Dear OpenMPI list, I am using a MPI-parallelized simulation program, with a domain-decomposition in 6-Dimensions. In order to improve the scalability of my program I would like to know according to what preferences is MPI distributing the ranks when using MPI_Cart_create( reorder allowed).
To explain my inquiry, imagine a 3-dimensional solver in X-Y-M and 4 computing nodes, each nodes consist of 4 Quad-Core CPUs (4(Node)x[ 4(CPUs) x 4(Cores))] CPUs=64CPUs). Now I decompose all 3 dimensions by 4 (4x4x4 = 64) using MPI_Cart_create. MPI has now several possibilities to map the problem e.g. X-M (locally) on a node and Y across the nodes, or Y-M (locally) and X across the nodes. Now my question is, how can I tell MPI that I want to distribute X-Y locally while M is distributed across nodes. The reason is that X-Y communication ratio is much large (FFT) compared to M where we have only 2 communications per time-step via an Allreduce. An MPI implementation for the BlueGENE for example has an option called mapfile where on can tell MPI how to map the dimensions onto the Nodes. I did not found somethings similar for openmpi. Does anybody know how to achieve this mapping or could anybody tell me where I could find some examples or tutorials ? Thank you very much for your help and best wishes Paul Hilscher
