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:

I have recently built a cluster upon a Dell PowerEdge Server with a Debian 6.0 OS. This server is composed of
4 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 cluster
such that there are a pbs server/mom on 1 system board and 3 mom and the 3 others cards. So this leads to
48 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 options
of openMPI, or a bad network configuration ?
B. S.
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