john-users - Re: OpenMP not using all threads

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Date: Mon, 21 Nov 2011 03:09:23 +0400
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: OpenMP not using all threads

On Sun, Nov 20, 2011 at 03:21:16PM -0700, Stephen John Smoogen wrote:
> Most (if not all) Linode systems are virtualized systems

Yes.

> which I believe normally don't do well with hyperthreading.

I think you're wrong about this.

What I think Linode actually does here is limit the customer's use of
CPU time through giving them only 4 out of probably 16 logical CPUs (in
a dual quad-core Xeon machine).  Due to hyperthreading, these 4 actually
let the customer use up to approximately 40% of the 8 physical cores'
performance.  If they gave the customer 8 logical CPUs, this would allow
this single customer to almost fully (85% or so) use _both_ Xeon CPUs
installed in the machine (as long as there's otherwise-idle CPU time),
which Linode might not want to offer (or not at the same price).

> For high compute
> virtualization you usually need systems with cpu-pinning and such to
> get the performance that you get from bare metal.

CPU pinning is not that important for these machines and software.

Alexander