MPI_SCATTER Fortran Matrices by Rows

Question

What is the best way to scatter a Fortran 90 matrix by its rows rather than columns? That is, let's say I have a matrix a(4,50) and I want to MPI_SCATTER it onto two processes where each part is alocal(2,50), where rank 0 has rows 1 and 2, and rank 1 has 3 and 4. Now, in C, this is simple since arrays are row-major, but in Fortran 90 they are column-major.

I'm trying to avoid using TRANSPOSE to flip a before scattering (i.e, doubling the memory use), and I figure there must be a way in MPI to do this. Would it be MPI_TYPE_VECTOR? MPI_TYPE_CREATE_SUBARRAY?

Likewise, what if I have a 3d array b(4,50,3) and I want two scattered matrices of blocal(2,50,3) distributed as above?

Answer 1

Yes, MPI_TYPE_VECTOR and MPI_TYPE_CREATE_SUBARRAY are what you want. The former for your first problem, the latter for your second. Comment if you want me to write the calls for you !

Answer 2

Didn't most of the MPI data transfer calls have a stride argument? Set it to the size of the data type times the height of the matrix and there you go...

I've taken a look to the MPI reference and there wasn't a explicit argument to that, but if you go to the example 5.12, they show how to send strided ints with MPI_Scatterv and MPI_Gatherv.