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From: Goswin von Brederlow <goswin-v-b@web.de>
To: Jon Harrop <jonathandeanharrop@googlemail.com>
Cc: "'Goswin von Brederlow'" <goswin-v-b@web.de>,
	<caml-list@yquem.inria.fr>
Subject: Re: [Caml-list] SMP multithreading
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Date: Sat, 20 Nov 2010 21:57:52 +0100
In-Reply-To: <04be01cb88a9$d2df4810$789dd830$@com> (Jon Harrop's message of
	"Sat, 20 Nov 2010 11:55:26 -0000")
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X-Spam: no; 0.00; ocaml:01 parallelism:01 cheers:01 parallelism:01 ocaml:01 bigarray:01 mfg:98 threads:01 rewrite:01 caml-list:01 writes:01 data:02 data:02 restrict:02 string:02 

Jon Harrop <jonathandeanharrop@googlemail.com> writes:

>> This is actually a quick way to use multiple cores with ocaml. Find a
>> often called function that takes considerable time and offload it to C
>
> Or HLVM, F#, Scala, Clojure or any of the other languages that permit shared
> memory parallelism. C is particularly poor in this regard so I would not
> just restrict yourself to C...
>
> Cheers,
> Jon.

I'm not talking about any shared memory parallelism here. The
parallelism is completly restricted to the ocaml side. You just find
some single threaded job that takes long, rewrite it as external
function and release the ocaml lock while it is running.

For example in my code I compute the sha256 sum of a block of
data. Since I use a C library for sha256 anyway the function is already
external. All I had to do was switch the interface from using string to
Bigarray and add enter/leave_blocking_section(). After that multiple
threads can compute the sha256 sum for blocks of data in parallel and my
code run 3.7 times faster with 4 cores.

MfG
        Goswin