Cooperative vs preemtive scheduling of virtual theads

[Ron Pressler]

Preemptive scheduling is often mistakenly used to refer to time-sharing [1].
When execution can return to the scheduler without user code being aware of
it in any way, scheduling is preemptive.

Cooperative scheduling [2] is when code is required to explicitly yield
control back to the scheduler, like in the old 16-bit Windows or with an 
`await` keyword in some programming languages.

[1]: https://en.wikipedia.org/wiki/Preemption_(computing)#PREEMPTIVE

[2]: https://en.wikipedia.org/wiki/Cooperative_multitasking



On 8 June 2020 at 10:06:23, Andrew Haley (aph at redhat.com(mailto:aph at redhat.com)) wrote:

> On 06/06/2020 16:59, Alan Bateman wrote:
> > Virtual threads are pre-emptive, the scheduling points are locking or
> > blocking I/O operations. It's not cooperative in the sense that there
> > isn't the equivalent of an "await" in other languages.
>  
> Just a terminology nitpick, for clarity: every co-operative scheduler
> I ever used switched tasks at locks and blocking I/O operations. I
> believe this is standard terminology.
>  
> One of the problems with this way of thinking, though, is that beneath
> abstraction layers client code can't tell what might block. For
> example, any call might be to code that loads new classes, thus
> blocking. So, in principle, any call site can switch tasks.
>  
> --
> Andrew Haley (he/him)
> Java Platform Lead Engineer
> Red Hat UK Ltd.  
> https://keybase.io/andrewhaley
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>