Integrated: 8282441: [LOOM] The debug agent should attempt to free vthread ThreadNodes

[Chris Plummer]

On Mon, 10 Nov 2025 03:15:06 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> This is first of what will probably be 3 PRs to improve virtual thread ThreadNode handling in the debug agent, with a goal of improving performance and reducing footprint.
> 
> This PR focuses on purging virtual thread ThreadNodes in two places:
> 
> 1. Freeing the ThreadNode after handling a THREAD_START event for a virtual thread.
> 2. Doing a "GC" of virtual thread ThreadNodes just before doing a "suspend all"
> 
> At some point in the future I want to attempt to free the ThreadNodes after performing ThreadReference related commands, which will result in the creation of a ThreadNode if not already created. Another area is in handleReportEventCompositeCommand() when the thread is not null and we are using the SUSPEND_NONE policy. This will get more ThreadNodes freed immediately after handling an event.
> 
> Part of the challenge with this PR is that there are many places in the debug agent that expect a ThreadNode to already have been created for the virtual thread, but now it is common that they have not. The main way this has been addressed is by having findRunningThread() create the ThreadNode if it does not already exist.
> 
> At some point in the future I will probably add logic to only "GC" ThreadNodes after the number reaches some threshold, but right now I want to free them very aggressively so we'll catch any bugs where there debug agent expects a ThreadNode to exist, but possibly now it might not.
> 
> Tested by running all tier2, tier5, and tier6 CI svc tests.

This pull request has now been integrated.

Changeset: 2b5cd14e
Author:    Chris Plummer <cjplummer at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/2b5cd14ea5c571e0d55df8293e49b9a0db521a46
Stats:     244 lines in 1 file changed: 187 ins; 39 del; 18 mod

8282441: [LOOM] The debug agent should attempt to free vthread ThreadNodes

Reviewed-by: amenkov, sspitsyn

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PR: https://git.openjdk.org/jdk/pull/28211