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On 04/10/2022 15:15, Jonas Schlecht wrote:<br>
<blockquote type="cite" cite="mid:354D433A-0A76-4C1D-97CC-447CF8E12827@hxcore.ol">
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<p class="MsoNormal">Hi everybody,<br>
<br>
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. <br>
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? Do they
even do that? 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.
After all, the ForkJoinPool needs ForkJoinTasks to use the
work-stealing logic. Or am I mistaken here? </p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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. <br>
</p>
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</blockquote>
<br>
For starters, think of ForkJoinPool as a "better thread pool". It
has many advantages over a thread pool that uses a shared blocking
queue for all tasks.<br>
<br>
Another thing is a ForkJoinPool can be created in "async mode" which
is local FIFO scheduling. This is good for applications doing
message passing and also good for scheduling the tasks for virtual
threads.<br>
<br>
As others have pointed out, scheduling a virtual thread to execute
causes a special task for the thread to be pushed to one of the FJP
submission queues. A virtual thread T1 unparking virtual thread T2
will push the task for T2 to the submission queue of T1's carrier
(worker thread). It may be that T2's task is executed by the that
worker thread or it may be that some worker thread steals the task.<br>
<br>
There are a few other features of ForkJoinPool that you might want
to look into. One is that it parallelism can be dynamically changed
when workers are blocked - you'll see this is used to smooth over
cases where virtual threads temporarily pin their carrier during
file I/O operations. Another recent addition to ForkJoinPool is the
ability to submit a task without signalling, look for "lazySubmit".
This is used to reduce steals in very specific cases such as when a
thread is unparked while parking.<br>
<br>
-Alan<br>
<br>
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