This section discusses multi-threaded FFTW wrappers only. MPI FFTW wrappers, available only with Intel MKL for the Linux* and Windows* operating systems, are described in a separate section.
Unlike the original FFTW interface, every computational function in the FFTW2 interface to Intel MKL provides multithreaded computation by default, with the number of threads defined by the number of processors available on the system (see section "Managing Performance and Memory" in the Intel MKL User's Guide). To limit the number of threads that use the FFTW interface, call the threaded FFTW computational functions:
void fftw_threads(int nthreads, fftw_plan plan, int howmany, fftw_complex *in, int istride, int idist, fftw_complex *out, int ostride, int odist);
void fftw_threads_one(int nthreads, rfftwnd_plan plan, fftw_complex *in, fftw_complex *out);
...
void rfftwnd_threads_real_to_complex( int nthreads, rfftwnd_plan plan, int howmany, fftw_real *in, int istride, int idist, fftw_complex *out, int ostride, int odist);
Compared to its non-threaded counterpart, every threaded computational function has threads_ as the second part of its name and additional first parameter nthreads. Set the nthreads parameter to the thread limit to ensure that the computation requires at most that number of threads.