Cuda program not working - cuda

Question

i'm a beginner in cuda programming. I'm trying an own easy code but it's not working and I don't know what else to do.
My code:
#include <mpi.h>
#include <cuda.h>
#include <stdio.h>
#include <sys/wait.h>
// Prototypes
__global__ void helloWorld(char*);
__device__ int getGlobalIdx_2D_2D();
// Host function
int main(int argc, char** argv)
{
unsigned int i, N, gridX, gridY, blockX, blockY;
N = 4096000;
char *str = (char *) malloc(N*sizeof(char));
for(i=0; i < N; i++) str[i]='c';
MPI_Init (&argc, &argv);
char *d_str;
size_t size = (size_t) N*sizeof(char);
cudaMalloc((void**)&d_str, size);
cudaMemcpy(d_str, str, size, cudaMemcpyHostToDevice);
gridX = 100;
gridY = 10;
blockX = blockY = 64;
dim3 dimGrid(gridX, gridY); // 4096 chars per block
dim3 dimBlock(blockX, blockY); // one thread per character, 2D
printf("dimGrid(%d, %d)\t", gridX, gridY);
printf("dimBlock(%d, %d)\t", blockX, blockY);
helloWorld<<< dimGrid, dimBlock >>>(d_str);
cudaMemcpy(str, d_str, size, cudaMemcpyDeviceToHost);
cudaThreadSynchronize();
MPI_Barrier (MPI_COMM_WORLD);
cudaFree(d_str);
printf("\nRes:\n");
for(i = 0; i < N; i++) printf("\t[%u] %c\n", i, str[i]);
MPI_Finalize ();
free(str);
return 0.0;
}
// Device kernel
__global__ void helloWorld(char* str)
{
// determine where in the thread grid we are
int pos = getGlobalIdx_2D_2D();
if (pos % 2 == 0) str[pos] -= 2;
else str[pos] += 8;
}
__device__ int getGlobalIdx_2D_2D()
{
int blockId = blockIdx.x + blockIdx.y * gridDim.x;
int threadId = blockId * (blockDim.x * blockDim.y) +
(threadIdx.y * blockDim.x) + threadIdx.x;
return threadId;
}
My desired output is: jajajajajajaja... x4096000
I've read that '%' operation is not efficient, but I don't think is the problem there.
Thanks!

Answer 1

You are performing absolutely no CUDA error checking, it is really beneficial to do so. Once you enable it you can find that block dimensions 64 x 64 are invalid as it results into 4096 threads within one block, which is not a valid configuration.