preemptive vs cooperative: attempt at formalization

[Simon Marlow]

On 12 April 2006 11:41, Malcolm Wallace wrote:

> John Meacham <john at repetae.net> wrote:
> 
>> In a concurrent implementation, a thread performing an infinite loop
>> with no IO or interaction with the outside world can potentially
>> stall switching to another thread forever, in FP, we usually denote
>> an infinite loop by _|_. so I think the first difference would be:
> 
> By infinite loop, you mean both non-terminating, and non-productive. 
> A non-terminating but productive pure computation (e.g. ones =
> 1:ones) is not necessarily a problem.

That's slightly odd terminology.  ones = 1:ones  is definitely
terminating.  (length ones) is not, though. 

>  Why?  Because ultimately the
> demand that forces production of the infinite data structure must
> come from somewhere, and that somewhere must essentially be I/O. 
> (The only alternative consumers are terminating pure (no problem),
> non-terminating productive pure (so look upward to its demand), or an
> unproductive non-terminating computation, which brings us full
> circle.) 
> 
> It is a curious artifact that in certain multi-threaded
> implementations, a non-terminating non-productive thread does not
> make the entire system unproductive,

Wow.  I wish I knew what that meant :)

Maybe you could expand on in what sense you mean non-terminating, and
what "productive" means?  Is there something we need to worry about
here?

Cheers,
	Simon