An MPI program can deadlock if one process is waiting for a
message from another process that never gets sent. To avoid deadlocks
when using FFTW's MPI routines, it is important to know which
functions are collective: that is, which functions must
always be called in the same order from every
process in a given communicator. (For example, MPI_Barrier is
the canonical example of a collective function in the MPI standard.)
The functions in FFTW that are always collective are: every
function beginning with ‘fftw_mpi_plan’, as well as
fftw_mpi_broadcast_wisdom and fftw_mpi_gather_wisdom.
Also, the following functions from the ordinary FFTW interface are
collective when they are applied to a plan created by an
‘fftw_mpi_plan’ function: fftw_execute,
fftw_destroy_plan, and fftw_flops.