Hi Ludovic,
l...@gnu.org (Ludovic Courtès) writes:
> Mark H Weaver <m...@netris.org> skribis:
>
>> It only makes sense to use 'par-map' when the procedure is fairly
>> expensive to compute.
>
> Indeed.
>
>> There is inevitably a lot of overhead in creating and joining the
>> threads.
>
> We use a thread pool, so there’s no such cost.
Sorry, I was using the term 'threads' not in the sense of OS-level
threads, but in a more general sense. I should have been more clear.
What I meant is that from the user's perspective, threads are being
created and joined, and even if you build those using a pool of OS-level
threads, this inevitably involves thread synchronization, which is very
expensive on modern architectures. So I maintain that there _is_ such a
cost, and it can't be avoided.
The point I was really trying to make here, in the simplest possible
terms, is that it will *never* make sense to replace all uses of 'map'
with 'par-map' wherever it is safe to do so.
> But there are other costs. When delimited continuations are used, we’re
> on the slow path. Also, Guile’s fat mutexes & co. are terribly
> inefficient. And finally, there may be contention on the futexes mutex
> (esp. when the computations is too small.)
Indeed, I wouldn't be surprised if we could improve this by an order of
magnitude. More items for my TODO list :)
> So yes, there’s room for improvement. Yet, it should be fruitful,
> provided you use it for reasonably long computations, as Mark outlines.
Regards,
Mark
