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Subject: Re: [OMPI users] Working with a CellBlade cluster
From: Gilbert Grosdidier (grodid_at_[hidden])
Date: 2008-10-29 12:36:41


Thank you very much Mi and Lenny for your detailed replies.

 I believe I can summarize the infos to allow for
'Working with a QS22 CellBlade cluster' like this:
- Yes, messages are efficiently handled with "-mca btl openib,sm,self"
- Better to go to the OMPI-1.3 version ASAP
- It is currently more efficient/easy to use numactl to control
processor affinity on a QS22.

 So far so good.

 One question remains: how could I monitor in details message passing
thru IB (on one side) and thru SM (on the other side) thru the use of mca
parameters, please ? Additionnal info about the verbosity level
of this monitoring will be highly appreciated ... A lengthy travel
inside the list of such parameters provided by ompi_info did not
enlighten me (there are so many xxx_sm_yyy type params that I don't know which
could be the right one ;-)

 Thanks in advance for your hints, Best Regards, Gilbert.

On Thu, 23 Oct 2008, Mi Yan wrote:

>
> 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
>
>
>
>
> "Lenny
> Verkhovsky"
> <lenny.verkhovsky To
> @gmail.com> "Open MPI Users"
> Sent by: <users_at_[hidden]>
> users-bounces_at_ope cc
> n-mpi.org
> Subject
> Re: [OMPI users] Working with a
> 10/23/2008 05:48 CellBlade cluster
> AM
>
>
> Please respond to
> Open MPI Users
> <users_at_open-mpi.o
> rg>
>
>
>
>
>
>
> 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_at_[hidden]>
> wrote:
> Working with a CellBlade cluster (QS22), the requirement is to have one
> instance of the executable running on each socket of the blade (there are
> 2
> sockets). The application is of the 'domain decomposition' type, and each
> instance is required to often send/receive data with both the remote
> blades and
> the neighbor socket.
>
> Question is : which specification must be used for the mca btl component
> to force 1) shmem type messages when communicating with this neighbor
> socket,
> while 2) using openib to communicate with the remote blades ?
> Is '-mca btl sm,openib,self' suitable for this ?
>
> Also, which debug flags could be used to crosscheck that the messages
> are
> _actually_ going thru the right channel for a given channel, please ?
>
> We are currently using OpenMPI 1.2.5 shipped with RHEL5.2 (ppc64).
> Which version do you think is currently the most optimised for these
> processors and problem type ? Should we go towards OpenMPI 1.2.8
> instead ?
> Or even try some OpenMPI 1.3 nightly build ?
>
> Thanks in advance for your help, Gilbert.
>
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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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