cuFFT performance decrease with CUDA 7.0

Recently I upgraded my Linux box (with a GTX 750 Ti) from CUDA 6.5 to 7.0 and noticed a performance decrease in cuFFT.

For example, the simple program below reports an average runtime of 0.550ms with 6.5 and 0.770ms with 7.0.

Downgrading to 6.5 brings back the 0.550ms. Upgrading to 7.0 yields 0.770ms again.

Here is the code:

#include <math.h>
#include <stdio.h>
#include <sys/time.h>

#include <cuda.h>
#include <cufft.h>

#define PI 3.141592653589793
#define NFFT 128
#define BATCH 1000
#define NRUNS 1000

inline double seconds()
    struct timeval tv;
    gettimeofday(&tv, NULL);
    return (double)tv.tv_sec + (double)tv.tv_usec * 1.e-6;

int main(int argc, char* argv[])

    int n = BATCH * NFFT;
    cufftComplex* array = (cufftComplex*)malloc(n*sizeof(cufftComplex));
    for (int i=0; i<n; i++)
        array[i].x = cos(2.*PI*i/NFFT);
        array[i].y = sin(2.*PI*i/NFFT);

    cufftComplex* d_array;
    cudaMalloc(&d_array, n*sizeof(cufftComplex));
    cudaMemcpy(d_array, array, n*sizeof(cufftComplex), cudaMemcpyHostToDevice);

    double runtime = 0.0;
    for (int i=0; i<NRUNS+1; i++)
        double t0 = seconds();

        cufftHandle plan;
        int nfft[] = {NFFT};
        cufftPlanMany(&plan, 1, nfft, NULL, 1, NFFT, NULL, 1, NFFT, CUFFT_C2C, BATCH);

        cufftComplex* d_fft;
        cudaMalloc(&d_fft, n*sizeof(cufftComplex));
        cufftExecC2C(plan, d_array, d_fft, CUFFT_FORWARD);


        if (i == 0) continue; // first run is just to warm up

        double t1 = seconds();
        runtime += t1-t0;

    printf("Avg. runtime (s): %.6f\n", runtime/NRUNS);



    return 0;

Compiled with:
nvcc -arch=sm_50 -lcufft -o test

Supposedly, cuFFT should be faster in 7.0:
(see “cuFFT Performance Improvements”)

Am I doing something wrong? Or am I forgetting something?

I modified the code to time each function and here are the results:

| Runtimes (s)   | CUDA 6.5 | CUDA 7.0 |
| cufftPlanMany  | 0.000366 | 0.000590 | significant
| cudaMalloc     | 0.000061 | 0.000061 | negligible
| cufftExecC2C   | 0.000013 | 0.000013 | negligible
| cufftDestroy   | 0.000069 | 0.000068 | negligible
| cudaFree       | 0.000041 | 0.000037 | negligible
| Total          | 0.000550 | 0.000770 |

The issue can be narrowed down to cufftPlanMany, but why is plan generation slower in CUDA 7.0? Is it supposed to be?

You may want to consider filing a bug if the slowdown affects your application’s run time outside a noise limit of 5%.

I have no insight into the internals of CUFFT, and don’t know what specifically changed in CUDA 7.0, but one plausible hypothesis is that the addition of various kind of kernels with increased performance has led to a greater complexity of the selection and configuration process performed by cufftPlanMany().

Since cufftPlanMany() presumably comprises only host-side activity, its speed will depend on the host system’s performance, whereas the actual FFT execution runs on the GPU and and performance is determined by the device. The optimizations in CUDA 7.0 were presumably with regard to the latter part (i.e. device code)