Question

I made a Dll file in visual C++ to compute modulus of an array of complex numbers in CUDA. The array is type of cufftComplex. I then called the Dll in LabVIEW to check the accuracy of the result. I’m receiving an incorrect result. Could anyone tell me what is wrong with the following code, please? I think there should be something wrong with my kernel function(the way I am retrieving the cufftComplex data should be incorrect).

#include <math.h>
#include <cstdlib>
#include <cuda_runtime.h>
#include <cufft.h>

extern "C" __declspec(dllexport) void Modulus(cufftComplex *digits,float *result);


__global__ void ModulusComputation(cufftComplex *a, int N, float *temp)
{
  int idx = blockIdx.x*blockDim.x + threadIdx.x;
  if (idx<N) 
    { 
      temp[idx] = sqrt((a[idx].x * a[idx].x) + (a[idx].y * a[idx].y));
    }

}



void Modulus(cufftComplex *digits,float *result)
{
  #define N 1024
  cufftComplex *d_data;
  float *temp;

  size_t size = sizeof(cufftComplex)*N;

  cudaMalloc((void**)&d_data, size);
  cudaMalloc((void**)&temp, sizeof(float)*N);

  cudaMemcpy(d_data, digits, size, cudaMemcpyHostToDevice);

  int blockSize = 16;
  int nBlocks = N/blockSize;
  if( N % blockSize != 0 )
      nBlocks++;

  ModulusComputation <<< nBlocks, blockSize >>> (d_data, N,temp);

  cudaMemcpy(result, temp, size, cudaMemcpyDeviceToHost);

  cudaFree(d_data);
  cudaFree(temp);

}

Answer 1

In the final cudaMemcpy in your code, you have:

 cudaMemcpy(result, temp, size, cudaMemcpyDeviceToHost); 

It should be:

 cudaMemcpy(result, temp, sizeof(float)*N, cudaMemcpyDeviceToHost); 

If you had included error checking for your cuda calls, you would have seen this cuda call (as originally written) throw an error.

There’s other comments that could be made. For example your block size (16) should be an integral multiple of 32. But this does not prevent proper operation.