CUDA string search in large file, wrong result - parallel-processing

Question

I am working on simple naive string search in CUDA.
I am new in CUDA. It works fine fol smaller files ( aprox. ~1MB ). After I make these files bigger ( ctrl+a ctrl+c several times in notepad++ ), my program's results are higher ( about +1% ) than a
grep -o text file_name | wc -l
It is very simple function, so I don't know what could cause this. I need it to work with larger files ( ~500MB ).
Kernel code ( gpuCount is a __device__ int global variable ):
__global__ void stringSearchGpu(char *data, int dataLength, char *input, int inputLength){
int id = blockDim.x*blockIdx.x + threadIdx.x;
if (id < dataLength)
{
int fMatch = 1;
for (int j = 0; j < inputLength; j++)
{
if (data[id + j] != input[j]) fMatch = 0;
}
if (fMatch)
{
atomicAdd(&gpuCount, 1);
}
}
}
This is calling the kernel in main function:
int blocks = 1, threads = fileSize;
if (fileSize > 1024)
{
blocks = (fileSize / 1024) + 1;
threads = 1024;
}
clock_t cpu_start = clock();
// kernel call
stringSearchGpu<<<blocks, threads>>>(cudaBuffer, strlen(buffer), cudaInput, strlen(input));
cudaDeviceSynchronize();
After this I just copy the result to Host and print it.
Can anyone please help me with this?

Answer 1

First of all, you should always check return values of CUDA functions to check for errors. Best way to do so would be the following:
#define gpuErrchk(ans) { gpuAssert((ans), __FILE__, __LINE__); }
inline void gpuAssert(cudaError_t code, const char *file, int line, bool abort=true)
{
if (code != cudaSuccess)
{
fprintf(stderr,"GPUassert: %s %s %d\n", cudaGetErrorString(code), file, line);
if (abort) exit(code);
}
}
Wrap your CUDA calls, such as:
gpuErrchk(cudaDeviceSynchronize());
Second, your kernel accesses out of bounds memory. Suppose, dataLength=100, inputLength=7 and id=98. In your kernel code:
if (id < dataLength) // 98 is less than 100, so condition true
{
int fMatch = 1;
for (int j = 0; j < inputLength; j++) // j runs from [0 - 6]
{
// if j>1 then id+j>=100, which is out of bounds, illegal operation
if (data[id + j] != input[j]) fMatch = 0;
}
Change the condition to something like:
if (id < dataLength - inputLength)