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Subject: Re: [OMPI users] Running OpenMPI on SGI Altix with 4096 cores : very poor performance
From: Gilbert Grosdidier (Gilbert.Grosdidier_at_[hidden])
Date: 2011-01-07 10:41:37


I'll very soon give a try to using Hyperthreading with our app,
and keep you posted about the improvements, if any.

  Our current cluster is made out of 4-core dual-socket Nehalem nodes.

  Cheers, Gilbert.

Le 7 janv. 11 à 16:17, Tim Prince a écrit :

> On 1/7/2011 6:49 AM, Jeff Squyres wrote:
>>
>> My understanding is that hyperthreading can only be activated/
>> deactivated at boot time -- once the core resources are allocated
>> to hyperthreads, they can't be changed while running.
>>
>> Whether disabling the hyperthreads or simply telling Linux not to
>> schedule on them makes a difference performance-wise remains to be
>> seen. I've never had the time to do a little benchmarking to
>> quantify the difference. If someone could rustle up a few cycles
>> (get it?) to test out what the real-world performance difference is
>> between disabling hyperthreading in the BIOS vs. telling Linux to
>> ignore the hyperthreads, that would be awesome. I'd love to see
>> such results.
>>
>> My personal guess is that the difference is in the noise. But
>> that's a guess.
>>
> Applications which depend on availability of full size instruction
> lookaside buffer would be candidates for better performance with
> hyperthreads completely disabled. Many HPC applications don't
> stress ITLB, but some do.
> Most of the important resources are allocated dynamically between
> threads, but the ITLB is an exception.
> We reported results of an investigation on Intel Nehalem 4-core
> hyperthreading where geometric mean performance of standard
> benchmarks for certain commercial applications was 2% better with
> hyperthreading disabled at boot time, compared with best 1 rank per
> core scheduling with hyperthreading enabled. Needless to say, the
> report wasn't popular with marketing. I haven't seen an equivalent
> investigation for the 6-core CPUs, where various strange performance
> effects have been noted, so, as Jeff said, the hyperthreading effect
> could be "in the noise."
>
>
> --
> Tim Prince
>
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> users_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/users

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   Gilbert Grosdidier                 Gilbert.Grosdidier_at_[hidden]
   LAL / IN2P3 / CNRS                 Phone : +33 1 6446 8909
   Faculté des Sciences, Bat. 200     Fax   : +33 1 6446 8546
   B.P. 34, F-91898 Orsay Cedex (FRANCE)
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