Defining a Cartesian Topology

• The routine MPI_Cart_create creates a Cartesian decomposition of the processes, with the number of dimensions given by the ndim argument. It returns a new communicator (in comm2d in example below) with the same processes as in the input communicator, but different topology.

• ndim = 2;

• dims[0] = 3; dims[1] = 4;

• periods[0] = 0; periods[1] = 0; // periodic is false

• reorder = 1; // reordering is true

• ierr = MPI_Cart_create (MPI_COMM_WORLD, ndim,

• dims, periods, reorder, &comm2d);

  • where reorder specifies that it’s o.k. to reorder the default process rank in order to achieve a good embedding (with good communication times between neighbors).

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