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Subject: Re: [OMPI users] Question about OpenMPI automatically spawns multiple threads for each task
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-05-17 14:34:18


Open MPI's OpenFabrics support will spawn up to two additional blocking threads (they wait for asynchronous verbs events of various flavors). They consume a few resources, but typically are not used much. They don't cause any noticeable change in performance.

On May 17, 2010, at 12:38 PM, Ping Luo wrote:

>
> Hello,
>
> I found when running am MPI program that is linked against OpenMPI library,
> for each MPI task, OpenMPI will spawn three threads, as the sample shown
> below:
>
>
> $ ps axms
> ...
> 13536 3565 0000000000000000 - - - - pts/14 0:00 mpirun -n 2 ./a.out
> 13536 - 0000000000000000 0000000000000000 0000000000000000 0000000180015ee2 S+ - 0:00 -
> 13536 3566 0000000000000000 - - - - pts/14 0:00 ./a.out
> 13536 - 0000000000000000 0000000000010000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> 13536 - 0000000000000000 0000000000000000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> 13536 - 0000000000000000 0000000000000000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> 13536 3567 0000000000000000 - - - - pts/14 0:00 ./a.out
> 13536 - 0000000000000000 0000000000010000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> 13536 - 0000000000000000 0000000000000000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> 13536 - 0000000000000000 0000000000000000 0000000000000006 00000001800104e0 SLl+ - 0:00 -
> ...
>
>
> Does OpenMPI always spawn three threads for each MPI task by default? Can the number
> of threads be changed? Why is it implemented this way? Is there any case that it will
> degrade performance because of over-subscription?
>
> Thanks,
>
> --
> Ping Luo
> 105 CSA, Building #517
> Texas A&M University
> Email: pingluo_at_[hidden]
> Phone: (979)862-3107
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-- 
Jeff Squyres
jsquyres_at_[hidden]
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