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 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 ? > > Regards. > > B. S. > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users
