Version française
Home     About     Download     Resources     Contact us    

This site is updated infrequently. For up-to-date information, please visit the new OCaml website at

Browse thread
Multiprocessor support in OCaml
[ Home ] [ Index: by date | by threads ]
[ Search: ]

[ Message by date: previous | next ] [ Message in thread: previous | next ] [ Thread: previous | next ]
Date: 2007-04-22 (11:55)
From: Don Syme <Don.Syme@m...>
Subject: RE: [Caml-list] Multiprocessor support in OCaml

Just to mention there is a way of getting multiple concurrently executing OCaml threads in a program, which I discovered a while back: you can statically link multiple independent copies of the OCaml runtime, each into its own DLL (on Windows). This allows multiple independent OCaml threads to run concurrently.

I presume this technique works well enough for SMP up to 2-4 processors, though have never done any serious performance testing.

The OCaml programs must not, of course, trade OCaml values, but can communicate in-process by other means (e.g. shared C memory or some other message passing technique).


P.S. I've only used this technique on Windows.

-----Original Message-----
From: [] On Behalf Of Xavier Leroy
Sent: 22 April 2007 11:30
To: Jason Ganetsky
Subject: Re: [Caml-list] Multiprocessor support in OCaml

> 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

Caml-list mailing list. Subscription management:
Beginner's list:
Bug reports: