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Date: Wed, 30 Jun 2010 14:38:23 +0000 (UTC)
From: Georg Wittig <>
Subject: Re: OpenMP vs lots of cores

Am Wed, 30 Jun 2010 09:55:42 +0200 schrieb Magnum, P.I.:

> I haven't had the time to try the OpenMP stuff but I sure will. How far
> will it scale in the current versions? I mean, what if you had 96 cores
> on a single host? It won't use all of them, right? Or would it, given
> enough what? Hashes? Salts? I suppose even if it can use all of them,
> the overhead will at some point make it ineffective, perhaps even just
> slow it down.
> Obviously though, MPI and MP could be used together. Say we have 96
> cores and fire off 12 MPI nodes using 8 MP threads each. Naturally some
> testing would be needed to find the best mix.

Under Fedora 13 it uses all CPUs by default. You can limit the number of 
CPUs by setting the environment variable OMP_NUM_THREADS before calling 
john. More details: "info libgomp"

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