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cannam@95: An MPI program can deadlock if one process is waiting for a
cannam@95: message from another process that never gets sent. To avoid deadlocks
cannam@95: when using FFTW's MPI routines, it is important to know which
cannam@95: functions are collective: that is, which functions must
cannam@95: always be called in the same order from every
cannam@95: process in a given communicator. (For example, MPI_Barrier is
cannam@95: the canonical example of a collective function in the MPI standard.)
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The functions in FFTW that are always collective are: every
cannam@95: function beginning with ‘fftw_mpi_plan’, as well as
cannam@95: fftw_mpi_broadcast_wisdom and fftw_mpi_gather_wisdom.
cannam@95: Also, the following functions from the ordinary FFTW interface are
cannam@95: collective when they are applied to a plan created by an
cannam@95: ‘fftw_mpi_plan’ function: fftw_execute,
cannam@95: fftw_destroy_plan, and fftw_flops.
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