Make my kernel run for longer !

Hello everyone,

I have a confusion about where to put my iteration to make it run longer, and I want to know the difference between those 2 approaches:

  1. Iterations inside the global function: for example vector addition
__global__ void
vectorAdd(const float *A, const float *B, float *C, int numElements)
    int i = blockDim.x * blockIdx.x + threadIdx.x;
    int nIter=1000;

    for (int k =0; k < nIter ; k++)
          if (i < numElements)
              C[i] = A[i] + B[i];

  1. outside the global function, in my main like this :
int nIter=1000;
   cudaEventRecord(start, 0);

   for (int j = 0; j < nIter; j++)
        vectorAdd<<<blocksPerGrid, threadsPerBlock>>>(d_A, d_B, d_C, numElements) ;

   cudaEventRecord(stop, 0);
   cudaEventElapsedTime(&ms, start, stop);

Q1: there is no overhead for the second approach ?
Q2: Or do you think it similar ?
Q3: which approach to adopt to be sure that I’m repeating my kernel ?

Thank you in advance for your response,

Neither one of these approaches is doing anything sensible for iterations beyond the first. The first approach will launch (at least/presumably) a single kernel that takes a longer time. The second approach will launch many kernels, each of which takes a shorter time, but in aggregate the time consumed may be longer than this first approach.

If you want to “repeat” a kernel, you would need a loop similar to the one in your second approach. Since you have not shown the host code for the first, or the device code for the second, my answer is imprecise.

Rather than asking about it here, if you learn to use a GPU profiler, the differences in your approaches will be readily evident with some study.