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Subject: Re: [OMPI users] 2 to 1 oversubscription
From: Mark Borgerding (markb_at_[hidden])
Date: 2009-07-13 13:24:54


Here's my advice: Don't trust anyones advice. Benchmark it yourself and
see.

The problems vary so wildly that only you can tell if your problem will
benefit from oversubscription. It really depends on too many factors to
accurately predict: schedulers, memory usage, network/interconnect
hardware, disk seek times, and probably a hundred other things.

I've even seen mixed results from oversubscribing within a single
algorithm. (Granted this is mostly with the older generation
hyperthreading, so I'm not sure how things fare with nehalem). The most
notable effect I've observed is related to cache use. If the problem
fits in cache it is much faster. With cores sharing cache it can even
be advantageous to *undersubscribe* the problem. i.e. schedule 2
processes on a quad core so each can have the full cache.

-- Mark Borgerding

Klymak Jody wrote:
> 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
>>
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
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-- 
Mark Borgerding
3dB Labs, Inc
Innovate.  Develop.  Deliver.