Re: 回复：Virtual thread hang and all threads stop running on JDK21

[Alan Bateman]

On 05/06/2024 10:37, 何品(虎鸣) wrote:
> Thanks,
> 1. it hung when we are sharing the common usage of the Virtualthread 
> (the default scheduler) cross two modules.
> 2. after that, I try to hack the virtual thread builder with separated 
> ThreadPoolExecutor.
> 3. but still , it hung.
>
> when it hung, `jcmd Thread.print` prints nothing. and dump programly 
> print nothing too.
>
> After switch back to normal thread, it never hang.
> Some information:
> module A is using `Object.notifyAll, Object.wait` and module B is 
> using `CompletableFuture.get` (may > 100 times in one rune)
> I was thinking if that could be a problem of notification missing, 
> where in Module A, the concurrency is 3000， protected by a semaphore, 
> but the underling Carrier Thread is only 128, and 3000 > 128.

If it's using synchronized/Object.wait then this may be related to 
pinning. When there is both object monitors and j.u.concurrent locks in 
play then it's possible to create deadlock scenarios due to starvation, 
or selecting a successor or thread to wakeup and the thread can't 
continue because there are no carriers available. Object.wait will 
temporarily increase parallelism to smooth and help some cases but it 
may not help you here, and does nothing when the scheduler has been 
changed to be something other than a ForkJoinPool instance.

When you say "prints nothing" then you mean this literally or do you 
mean that jcmd is hung too? If so that's a hint that may be the lock for 
standard output is held by a virtual thread that can't continue because 
there are no carriers available.

It would be interesting to try the latest Loom EA builds which has 
changes to the object monitor implementation that doesn't pin. Would you 
have time to try these builds out?

-Alan

[1] https://jdk.java.net/loom/
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