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(fftw.info)How Many Threads to Use?

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How Many Threads to Use?
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   There is a fair amount of overhead involved in spawning and
synchronizing threads, so the optimal number of threads to use depends
upon the size of the transform as well as on the number of processors
you have.

   As a general rule, you don't want to use more threads than you have
processors.  (Using more threads will work, but there will be extra
overhead with no benefit.)  In fact, if the problem size is too small,
you may want to use fewer threads than you have processors.

   You will have to experiment with your system to see what level of
parallelization is best for your problem size.  Useful tools to help you
do this are the test programs that are automatically compiled along with
the threads libraries, `fftw_threads_test' and `rfftw_threads_test' (in
the `threads' subdirectory).  These take the same arguments as the
other FFTW test programs (see `tests/README'), except that they also
take the number of threads to use as a first argument, and report the
parallel speedup in speed tests.  For example,

     fftw_threads_test 2 -s 128x128

   will benchmark complex 128x128 transforms using two threads and
report the speedup relative to the uniprocessor transform.

   For instance, on a 4-processor 200MHz Pentium Pro system running
Linux 2.2.0, we found that the "crossover" point at which 2 threads
became beneficial for complex transforms was about 4k points, while 4
threads became beneficial at 8k points.