The following sample program shows how to employ internal OpenMP* threading in Intel® oneAPI Math Kernel Library (oneMKL) for FFT computation.
To specify the number of threads inside Intel® oneAPI Math Kernel Library (oneMKL), use the following settings:
set MKL_NUM_THREADS = 1 for one-threaded mode;
set MKL_NUM_THREADS = 4 for multi-threaded mode.
/* C99 example */
#include "mkl_dfti.h"
float data[200][100];
DFTI_DESCRIPTOR_HANDLE fft = NULL;
MKL_LONG dim_sizes[2] = {200, 100};
/* ...put values into data[i][j] 0<=i<=199, 0<=j<=99 */
DftiCreateDescriptor(&fft, DFTI_SINGLE, DFTI_REAL, 2, dim_sizes);
DftiCommitDescriptor(fft);
DftiComputeForward(fft, data);
DftiFreeDescriptor(&fft);
The following Example “Using Parallel Mode with Multiple Descriptors Initialized in a Parallel Region” and Example “Using Parallel Mode with Multiple Descriptors Initialized in One Thread” illustrate a parallel customer program with each descriptor instance used only in a single thread.
Specify the number of OpenMP threads for Example “Using Parallel Mode with Multiple Descriptors Initialized in a Parallel Region” like this:
set MKL_NUM_THREADS = 1 for Intel® oneAPI Math Kernel Library (oneMKL) to work in the single-threaded mode (recommended);
set OMP_NUM_THREADS = 4 for the customer program to work in the multi-threaded mode.
Note that in this example, the program can be transformed to become single-threaded at the customer level but using parallel mode within Intel® oneAPI Math Kernel Library (oneMKL). To achieve this, you must set the parameter DFTI_NUMBER_OF_TRANSFORMS = 4 and to set the corresponding parameter DFTI_INPUT_DISTANCE = 5000.
program fft2d_private_descr_main
use mkl_dfti
integer nth, len(2)
! 4 OMP threads, each does 2D FFT 50x100 points
parameter (nth = 4, len = (/50, 100/))
complex x(len(2)*len(1), nth)
type(dfti_descriptor), pointer :: myFFT
integer th, myStatus
! assume x is initialized and do 2D FFTs
!$OMP PARALLEL DO SHARED(len, x) PRIVATE(myFFT, myStatus)
do th = 1, nth
myStatus = DftiCreateDescriptor (myFFT, DFTI_SINGLE, DFTI_COMPLEX, 2, len)
myStatus = DftiCommitDescriptor (myFFT)
myStatus = DftiComputeForward (myFFT, x(:, th))
myStatus = DftiFreeDescriptor (myFFT)
end do
!$OMP END PARALLEL DO
end
Specify the number of OpenMP threads for Example “Using Parallel Mode with Multiple Descriptors Initialized in One Thread” like this:
set MKL_NUM_THREADS = 1 for Intel® oneAPI Math Kernel Library (oneMKL) to work in the single-threaded mode (obligatory);
set OMP_NUM_THREADS = 4 for the customer program to work in the multi-threaded mode.
program fft2d_array_descr_main
use mkl_dfti
integer nth, len(2)
! 4 OMP threads, each does 2D FFT 50x100 points
parameter (nth = 4, len = (/50, 100/))
complex x(len(2)*len(1), nth)
type thread_data
type(dfti_descriptor), pointer :: FFT
end type thread_data
type(thread_data) :: workload(nth)
integer th, status, myStatus
do th = 1, nth
status = DftiCreateDescriptor (workload(th)%FFT, DFTI_SINGLE, DFTI_COMPLEX, 2, len)
status = DftiCommitDescriptor (workload(th)%FFT)
end do
! assume x is initialized and do 2D FFTs
!$OMP PARALLEL DO SHARED(len, x, workload) PRIVATE(myStatus)
do th = 1, nth
myStatus = DftiComputeForward (workload(th)%FFT, x(:, th))
end do
!$OMP END PARALLEL DO
do th = 1, nth
status = DftiFreeDescriptor (workload(th)%FFT)
end do
end
The following Example “Using Parallel Mode with a Common Descriptor” illustrates a parallel customer program with a common descriptor used in several threads.
program fft2d_shared_descr_main
use mkl_dfti
integer nth, len(2)
! 4 OMP threads, each does 2D FFT 50x100 points
parameter (nth = 4, len = (/50, 100/))
complex x(len(2)*len(1), nth)
type(dfti_descriptor), pointer :: FFT
integer th, status, myStatus
status = DftiCreateDescriptor (FFT, DFTI_SINGLE, DFTI_COMPLEX, 2, len)
status = DftiCommitDescriptor (FFT)
! assume x is initialized and do 2D FFTs
!$OMP PARALLEL DO SHARED(len, x, FFT) PRIVATE(myStatus)
do th = 1, nth
myStatus = DftiComputeForward (FFT, x(:, th))
end do
!$OMP END PARALLEL DO
status = DftiFreeDescriptor (FFT)
end