MPI Plan Creation (FFTW 3.3.10)

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#### 6.12.5 MPI Plan Creation

#### Complex-data MPI DFTs

Plans for complex-data DFTs (see 2d MPI example) are created by:

    fftw_plan fftw_mpi_plan_dft_1d(ptrdiff_t n0, fftw_complex *in, fftw_complex *out,
                                   MPI_Comm comm, int sign, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_2d(ptrdiff_t n0, ptrdiff_t n1,
                                   fftw_complex *in, fftw_complex *out,
                                   MPI_Comm comm, int sign, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_3d(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
                                   fftw_complex *in, fftw_complex *out,
                                   MPI_Comm comm, int sign, unsigned flags);
    fftw_plan fftw_mpi_plan_dft(int rnk, const ptrdiff_t *n,
                                fftw_complex *in, fftw_complex *out,
                                MPI_Comm comm, int sign, unsigned flags);
    fftw_plan fftw_mpi_plan_many_dft(int rnk, const ptrdiff_t *n,
                                     ptrdiff_t howmany, ptrdiff_t block, ptrdiff_t tblock,
                                     fftw_complex *in, fftw_complex *out,
                                     MPI_Comm comm, int sign, unsigned flags);

These are similar to their serial counterparts (see Complex DFTs) in specifying the dimensions, sign, and flags of the transform. The `comm` argument gives an MPI communicator that specifies the set of processes to participate in the transform; plan creation is a collective function that must be called for all processes in the communicator. The `in` and `out` pointers refer only to a portion of the overall transform data (see MPI Data Distribution) as specified by the ‘`local_size`’ functions in the previous section. Unless `flags` contains `FFTW_ESTIMATE`, these arrays are overwritten during plan creation as for the serial interface. For multi-dimensional transforms, any dimensions `> 1` are supported; for one-dimensional transforms, only composite (non-prime) `n0` are currently supported (unlike the serial FFTW). Requesting an unsupported transform size will yield a `NULL` plan. (As in the serial interface, highly composite sizes generally yield the best performance.)

The advanced-interface `fftw_mpi_plan_many_dft` additionally allows you to specify the block sizes for the first dimension (`block`) of the n0 × n1 × n2 × … × nd-1 input data and the first dimension (`tblock`) of the n1 × n0 × n2 ×…× nd-1 transposed data (at intermediate steps of the transform, and for the output if `FFTW_TRANSPOSED_OUT` is specified in `flags`). These must be the same block sizes as were passed to the corresponding ‘`local_size`’ function; you can pass `FFTW_MPI_DEFAULT_BLOCK` to use FFTW’s default block size as in the basic interface. Also, the `howmany` parameter specifies that the transform is of contiguous `howmany`-tuples rather than individual complex numbers; this corresponds to the same parameter in the serial advanced interface (see Advanced Complex DFTs) with `stride = howmany` and `dist = 1`.

#### MPI flags

The `flags` can be any of those for the serial FFTW (see Planner Flags), and in addition may include one or more of the following MPI-specific flags, which improve performance at the cost of changing the output or input data formats.

- `FFTW_MPI_SCRAMBLED_OUT`, `FFTW_MPI_SCRAMBLED_IN`: valid for 1d transforms only, these flags indicate that the output/input of the transform are in an undocumented “scrambled” order. A forward `FFTW_MPI_SCRAMBLED_OUT` transform can be inverted by a backward `FFTW_MPI_SCRAMBLED_IN` (times the usual 1/N normalization). See One-dimensional distributions.
- `FFTW_MPI_TRANSPOSED_OUT`, `FFTW_MPI_TRANSPOSED_IN`: valid for multidimensional (`rnk > 1`) transforms only, these flags specify that the output or input of an n0 × n1 × n2 × … × nd-1 transform is transposed to n1 × n0 × n2 ×…× nd-1 . See Transposed distributions.

#### Real-data MPI DFTs

Plans for real-input/output (r2c/c2r) DFTs (see Multi-dimensional MPI DFTs of Real Data) are created by:

    fftw_plan fftw_mpi_plan_dft_r2c_2d(ptrdiff_t n0, ptrdiff_t n1,
                                       double *in, fftw_complex *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_r2c_2d(ptrdiff_t n0, ptrdiff_t n1,
                                       double *in, fftw_complex *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_r2c_3d(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
                                       double *in, fftw_complex *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_r2c(int rnk, const ptrdiff_t *n,
                                    double *in, fftw_complex *out,
                                    MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_c2r_2d(ptrdiff_t n0, ptrdiff_t n1,
                                       fftw_complex *in, double *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_c2r_2d(ptrdiff_t n0, ptrdiff_t n1,
                                       fftw_complex *in, double *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_c2r_3d(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
                                       fftw_complex *in, double *out,
                                       MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_dft_c2r(int rnk, const ptrdiff_t *n,
                                    fftw_complex *in, double *out,
                                    MPI_Comm comm, unsigned flags);

Similar to the serial interface (see Real-data DFTs), these transform logically n0 × n1 × n2 × … × nd-1 real data to/from n0 × n1 × n2 × … × (nd-1/2 + 1) complex data, representing the non-redundant half of the conjugate-symmetry output of a real-input DFT (see Multi-dimensional Transforms). However, the real array must be stored within a padded n0 × n1 × n2 × … × [2 (nd-1/2 + 1)] array (much like the in-place serial r2c transforms, but here for out-of-place transforms as well). Currently, only multi-dimensional (`rnk > 1`) r2c/c2r transforms are supported (requesting a plan for `rnk = 1` will yield `NULL`). As explained above (see Multi-dimensional MPI DFTs of Real Data), the data distribution of both the real and complex arrays is given by the ‘`local_size`’ function called for the dimensions of the complex array. Similar to the other planning functions, the input and output arrays are overwritten when the plan is created except in `FFTW_ESTIMATE` mode.

As for the complex DFTs above, there is an advance interface that allows you to manually specify block sizes and to transform contiguous `howmany`-tuples of real/complex numbers:

    fftw_plan fftw_mpi_plan_many_dft_r2c
                  (int rnk, const ptrdiff_t *n, ptrdiff_t howmany,
                   ptrdiff_t iblock, ptrdiff_t oblock,
                   double *in, fftw_complex *out,
                   MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_many_dft_c2r
                  (int rnk, const ptrdiff_t *n, ptrdiff_t howmany,
                   ptrdiff_t iblock, ptrdiff_t oblock,
                   fftw_complex *in, double *out,
                   MPI_Comm comm, unsigned flags);

#### MPI r2r transforms

There are corresponding plan-creation routines for r2r transforms (see More DFTs of Real Data), currently supporting multidimensional (`rnk > 1`) transforms only (`rnk = 1` will yield a `NULL` plan):

    fftw_plan fftw_mpi_plan_r2r_2d(ptrdiff_t n0, ptrdiff_t n1,
                                   double *in, double *out,
                                   MPI_Comm comm,
                                   fftw_r2r_kind kind0, fftw_r2r_kind kind1,
                                   unsigned flags);
    fftw_plan fftw_mpi_plan_r2r_3d(ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t n2,
                                   double *in, double *out,
                                   MPI_Comm comm,
                                   fftw_r2r_kind kind0, fftw_r2r_kind kind1, fftw_r2r_kind kind2,
                                   unsigned flags);
    fftw_plan fftw_mpi_plan_r2r(int rnk, const ptrdiff_t *n,
                                double *in, double *out,
                                MPI_Comm comm, const fftw_r2r_kind *kind,
                                unsigned flags);
    fftw_plan fftw_mpi_plan_many_r2r(int rnk, const ptrdiff_t *n,
                                     ptrdiff_t iblock, ptrdiff_t oblock,
                                     double *in, double *out,
                                     MPI_Comm comm, const fftw_r2r_kind *kind,
                                     unsigned flags);

The parameters are much the same as for the complex DFTs above, except that the arrays are of real numbers (and hence the outputs of the ‘`local_size`’ data-distribution functions should be interpreted as counts of real rather than complex numbers). Also, the `kind` parameters specify the r2r kinds along each dimension as for the serial interface (see Real-to-Real Transform Kinds). See Other Multi-dimensional Real-data MPI Transforms.

#### MPI transposition

FFTW also provides routines to plan a transpose of a distributed `n0` by `n1` array of real numbers, or an array of `howmany`-tuples of real numbers with specified block sizes (see FFTW MPI Transposes):

    fftw_plan fftw_mpi_plan_transpose(ptrdiff_t n0, ptrdiff_t n1,
                                      double *in, double *out,
                                      MPI_Comm comm, unsigned flags);
    fftw_plan fftw_mpi_plan_many_transpose
                    (ptrdiff_t n0, ptrdiff_t n1, ptrdiff_t howmany,
                     ptrdiff_t block0, ptrdiff_t block1,
                     double *in, double *out, MPI_Comm comm, unsigned flags);

These plans are used with the `fftw_mpi_execute_r2r` new-array execute function (see Using MPI Plans), since they count as (rank zero) r2r plans from FFTW’s perspective.

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