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Re: [Caml-list] native threads not parallel?
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Date: -- (:)
From: shivkumar chandrasekaran <shiv@e...>
Subject: Re: [Caml-list] native threads not parallel?
On Thu, 2003-02-20 at 16:15, Jacques Garrigue wrote:
> From: Shivkumar Chandrasekaran <>
> > not. But I seem to have the same problem as discussed by Markus Mottl 
> > (see attachment below) on Mac OS X 10.2.4 on a dual processor G4 
> > machine. An earlier message in that thread mentioned that there was no 
> > such problem on dual-processor Linux machines.
> Let's answer again, since this is a frequent question: ocaml threads
> do not not support parrallelism. Only one ocaml thread will run at a
> time, independently of the number of processors, and this on any
> architecture/OS.

Sorry, but let me ask again. I *know* that ocaml threads cannot use
multiple processors. That was not the subject of the thread I cited. I
should have been clearer.

If I am recalling correctly, Xavier has mentioned before that in
*native-code* (see subject) ocaml will allow C code to run in parallel.
Markus' email was precisely on that point as was mine. I have C code
that I would like to execute on a processor different from the ocaml
thread one. Apparently, as I gather from the cited email of Markus
Mottl, this did occur (at least on some dual processor Linux machines)
when the corresponding C code was bracketed with "enter/leaving_blocking
section ()" calls, and, *I assume*, calling the C-code from a separate
ocaml thread using Thread.create.

I understand the dangers about calling back into caml from the bracketed
C-code. The preceding thread I cited discussed this as well, as you



> > > From :Markus Mottl < >
> > > The only thing I can imagine is that OCaml somehow changes
> > > scheduling so that the threads cannot run in parallel anymore, even if
> > > "enter_blocking_section" is used. Is this possible? Or am I just making
> > > some stupid mistake?
> enter_blocking_section only happens in C code, and is followed by a
> leave_blocking_section before returning to caml code. Since the
> leave_blocking_section reacquires the lock, no more than one ocaml
> thread can run simultaneously.
> Easy way to crash the runtime: call a caml callback immediately after
> an enter_blocking_section.  The two caml threads will start to
> allocate simultaneously on the same heap, without any locking...
> Jacques Garrigue
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