Chris@42: Previous: Transposed distributions, Up: MPI Data Distribution [Contents][Index]
Chris@42:For one-dimensional distributed DFTs using FFTW, matters are slightly Chris@42: more complicated because the data distribution is more closely tied to Chris@42: how the algorithm works. In particular, you can no longer pass an Chris@42: arbitrary block size and must accept FFTW’s default; also, the block Chris@42: sizes may be different for input and output. Also, the data Chris@42: distribution depends on the flags and transform direction, in order Chris@42: for forward and backward transforms to work correctly. Chris@42:
Chris@42:ptrdiff_t fftw_mpi_local_size_1d(ptrdiff_t n0, MPI_Comm comm, Chris@42: int sign, unsigned flags, Chris@42: ptrdiff_t *local_ni, ptrdiff_t *local_i_start, Chris@42: ptrdiff_t *local_no, ptrdiff_t *local_o_start); Chris@42:
This function computes the data distribution for a 1d transform of
Chris@42: size n0 with the given transform sign and flags.
Chris@42: Both input and output data use block distributions. The input on the
Chris@42: current process will consist of local_ni numbers starting at
Chris@42: index local_i_start; e.g. if only a single process is used,
Chris@42: then local_ni will be n0 and local_i_start will
Chris@42: be 0. Similarly for the output, with local_no numbers
Chris@42: starting at index local_o_start. The return value of
Chris@42: fftw_mpi_local_size_1d will be the total number of elements to
Chris@42: allocate on the current process (which might be slightly larger than
Chris@42: the local size due to intermediate steps in the algorithm).
Chris@42:
As mentioned above (see Load balancing), the data will be divided
Chris@42: equally among the processes if n0 is divisible by the
Chris@42: square of the number of processes. In this case,
Chris@42: local_ni will equal local_no. Otherwise, they may be
Chris@42: different.
Chris@42:
For some applications, such as convolutions, the order of the output
Chris@42: data is irrelevant. In this case, performance can be improved by
Chris@42: specifying that the output data be stored in an FFTW-defined
Chris@42: “scrambled” format. (In particular, this is the analogue of
Chris@42: transposed output in the multidimensional case: scrambled output saves
Chris@42: a communications step.) If you pass FFTW_MPI_SCRAMBLED_OUT in
Chris@42: the flags, then the output is stored in this (undocumented) scrambled
Chris@42: order. Conversely, to perform the inverse transform of data in
Chris@42: scrambled order, pass the FFTW_MPI_SCRAMBLED_IN flag.
Chris@42:
Chris@42:
Chris@42:
In MPI FFTW, only composite sizes n0 can be parallelized; we
Chris@42: have not yet implemented a parallel algorithm for large prime sizes.
Chris@42:
Chris@42: Previous: Transposed distributions, Up: MPI Data Distribution [Contents][Index]
Chris@42: