RFR: 8299426: Heap dump does not contain virtual Thread stack references [v2]

[Alex Menkov]

On Thu, 30 Nov 2023 05:29:46 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> > There is a check for virtual thread liveness which has to be good enough in general:
> > ```
> > 1617   static bool should_dump_vthread(oop vt) {
> > 1618     return java_lang_VirtualThread::state(vt) != java_lang_VirtualThread::NEW
> > 1619         && java_lang_VirtualThread::state(vt) != java_lang_VirtualThread::TERMINATED;
> > 1620   }
> > . . .
> > 1919     if (java_lang_VirtualThread::is_instance(o) && ThreadDumper::should_dump_vthread(o)) {
> > 1920       _vthread_dumper->dump_vthread(o, writer());
> > 1921     }
> > ```
> 
> That should in general take care of most unreachable virtual threads, but technically I don't think a virtual thread has to reach the TERMINATED state in order to become unreachable. However, it will never get scheduled.

I'm not sure I understand the scenario. The state is set to TERMINATED after the thread completes its execution.
So a virtual thread was scheduled, mounted, did some work (as state != NEW) and then scheduler unmounts it and decides to not schedule it again and just "loses" it?
This does not look like a real scenario for me, but anyway I think that's fine to report such unreachable virtual threads until GC collects the objects.

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PR Comment: https://git.openjdk.org/jdk/pull/16665#issuecomment-1834534568