OpenCV Cuda DFT extremely slow


I’m just running some basic testing metrics CPU vs GPU for OpenCV 4.5.2 with Cuda 11.4 on Ubuntu 21.XX

Each DFT is taking 80ms which is definitely wrong. I did a Nsight check and found everything at the hardware level looking good, no memory copies happening. But on the API side, CUDA is running a CUModuleLoad 80ms every single loop here. I’m not understanding why it’s doing that?

Any thoughts on how to proceed or what I may have gotten wrong? Thanks, I don’t think this happened with my previous environment that got nuked.

Number of CUDA devices: 1
Total time for GPU-DFT: 0.94756
Total time for GPU-DFT: 0.822355
Total time for GPU-DFT: 0.817892
Total time for GPU-DFT: 0.81832
Total time for GPU-DFT: 0.815274
Total time for GPU-DFT: 0.811999
Total time for GPU-DFT: 0.804606
Total time for GPU-DFT: 0.796385
Total time for GPU-DFT: 0.798238
Total time for GPU-DFT: 0.792199
Total time for GPU-DFT: 0.791789
Total time for GPU-DFT: 0.828016
Total time for CPU-DFT: 0.0381671
Total time for CPU-DFT: 0.0340495
Total time for CPU-DFT: 0.0344006
Total time for CPU-DFT: 0.0340798
Total time for CPU-DFT: 0.0342437
Total time for CPU-DFT: 0.0343547
Total time for CPU-DFT: 0.0339979
Total time for CPU-DFT: 0.0341221
Total time for CPU-DFT: 0.0345065
Total time for CPU-DFT: 0.0341161
Total time for CPU-DFT: 0.034196

Standard tutorial code at the bottom

int main()
        cv::Mat image =cv::imread("720p_ifft.png", cv::COLOR_BGR2GRAY);

        int height, width;
        height = image.rows;
        width = image.cols;
        cv::Size s = image.size();
        height = s.height;
        width = s.width;

        //Convert to 32-bit floating point
        int device =cv::cuda::getCudaEnabledDeviceCount();
        std::cout<<"Number of CUDA devices: "<< device << std::endl;
        int getD = cv::cuda::getDevice();
        //Get dft size
        int h =cv:: getOptimalDFTSize( image.rows );
        int w =cv:: getOptimalDFTSize( image.cols );
        cv::Size dftsize(h,w);
        cv::Mat sizedimage;
        cv::Mat transform = cv::Mat(h,w/2+1,CV_32FC2);
        //Resize Image
        //Upload image to GpuMat
        cv::cuda::GpuMat gputransform = cv::cuda::GpuMat(h,w/2+1,CV_32FC2);
        cv::cuda::GpuMat gpuimage;
        for(int i=0;i<=11;i++){
            double t = (double)cv::getTickCount();
            t = ((double)cv::getTickCount() - t)/cv::getTickFrequency();
            std::cout<<"Total time for GPU-DFT: "<<t << std::endl;
            //Download transformed image to CPU
        for(int j=0;j<=10;j++){
            cv::Mat cputransform = cv::Mat(h,w/2+1,CV_32FC2);
            double totalcputime = (double)cv::getTickCount();
            totalcputime = ((double)cv::getTickCount() - totalcputime)/cv::getTickFrequency();
            std::cout<<"Total time for CPU-DFT: "<<totalcputime<<std::endl;
return 0;

In the future, please use the tools in the editor window to format your code correctly.

Looking at your output, I would say each DFT seems to be taking ~800ms, not 80ms.

I’m not that familiar with the internals of OpenCV. Its not maintained or supported by NVIDIA, and although it uses CUDA under the hood, this is not a CUDA programming question.

An application that uses cuModuleLoad is using the driver API under the hood. After the first time you run that function, subsequent calls to that function for the same module (presumably the case here) should be able to pull what is needed out of the JIT cache, and not recompile anything. So the first thing I would check is whether your JIT cache is working correctly. There are various questions on these forums about the JIT cache. (I would also want to be certain that if/when you built your OpenCV CUDA libraries, you did not inadvertently specify the -G switch, although this seems unlikely.)


In case DFT is calling the CUFFT library under the hood (seems quite plausible), see here. I don’t know how you’d work around that exactly, since you may not have explicit control over the call to cufftDestroy.

1 Like

Thanks Robert, I appreciate the response here.

I think I’m going to revert back to Cuda 11.0 and the respective libraries to get the performance back.

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