Re: [Caml-list] Multiprocessor support in OCaml

[Xavier Leroy <Xavier.Leroy@inria.fr>]

> Anyway, I have recently written an OCaml thread pool implementation, on
> top of the Thread and Event modules. I did this for the purpose of
> exploiting an SMP system I have, and was a disappointed to read today
> that OCaml doesn't support multiprocessor systems.

You are correct that OCaml *threads* do not exploit multiprocessing.
Basically, only one OCaml thread can run at a time.

You can still get parallelism in several ways.  First, external C
libraries called from OCaml can run in parallel with OCaml code
provided the OCaml/C interface for these libraries makes uses of the
"blocking section" mechanism.  Second, process-level parallelism works
very well with programs written in message-passing style, using e.g.
OcamlMPI or OCamlP3L.

> I played around with it a little, and discovered that by liberally
> calling Thread.yield, I do cajole my threads into running on multiple
> processors.

This is an illusion.  Thread.yield gives more opportunities to the OS
scheduler to reschedule a Caml thread on a different processor, but
you're not gaining parallelism this way and you might actually lose
performance (because of cache ping-pong effects and the like).

- Xavier Leroy