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 ?