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Subject: Re: [OMPI devel] poor btl sm latency
From: Ralph Castain (rhc.openmpi_at_[hidden])
Date: 2012-02-13 10:22:47


Few thoughts

1. Bind to socket is broken in 1.5.4 - fixed in next release

2. Add --report-bindings to cmd line and see where it thinks the procs are bound

3. Sounds lime memory may not be local - might be worth checking mem binding.

Sent from my iPad

On Feb 13, 2012, at 7:07 AM, Matthias Jurenz <matthias.jurenz_at_[hidden]> wrote:

> Hi Sylvain,
>
> thanks for the quick response!
>
> Here some results with enabled process binding. I hope I used the parameters
> correctly...
>
> bind two ranks to one socket:
> $ mpirun -np 2 --bind-to-core ./all2all
> $ mpirun -np 2 -mca mpi_paffinity_alone 1 ./all2all
>
> bind two ranks to two different sockets:
> $ mpirun -np 2 --bind-to-socket ./all2all
>
> All three runs resulted in similar bad latencies (~1.4us).
> :-(
>
>
> Matthias
>
> On Monday 13 February 2012 12:43:22 sylvain.jeaugey_at_[hidden] wrote:
>> Hi Matthias,
>>
>> You might want to play with process binding to see if your problem is
>> related to bad memory affinity.
>>
>> Try to launch pingpong on two CPUs of the same socket, then on different
>> sockets (i.e. bind each process to a core, and try different
>> configurations).
>>
>> Sylvain
>>
>>
>>
>> De : Matthias Jurenz <matthias.jurenz_at_[hidden]>
>> A : Open MPI Developers <devel_at_[hidden]>
>> Date : 13/02/2012 12:12
>> Objet : [OMPI devel] poor btl sm latency
>> Envoyé par : devel-bounces_at_[hidden]
>>
>>
>>
>> Hello all,
>>
>> on our new AMD cluster (AMD Opteron 6274, 2,2GHz) we get very bad
>> latencies
>> (~1.5us) when performing 0-byte p2p communication on one single node using
>> the
>> Open MPI sm BTL. When using Platform MPI we get ~0.5us latencies which is
>> pretty good. The bandwidth results are similar for both MPI
>> implementations
>> (~3,3GB/s) - this is okay.
>>
>> One node has 64 cores and 64Gb RAM where it doesn't matter how many ranks
>> allocated by the application. We get similar results with different number
>> of
>> ranks.
>>
>> We are using Open MPI 1.5.4 which is built by gcc 4.3.4 without any
>> special
>> configure options except the installation prefix and the location of the
>> LSF
>> stuff.
>>
>> As mentioned at http://www.open-mpi.org/faq/?category=sm we tried to use
>> /dev/shm instead of /tmp for the session directory, but it had no effect.
>> Furthermore, we tried the current release candidate 1.5.5rc1 of Open MPI
>> which
>> provides an option to use the SysV shared memory (-mca shmem sysv) - also
>> this
>> results in similar poor latencies.
>>
>> Do you have any idea? Please help!
>>
>> Thanks,
>> Matthias
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
> _______________________________________________
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