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Subject: Re: [OMPI users] 2 to 1 oversubscription
From: Klymak Jody (jklymak_at_[hidden])
Date: 2009-07-11 18:20:34


Hi Robert,

I ran some very crude tests and found that things slowed down once you
got over 8 cores at a time. However, they didn't slow down by 50% if
you went to 16 processes. Sadly, the tests were so crude, I did not
keep good notes (it appears).

I'm running a gcm, so my benchmarks may not be very useful to most
folks. If there was an easy-to-compile benhmark that I could run on
my cluster, I'd be curious what the results are too.

Thanks, Jody

On 11-Jul-09, at 2:16 PM, Robert Kubrick wrote:

> The Open MPI FAQ recommends not to oversubscribe the available cores
> for best performances, but is this still true? The new Nehalem
> processors are built to run 2 threads on each core. On a 8 sockets
> systems, that sums up to 128 threads that Intel claims can be run
> without significant performance degradation. I guess the last word
> is to those who have tried to run some benchmarks and applications
> on the new Intel processors. Any experience to share?
>
> http://www.open-mpi.org/faq/?category=running#oversubscribing
> http://en.wikipedia.org/wiki/Simultaneous_multithreading
> http://communities.intel.com/community/openportit/server/blog/2009/06/11/nehalem-ex-brings-new-economics-to-scalable-systems
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