdk/compare/875ef400...1de99906

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26479/files
  - new: https://git.openjdk.org/jdk/pull/26479/files/9f130030..1de99906

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26479&range=20
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26479&range=19-20

  Stats: 5674 lines in 149 files changed: 3285 ins; 1418 del; 971 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