[jdk26] Integrated: 8375130: [BACKOUT] Scalability issue when submitting virtual threads with almost empty tasks

[Alan Bateman]

On Tue, 13 Jan 2026 09:22:14 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> This PR is for the jdk26 branch, not main line.
> 
> The changes in [JDK-8360046](https://bugs.openjdk.org/browse/JDK-8360046) improve the scalability of virtual threads but introduce a subtle issue with signal filtering and a regression in some SPECjbb2015 runs. [pull/28797](https://git.openjdk.org/jdk/pull/28797) is in progress for main line to address these issues.
> 
> For the jdk26 branch, the proposal is to backout the changes to ForkJoinPool from JDK-8360046 but leave the small change to setParallelism to trigger worker activation and the small change to VirtualThread to submit(ForkJoinTask) consistently.  As part of the change, the acquire fence change to address [JDK-8372835](https://bugs.openjdk.org/browse/JDK-8372835) are proposed to be included, to avoid bringing back an issue that was fixed by JDK-8360046. So a somewhat complicated story due to jdk26 being in RDP1.
> 
> 
> Testing: test1-5, repeat testing of java/util/concurrent and java/lang/Thread/virtual tests.

This pull request has now been integrated.

Changeset: 283da4dd
Author:    Alan Bateman <alanb at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/283da4ddcbd7e0f46d996c698c9c7195c6545197
Stats:     467 lines in 2 files changed: 143 ins; 140 del; 184 mod

8375130: [BACKOUT] Scalability issue when submitting virtual threads with almost empty tasks

Reviewed-by: vklang

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PR: https://git.openjdk.org/jdk/pull/29187