Thanks for your answer.You are right.I've tried upon 4 nodes with 6 processes and things are worst.So do you suggest that unique thing to do is to order an infiniband switch or is there a possibility to enhancesomething by tuning mca parameters ?
De : Ralph Castain <firstname.lastname@example.org>
À : Dugenoux Albert <email@example.com>; Open MPI Users <firstname.lastname@example.org>
Envoyé le : Mardi 10 juillet 2012 16h47
Objet : Re: [OMPI users] Bad parallel scaling using Code Saturne with openmpi
I suspect it mostly reflects communication patterns. I don't know anything about Saturne, but shared memory is a great deal faster than TCP, so the more processes sharing a node the better. You may also be hitting some natural boundary in your model - perhaps with 8 processes/node you wind up with more processes that cross the node boundary, further increasing the communication requirement.
Do things continue to get worse if you use all 4 nodes with 6 processes/node?
On Jul 10, 2012, at 7:31 AM, Dugenoux Albert wrote:
_______________________________________________Hi.I have recently built a cluster upon a Dell PowerEdge Server with a Debian 6.0 OS. This server is composed of4 system board of 2 processors of hexacores. So it gives 12 cores per system board.The boards are linked with a local Gbits switch.In order to parallelize the software Code Saturne, which is a CFD solver, I have configured the clustersuch that there are a pbs server/mom on 1 system board and 3 mom and the 3 others cards. So this leads to48 cores dispatched on 4 nodes of 12 CPU. Code saturne is compiled with the openmpi 1.6 version.When I launch a simulation using 2 nodes with 12 cores, elapse time is good and network traffic is not full.But when I launch the same simulation using 3 nodes with 8 cores, elapse time is 5 times the previous one.I both cases, I use 24 cores and network seems not to be satured.I have tested several configurations : binaries in local file system or on a NFS. But results are the same.I have visited severals forums (in particular http://www.open-mpi.org/community/lists/users/2009/08/10394.php)and read lots of threads, but as I am not an expert at clusters, I presently do not see where it is wrong !Is it a problem in the configuration of PBS (I have installed it from the deb packages), a subtile compilation optionsof openMPI, or a bad network configuration ?Regards.B. S.
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