Question Regarding JEP 444: Virtual Threads and Use of ForkJoinPool

[David Holmes]

On 14/06/2023 10:31 am, Bazlur Rahman wrote:
> My understanding is that the ForkJoinPool usually adopts a 
> divide-and-conquer approach, breaking a larger task into several smaller 
> subtasks. In this context, the task is essentially a virtual thread. I 
> am interested to know how the divide-and-conquer algorithm plays out 
> when the ForkJoinPool tries to break down a virtual thread into smaller 
> units.

It isn't the FJP that does that, it is the use of ForkJoinTasks and the 
associated APIs by the programmer that does that.

Cheers,
David
-----

> *Thank you,*
> *-*
> *A N M Bazlur Rahman*
> 
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> 
> On Tue, Jun 13, 2023 at 8:13 PM David Holmes <david.holmes at oracle.com 
> <mailto:david.holmes at oracle.com>> wrote:
> 
>     Hi,
> 
>     On 14/06/2023 9:50 am, Bazlur Rahman wrote:
>      > Hello,
>      >
>      > I was reading JEP 444, which discusses the implementation of Virtual
>      > Threads. I've observed that the JDK's virtual thread scheduler
>     employs a
>      > work-stealing ForkJoinPool operating in FIFO mode, which is distinct
>      > from the common pool operating in LIFO mode.
>      >
>      > This has sparked my curiosity because ForkJoinPool, to my
>     understanding,
>      > has not traditionally been designed to handle I/O-bound tasks.
>     However,
>      > the design of virtual threads seems to be aimed precisely at
>     addressing
>      > such tasks.
> 
>     Yes and it is the virtual threads mechanism that handles the I/O-bound
>     tasks, not the underlying FJP. Where a regular thread in the FJP would
>     block on an I/O operation and become unavailable for further use, it is
>     the virtual thread that blocks, allowing the carrier thread in the FJP
>     to continue executing other virtual threads.
> 
>     Cheers,
>     David
>     -----
> 
>      > Therefore, I'm curious as to why Java has opted to use the
>     ForkJoinPool
>      > for scheduling virtual threads. I would greatly appreciate it if you
>      > could shed some light on what makes this design choice effective in
>      > managing I/O-bound tasks, despite the ForkJoinPool's traditional
>     usage
>      > patterns.
>      >
>      > Thank you for your time and consideration. I look forward to your
>      > insights on this subject.
>      > *Thank you,*
>      >
>      > *-*
>      > *A N M Bazlur Rahman*
>      >
>      > ---
>      > /"And say: 'My Lord, increase me in knowledge.'" - Quran 20:114./
>      >
>      > *Java Champion*
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