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Subject: Re: [OMPI users] Hybrid program
From: Ralph Castain (rhc_at_[hidden])
Date: 2008-11-18 11:46:51


Unfortunately, paffinity doesn't know anything about assigning threads
to cores. This is actually a behavior of Linux, which only allows
paffinity to be set at the process level. So, when you set paffinity
on a process, you bind all threads of that process to the specified
core(s). You cannot specify that a thread be given a specific core.

In this case, your two threads/process are sharing the same core and
thus contending for it. As you'd expect in that situation, one thread
gets the vast majority of the attention, while the other thread is
mostly idle.

If you can upgrade to the beta 1.3 release, try using the slot mapping
to assign multiple cores to each process. This will ensure that the
threads for that process have exclusive access to those cores, but
will not bind a particular thread to one core - the threads can "move
around" across the specified set of cores. Your threads will then be
able to run without interfering with each other.

Ralph

On Nov 18, 2008, at 9:18 AM, Gabriele Fatigati wrote:

> Dear OpenMPI developers,
> i have a strange problem with mixed program MPI+OPENMP over OpenMPI
> 1.2.6. I'm using PJL TASK GEOMETRY in LSF Scheduler, setting 2 MPI
> process every compute node, and 2 OMP threads per process. Using
> paffinity and maffinity, i've noted that over every node, i have 2
> thread that works 100%, and 2 threads doesn't works, or works very
> few.
>
> If i disable paffinity and maffinity, 4 threads works well, without
> load imbalance.
> I don't understand this issue: paffinity and maffinity should map
> every thread over a specific core, optimizing the cache flow, but i
> have this without settings there!
>
> Can i use paffinity and maffinity in mixed MPI+OpenMP program? Or it
> works only over MPI thread?
>
> Thanks in advance.
>
>
> --
> Ing. Gabriele Fatigati
>
> CINECA Systems & Tecnologies Department
>
> Supercomputing Group
>
> Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy
>
> www.cineca.it Tel: +39 051 6171722
>
> g.fatigati_at_[hidden]
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