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

[Viktor Klang]

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 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.

Thanks Alan!

-------------

Marked as reviewed by vklang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29187#pullrequestreview-3656531482