RFR: 8371380: [LOOM] The debug agent should avoid enabling VIRTUAL_THREAD_START/END events when possible

[Alex Menkov]

On Tue, 25 Nov 2025 02:25:02 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> Don't enabled VIRTUAL_THREAD_START/END events unless absolutely necessary. Solves performance issues when trying to debug apps that create a lot of virtual threads. Details in first comment.
> 
> With these changes the Skynet benchmark no longer shows any slowdown when launching with debugging enabled or when attaching the debugger.
> 
> Tested with all tier2, tier3, tier5, and tier6 CI testing (with filters to only run svc tests).

src/jdk.jdwp.agent/share/native/libjdwp/eventFilter.c line 1296:

> 1294: {
> 1295:     jvmtiError error = JVMTI_ERROR_NONE;
> 1296:     int ei = NODE_EI(node);

Suggestion:

    EventIndex ei = NODE_EI(node);

src/jdk.jdwp.agent/share/native/libjdwp/eventFilter.c line 1394:

> 1392:     jvmtiError error2 = JVMTI_ERROR_NONE;
> 1393:     jthread thread;
> 1394:     int ei = NODE_EI(node);

Suggestion:

    EventIndex ei = NODE_EI(node);

src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c line 1691:

> 1689:      * a deferred event request.
> 1690:      */
> 1691:     if (gdata->virtualThreadStartEventsPermanentlyEnabled) {

Looks like this block can disable VITRUAL_THREAD_START when `rememberVThreadsWhenDisconnected` is set.

src/jdk.jdwp.agent/share/native/libjdwp/util.h line 90:

> 88:     jboolean includeVThreads;   /* If true, VM.AllThreads includes vthreads. */
> 89:     jboolean rememberVThreadsWhenDisconnected;
> 90:     jboolean virtualThreadStartEventsPermanentlyEnabled;

Could you please add description for this 2 flags (and maybe `vthreadsSupported` description also can be enhanced to see relations between the flags)

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PR Review Comment: https://git.openjdk.org/jdk/pull/28485#discussion_r2561596843
PR Review Comment: https://git.openjdk.org/jdk/pull/28485#discussion_r2561664461
PR Review Comment: https://git.openjdk.org/jdk/pull/28485#discussion_r2566654961
PR Review Comment: https://git.openjdk.org/jdk/pull/28485#discussion_r2566662313