From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: david.baelde@ens-lyon.org
Cc: Goswin von Brederlow <goswin-v-b@web.de>,
Savonet's developpers list <savonet-devl@lists.sourceforge.net>,
Caml Mailing List <caml-list@yquem.inria.fr>
Subject: Re: [Caml-list] Re: Random segfaults / out of memory
Date: Tue, 30 Mar 2010 17:57:10 +0200 [thread overview]
Message-ID: <4BB21F56.9070802@inria.fr> (raw)
In-Reply-To: <53c655921003300014s2c7b05eax24b91edbb0d0d9ec@mail.gmail.com>
> So, is it really forbidden to release the global lock in a noalloc function?
Yes. Actually, it is forbidden to call any function of the OCaml
runtime system from a noalloc function.
Explanation: ocamlopt-generated code caches in registers some global
variables of importance to the OCaml runtime system, such as the
current allocation pointer.
When calling a regular (no-"noalloc") C function from OCaml, these
global variables are updated with the cached values so that everything
goes well if the C function allocates, triggers a GC, or releases the
global lock (enabling a context switch).
This updating is skipped when the C function has been declared
"noalloc" -- this is why calls to "noalloc" functions are slightly
faster. The downside is that the runtime system is not in a
functioning state while within a "noalloc" C function, and must
therefore not be invoked.
The cost of updating global variables is small, so "noalloc" makes
sense only for short-running C functions (say, < 100 instructions) like
those from the math library (sin, cos, etc). If the C function makes
significant work (1000 instructions or more), just play it safe and
don't declare it "noalloc".
Hope this helps,
- Xavier Leroy
next prev parent reply other threads:[~2010-03-30 15:57 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-17 8:27 Rewriting the Digest module causes linking errors Goswin von Brederlow
2010-03-17 8:39 ` [Caml-list] " Mark Shinwell
2010-03-17 9:53 ` Goswin von Brederlow
2010-03-17 16:39 ` Random segfaults / out of memory [Was: Re: [Caml-list] Rewriting the Digest module causes linking errors] Goswin von Brederlow
2010-03-18 10:56 ` Random segfaults / out of memory Goswin von Brederlow
2010-03-30 7:14 ` [Caml-list] " David Baelde
2010-03-30 15:57 ` Xavier Leroy [this message]
2010-03-30 16:19 ` Markus Mottl
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