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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.
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ndim = 2;
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dims[0] = 3; dims[1] = 4;
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periods[0] = 0; periods[1] = 0; // periodic is false
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reorder = 1; // reordering is true
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ierr = MPI_Cart_create (MPI_COMM_WORLD, ndim,
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dims, periods, reorder, &comm2d);
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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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