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Date: Tue, 22 Nov 2011 00:15:45 +0100
From: magnum <>
Subject: OMP vs SSE (was: Cloud instances and Cisco 7)

2011-11-19 18:43, Solar Designer wrote:
> That said, latest -jumbo includes OpenMP parallelization for these
> hashes, which you may make use of.  This gives me 215k c/s on an
> 8-core (dual Xeon E5420 2.5 GHz).

What if that had been a single E5520 instead (with "4 cores, 8
threads"). Can that one run 8xSSE just like that? I have always assumed
(for no particular reason) that such a CPU would only have 4 SSE
"engines" but I'm not sure what to google to find out for sure. Maybe it
depends on CPU model? Or maybe this question doesn't even make sense?

If there is indeed such a limitation in HT, a couple more questions:

1. What happens if you overbook SSE? Really bad performance?
2. Is there a way to easily "detect" HT in our OMP init code, so we can
limit at the number of cores instead (I assume default OMP_NUM_THREADS
will be 8 for an E5520)?


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