DFTI performance improvement #32

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1. atlemann commented on Apr 9, 2026

   

   atlemann

   on Apr 9, 2026

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   I'm currently trying to create C# methods for calling the DftiCreateDescriptor in a library, since I need single precision 3D FFT and thought it was most flexible to just expose all the Dfti methods in C# and perhaps add them to this project. But I just can't get it to work in my lib. Every time my code hits the DftiCreateDescriptor from C#, it crashes immediately with a 132 exit code.
2. TSerious commented on Apr 10, 2026

   

   TSerious

   on Apr 10, 2026

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   @atlemann you can checkout my repo: oneMKL.FFT.NET
   It has most of the Dfti methods exposed.
3. atlemann commented on Apr 10, 2026

   

   atlemann

   on Apr 10, 2026

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   Looks very similar to what I tried. Perhaps it's some nuance with NixOS. Looks like you're targeting Windows?
4. TSerious commented on Apr 10, 2026

   

   TSerious

   on Apr 10, 2026

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   Yes, that's right.
   I already thought about integrating what I did into MKL.NET, but the last time I tried I was not able to get it compiling (see the first post in this request).
5. atlemann commented on Apr 10, 2026

   

   atlemann

   on Apr 10, 2026

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   Looks like some parts of the FFTW api's are available, e.g. fftwf_plan_dft_3d which gives you a pointer to a plan for single precision 3D FFT. https://www.intel.com/content/www/us/en/docs/onemkl/developer-reference-fortran/2023-0/using-fftw3-wrappers.html

   So you can define these:

   private const int FFTW_FORWARD = -1;
   private const int FFTW_BACKWARD = 1;
   private const uint FFTW_ESTIMATE = 64;
   
   [DllImport(Mkl.DllFile, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
   static extern IntPtr fftwf_plan_dft_3d(int nx, int ny, int nz, void* @in, void* @out, int sign, uint flags);
   
   [DllImport(Mkl.DllFile, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
   static extern void fftwf_execute(IntPtr plan);
   
   [DllImport(Mkl.DllFile, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
   static extern void fftwf_destroy_plan(IntPtr plan);

   and call it like this:

   private static void Compute3D(void* matrixData, int nx, int ny, int nz, bool forward)
   {
       var plan = IntPtr.Zero;
       try
       {
           int sign = forward ? FFTW_FORWARD : FFTW_BACKWARD;
           plan = fftwf_plan_dft_3d(nx, ny, nz, matrixData, matrixData, sign, FFTW_ESTIMATE); // FFTW_ESTIMATE is ignored, according to the docs
           if (plan == IntPtr.Zero)
               throw new InvalidOperationException("Failed to create FFTW plan");
   
           fftwf_execute(plan);
   
           // Scale for inverse FFT
           if (!forward)
           {
               var scale = 1.0f / ((float)nx * ny * nz);
               // Unsafe, since I've already fixed matixData outside this function
               Blas.Unsafe.scal(nx * ny * nz * 2, scale, (float*)matrixData, 1);
           }
       }
       finally
       {
           if (plan != IntPtr.Zero)
               fftwf_destroy_plan(plan);
       }
   }