FFTW 3.3.5: FFTW MPI Wisdom

FFTW’s “wisdom” facility (see Words of Wisdom-Saving Plans) can Chris@42: be used to save MPI plans as well as to save uniprocessor plans. Chris@42: However, for MPI there are several unavoidable complications. Chris@42:

First, the MPI standard does not guarantee that every process can Chris@42: perform file I/O (at least, not using C stdio routines)—in general, Chris@42: we may only assume that process 0 is capable of I/O.⁷ So, if we Chris@42: want to export the wisdom from a single process to a file, we must Chris@42: first export the wisdom to a string, then send it to process 0, then Chris@42: write it to a file. Chris@42:

Second, in principle we may want to have separate wisdom for every Chris@42: process, since in general the processes may run on different hardware Chris@42: even for a single MPI program. However, in practice FFTW’s MPI code Chris@42: is designed for the case of homogeneous hardware (see Load balancing), and in this case it is convenient to use the same wisdom Chris@42: for every process. Thus, we need a mechanism to synchronize the wisdom. Chris@42:

To address both of these problems, FFTW provides the following two Chris@42: functions: Chris@42:

Given a communicator comm, fftw_mpi_broadcast_wisdom Chris@42: will broadcast the wisdom from process 0 to all other processes. Chris@42: Conversely, fftw_mpi_gather_wisdom will collect wisdom from all Chris@42: processes onto process 0. (If the plans created for the same problem Chris@42: by different processes are not the same, fftw_mpi_gather_wisdom Chris@42: will arbitrarily choose one of the plans.) Both of these functions Chris@42: may result in suboptimal plans for different processes if the Chris@42: processes are running on non-identical hardware. Both of these Chris@42: functions are collective calls, which means that they must be Chris@42: executed by all processes in the communicator. Chris@42: Chris@42:

So, for example, a typical code snippet to import wisdom from a file Chris@42: and use it on all processes would be: Chris@42:

Chris@42:

{
Chris@42:     int rank;
Chris@42: 
Chris@42:     fftw_mpi_init();
Chris@42:     MPI_Comm_rank(MPI_COMM_WORLD, &rank);
Chris@42:     if (rank == 0) fftw_import_wisdom_from_filename("mywisdom");
Chris@42:     fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
Chris@42: }
Chris@42:

(Note that we must call fftw_mpi_init before importing any Chris@42: wisdom that might contain MPI plans.) Similarly, a typical code Chris@42: snippet to export wisdom from all processes to a file is: Chris@42: Chris@42:

Chris@42:

Footnotes

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In fact, Chris@42: even this assumption is not technically guaranteed by the standard, Chris@42: although it seems to be universal in actual MPI implementations and is Chris@42: widely assumed by MPI-using software. Technically, you need to query Chris@42: the MPI_IO attribute of MPI_COMM_WORLD with Chris@42: MPI_Attr_get. If this attribute is MPI_PROC_NULL, no Chris@42: I/O is possible. If it is MPI_ANY_SOURCE, any process can Chris@42: perform I/O. Otherwise, it is the rank of a process that can perform Chris@42: I/O ... but since it is not guaranteed to yield the same rank Chris@42: on all processes, you have to do an MPI_Allreduce of some kind Chris@42: if you want all processes to agree about which is going to do I/O. Chris@42: And even then, the standard only guarantees that this process can Chris@42: perform output, but not input. See e.g. Parallel Programming Chris@42: with MPI by P. S. Pacheco, section 8.1.3. Needless to say, in our Chris@42: experience virtually no MPI programmers worry about this.

Chris@42:

6.8 FFTW MPI Wisdom

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