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In particular, suppose that we have an n0 by n1 array in
Chris@10: row-major order, block-distributed across the n0 dimension. To
Chris@10: transpose this into an n1 by n0 array block-distributed
Chris@10: across the n1 dimension, we would create a plan by calling the
Chris@10: following function:
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fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1, Chris@10: double *in, double *out, Chris@10: MPI_Comm comm, unsigned flags); Chris@10:Chris@10:
Chris@10: The input and output arrays (in and out) can be the
Chris@10: same. The transpose is actually executed by calling
Chris@10: fftw_execute on the plan, as usual.
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The flags are the usual FFTW planner flags, but support
Chris@10: two additional flags: FFTW_MPI_TRANSPOSED_OUT and/or
Chris@10: FFTW_MPI_TRANSPOSED_IN. What these flags indicate, for
Chris@10: transpose plans, is that the output and/or input, respectively, are
Chris@10: locally transposed. That is, on each process input data is
Chris@10: normally stored as a local_n0 by n1 array in row-major
Chris@10: order, but for an FFTW_MPI_TRANSPOSED_IN plan the input data is
Chris@10: stored as n1 by local_n0 in row-major order. Similarly,
Chris@10: FFTW_MPI_TRANSPOSED_OUT means that the output is n0 by
Chris@10: local_n1 instead of local_n1 by n0.
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To determine the local size of the array on each process before and
Chris@10: after the transpose, as well as the amount of storage that must be
Chris@10: allocated, one should call fftw_mpi_local_size_2d_transposed,
Chris@10: just as for a 2d DFT as described in the previous section:
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ptrdiff_t fftw_mpi_local_size_2d_transposed Chris@10: (ptrdiff_t n0, ptrdiff_t n1, MPI_Comm comm, Chris@10: ptrdiff_t *local_n0, ptrdiff_t *local_0_start, Chris@10: ptrdiff_t *local_n1, ptrdiff_t *local_1_start); Chris@10:Chris@10:
Chris@10: Again, the return value is the local storage to allocate, which in
Chris@10: this case is the number of real (double) values rather
Chris@10: than complex numbers as in the previous examples.
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