Virtual Thread Scheduler - why ForkJoinPool?

[Florian Schmaus]

On 04.10.22 16:15, Jonas Schlecht wrote:
> Hi everybody,

Hi Jonas,

> I am currently writing my thesis which, for some part, also covers 
> Virtual Threads. So far, I get how everything works but I don’t 
> understand how the work-stealing nature of the ForkJoinPool is used by 
> Virtual Threads.
> I know the benefits of a work-stealing scheduler and how the 
> ForkJoinPool uses ForkJoinTasks. But how do Virtual Threads „fork“ tasks 
> which can be stolen by other threads?

Basically, VirtualThread.submitRunContination() [1] submits the virtual 
thread's continuation into the fork/join pool. This typically happens 
when a virtual thread is unparked, e.g., when it became runnable again 
after it was blocked on a synchronization primitive.

There are potentially more cases where the runtime systems puts a 
virtual thread (or a continuation of thereof) into the fork/join pool. I 
am sure the Loom experts can fill those in.


> As far as I 
> understand it, the same scheduling result could be achieved by using any 
> other thread pool with the amount of available CPU cores as the number 
> of threads.

That appears to be re-inventing the wheel. :)


> After all, the ForkJoinPool needs ForkJoinTasks to use the 
> work-stealing logic. Or am I mistaken here?

You are right: A ForkJoinPool would like to be feed with ForkJoinTasks. 
But a ForkJoinTask can easily be created from a Runnable via 
ForkJoinTask.adapt(Runnable).


> Could you maybe point me to some ressources that explain why you decided 
> to use the ForkJoinPool and how it is used? I couldn’t find any online.

Work-stealing scheduling is a perfect fit for concurrency platforms like 
Loom, and ForkJoinPool already implemented work-stealing. But I was not 
the one who decided this, so please do not consider this an 
authoritative answer.

I hope that I could help and point you into the right direction.

Best Wishes
  Florian

1: 
https://github.com/openjdk/loom/blob/3afb61b8fcc150595d6e5d2f7981b99dacf3b5be/src/java.base/share/classes/java/lang/VirtualThread.java#L233