Why AddVector CUDA c++ is not working? - c++

Question

I am trying to add 2 arrays using CUDA , but it didn't work .
I did all that it should be done:
1) I parallelized the VectorAdd function
2) I allocated memory to the GPu and moved the data to the GPU
3) And last thing i modified the function VectorAdd to run on the GPU
This is the code :
#define SIZE 1024
__global__ void VectorAdd(int *a, int *b, int *c, int n)
{
int i = threadIdx.x ;
if(i < n)
c[i] = a[i] + b[i];
}
int main()
{
int *a , *b , *c;
int *d_a , *d_b , *d_c;
a = (int *)malloc(SIZE * sizeof(int));
b = (int *)malloc(SIZE * sizeof(int));
c = (int *)malloc(SIZE * sizeof(int));
cudaMalloc( &d_a , SIZE * sizeof(int) );
cudaMalloc( &d_b , SIZE * sizeof(int) );
cudaMalloc( &d_c , SIZE * sizeof(int) );
for ( int i = 0 ; i < SIZE ; ++i)
{
a[i] = i ;
b[i] = i ;
c[i] = 0 ;
}
cudaMemcpy(d_a, a, SIZE *sizeof(int), cudaMemcpyHostToDevice);
cudaMemcpy(d_b, b, SIZE *sizeof(int), cudaMemcpyHostToDevice);
cudaMemcpy(d_c, c, SIZE *sizeof(int), cudaMemcpyHostToDevice);
VectorAdd<<< 1, SIZE >>>(d_a, d_b, d_c, SIZE);
cudaMemcpy(c, d_c, SIZE * sizeof(int), cudaMemcpyDeviceToHost);
for(int i = 0 ; i < 10 ; ++i)
{
printf("C[%d] = %d\n", i, c[i]);
}
free(a);
free(b);
free(c);
cudaFree(d_a);
cudaFree(d_b);
cudaFree(d_c);
return 0;
}
The output on the console is this :
c[0] = 0 , c[1] = 0 , c[2] = 0 , c[3] = 0 , c[4] = 0 ....
Why is that it should be :
c[0] = 0 ; c[1] = 2 ; c[2] = 4 ....

Answer 1

In your case the problem depends on your used gpu. Your kernel is launched with 1024 threads per block. Since your gpu is of compute capability 1.x only 512 or 768 threads per block are supported. A detailed list can be found in the official programming guide.
Because you didn't use proper cuda error checking, you weren't possible to get the error returned by the cuda runtime api. A good guide for cuda error checking is given by #talonmies in this SO answer/question.