1. MCA BTL parameters
With "-mca btl openib,self", both message between two Cell processors on one QS22 and messages between two QS22s go through IB.

With "-mca btl openib,sm,slef", message on one QS22 go through shared memory, message between QS22 go through IB,

Depending on the message size and other MCA parameters, it does not guarantee message passing on shared memory is faster than on IB. E.g. the bandwidth for 64KB message is 959MB/s on shared-memory and is 694MB/s on IB; the bandwidth for 4MB message is 539 MB/s and 1092 MB/s on IB. The bandwidth of 4MB message on shared memory may be higher if you tune some MCA parameter.

2. mpi_paffinity_alone
"mpi_paffinity_alone =1" is not a good choice for QS22. There are two sockets with two physical Cell/B.E. on one QS22. Each Cell/B.E. has two SMT threads. So there are four logical CPUs on one QS22. CBE Linux kernel maps logical cpu 0 and 1 to socket1 and maps logical cpu 1 and 2 to socket 2. If mpi_paffinity_alone is set to 1, the two MPI instances will be assigned to logical cpu 0 and cpu 1 on socket 1. I believe this is not what you want.

A temporaily solution to force the affinity on QS22 is to use "numactl", E.g. assuming the hostname is "qs22" and the executable is "foo". the following command can be used
mpirun -np 1 -H qs22 numactl -c0 -m0 foo : -np 1 -H qs22 numactl -c1 -m1 foo

In the long run, I wish CBE kernel export CPU topology in /sys and use PLPA to force the processor affinity.

Best Regards,
Mi

Inactive hide details for "Lenny Verkhovsky" <lenny.verkhovsky@gmail.com>"Lenny Verkhovsky" <lenny.verkhovsky@gmail.com>


          "Lenny Verkhovsky" <lenny.verkhovsky@gmail.com>
          Sent by: users-bounces@open-mpi.org

          10/23/2008 05:48 AM
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          Open MPI Users <users@open-mpi.org>


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Re: [OMPI users] Working with a CellBlade cluster

Hi,


If I understand you correctly the most suitable way to do it is by paffinity that we have in Open MPI 1.3 and the trank.
how ever usually OS is distributing processes evenly between sockets by it self.

There still no formal FAQ due to a multiple reasons but you can read how to use it in the attached scratch ( there were few name changings of the params, so check with ompi_info )

shared memory is used between processes that share same machine, and openib is used between different machines ( hostnames ), no special mca params are needed.

Best Regards
Lenny,



On Sun, Oct 19, 2008 at 10:32 AM, Gilbert Grosdidier <grodid@mail.cern.ch> wrote:

(See attached file: RANKS_FAQ.doc)_______________________________________________
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