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In this section, we collect a few tips on getting the best performance Chris@19: out of FFTW's MPI transforms. Chris@19: Chris@19:
First, because of the 1d block distribution, FFTW's parallelization is Chris@19: currently limited by the size of the first dimension. Chris@19: (Multidimensional block distributions may be supported by a future Chris@19: version.) More generally, you should ideally arrange the dimensions so Chris@19: that FFTW can divide them equally among the processes. See Load balancing. Chris@19: Chris@19: Chris@19:
Second, if it is not too inconvenient, you should consider working Chris@19: with transposed output for multidimensional plans, as this saves a Chris@19: considerable amount of communications. See Transposed distributions. Chris@19: Chris@19: Chris@19:
Third, the fastest choices are generally either an in-place transform
Chris@19: or an out-of-place transform with the FFTW_DESTROY_INPUT flag
Chris@19: (which allows the input array to be used as scratch space). In-place
Chris@19: is especially beneficial if the amount of data per process is large.
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Fourth, if you have multiple arrays to transform at once, rather than Chris@19: calling FFTW's MPI transforms several times it usually seems to be Chris@19: faster to interleave the data and use the advanced interface. (This Chris@19: groups the communications together instead of requiring separate Chris@19: messages for each transform.) Chris@19: Chris@19: Chris@19: Chris@19: