How to write CUDA global function for this? - cuda

Question

I want to convert the following function into CUDA.
void fun()
{
for(i = 0; i < terrainGridLength; i++)
{
for(j = 0; j < terrainGridWidth; j++)
{
//CODE of function
}
}
}
I wrote the function like this :
__global__ void fun()
{
int i = blockIdx.x * blockDim.x + threadIdx.x;
int j = blockIdx.y * blockDim.y + threadIdx.y;
if((i < terrainGridLength)&&(j<terrainGridWidth))
{
//CODE of function
}
}
I declared both terrainGridLength and terrainGridWidth as constants and assigned value 120 to both. And I am calling function like
fun<<<30,500>>>()
But i am not getting correct output.
Is the code which i wrote is correct?.I didn't understood much about the parellel execution of the code.Please explain me how the code will work and correct me if i done any mistakes.

Answer 1

You use y dimension which means you are using 2D array threads, so you cannot invoke the kernel with only:
int numBlock = 30;
int numThreadsPerBlock = 500;
fun<<<numBlock,numThreadsPerBlock>>>()
The invocation should be: (Note that now Blocks have 2D Threads)
dim3 dimGrid(GRID_SIZE, GRID_SIZE); // 2D Grids with size = GRID_SIZE*GRID_SIZE
dim3 dimBlocks(BLOCK_SIZE, BLOCK_SIZE); //2D Blocks with size = BLOCK_SIZE*BLOCK_SIZE
fun<<<dimGrid, dimBlocks>>>()
Refer to CUDA Programming Guide for further info, and also if you want to do 2D array or 3D, you better use cudaMalloc3D or cudaMallocPitch
As of your code, I think this would work (but I haven't tried though, hope you can grab the idea with this):
//main
dim3 dimGrid(1, 1); // 2D Grids with size = 1
dim3 dimBlocks(Width, Height); //2D Blocks with size = Height*Width
fun<<<dimGrid, dimBlocks>>>(Width, Height)
//kernel
__global__ void fun(int Width, int Height)
{
int i = blockIdx.x * blockDim.x + threadIdx.x;
int j = blockIdx.y * blockDim.y + threadIdx.y;
if((i < Width)&&(j<Height))
{
//CODE of function
}
}