Cuda not giving correct answer when array size is larger than 1,000,000

Question

I have wrote a simple sum reduction code which seems to work just fine until i increase array size to 1 million what can be the problem.

#define BLOCK_SIZE 128
#define ARRAY_SIZE 10000

cudaError_t addWithCuda(const long *input, long *output, int totalBlocks, size_t size);

__global__ void sumKernel(const long *input, long *output)
{
    int tid = threadIdx.x;
    int bid = blockDim.x * blockIdx.x;

    __shared__ long data[BLOCK_SIZE];

    if(bid+tid < ARRAY_SIZE)
           data[tid] = input[bid+tid];
    else
           data[tid] = 0;

     __syncthreads();

    for(int i = BLOCK_SIZE/2; i >= 1; i >>= 1)
    {
        if(tid < i)
        data[tid] += data[tid + i];
        __syncthreads(); 
    }

    if(tid == 0)
        output[blockIdx.x] = data[0];
}

int main()
{    
    int totalBlocks = ARRAY_SIZE/BLOCK_SIZE;

    if(ARRAY_SIZE % BLOCK_SIZE != 0)
        totalBlocks++;

    long *input = (long*) malloc(ARRAY_SIZE * sizeof(long) );
    long *output = (long*) malloc(totalBlocks * sizeof(long) );

    for(int i=0; i<ARRAY_SIZE; i++)
    {
        input[i] = i+1 ;
    }
// Add vectors in parallel.
        cudaError_t cudaStatus = addWithCuda(input, output, totalBlocks, ARRAY_SIZE);
        if (cudaStatus != cudaSuccess) {
             fprintf(stderr, "addWithCuda failed!");
             return 1;
        }

    long ans = 0;
    for(int i =0 ; i < totalBlocks ;i++)
    {
        ans = ans + output[i];
    }

    printf("Final Ans : %ld",ans);

// cudaDeviceReset must be called before exiting in order for profiling and
// tracing tools such as Nsight and Visual Profiler to show complete traces.
cudaStatus = cudaDeviceReset();
        if (cudaStatus != cudaSuccess) {
              fprintf(stderr, "cudaDeviceReset failed!");
              return 1;
         }

     getchar();

      return 0;
}

     // Helper function for using CUDA to add vectors in parallel.
     cudaError_t addWithCuda(const long *input, long *output, int totalBlocks, size_t size)
     {
          long *dev_input = 0;
          long *dev_output = 0;

          cudaError_t cudaStatus;

// Choose which GPU to run on, change this on a multi-GPU system.
           cudaStatus = cudaSetDevice(0);
         if (cudaStatus != cudaSuccess) {
             fprintf(stderr, "cudaSetDevice failed!  Do you have a CUDA-capable GPU installed?");
             goto Error;
     }

// Allocate GPU buffers for two vectors (one input, one output)    .

     cudaStatus = cudaMalloc((void**)&dev_input, size * sizeof(long));
     if (cudaStatus != cudaSuccess) {
         fprintf(stderr, "cudaMalloc failed!");
         goto Error;
         }

cudaStatus = cudaMalloc((void**)&dev_output, totalBlocks * sizeof(long));
if (cudaStatus != cudaSuccess) {
    fprintf(stderr, "cudaMalloc failed!");
    goto Error;
}

// Copy input vectors from host memory to GPU buffers.
cudaStatus = cudaMemcpy(dev_input, input, size * sizeof(long), cudaMemcpyHostToDevice);
if (cudaStatus != cudaSuccess) {
    fprintf(stderr, "cudaMemcpy failed!");
    goto Error;
}

cudaStatus = cudaMemcpy(dev_output, output, (totalBlocks) * sizeof(long), cudaMemcpyHostToDevice);
if (cudaStatus != cudaSuccess) {
    fprintf(stderr, "cudaMemcpy failed!");
    goto Error;
}

// Launch a kernel on the GPU with one thread for each element.
sumKernel<<<totalBlocks, BLOCK_SIZE>>>(dev_input, dev_output);

// cudaDeviceSynchronize waits for the kernel to finish, and returns
// any errors encountered during the launch.
cudaStatus = cudaDeviceSynchronize();
if (cudaStatus != cudaSuccess) {
    fprintf(stderr, "cudaDeviceSynchronize returned error code %d after launching addKernel!\n", cudaStatus);
    goto Error;
}

// Copy output vector from GPU buffer to host memory.
cudaStatus = cudaMemcpy(output, dev_output, totalBlocks * sizeof(long), cudaMemcpyDeviceToHost);
if (cudaStatus != cudaSuccess) {
    fprintf(stderr, "cudaMemcpy failed!");
    goto Error;
}

Error:
cudaFree(dev_input);
cudaFree(dev_output);

return cudaStatus;
}

and just for the reference if it has to do somthing with my GPU device, my GPU is GTXX 650ti. and here is the info about GPU:

Maximum number of threads per multiprocessor: 2048

Maximum number of threads per block: 1024

Maximum sizes of each dimension of a block: 1024 x 1024 x 64

Maximum sizes of each dimension of a grid: 2147483647 x 65535 x 65535

Maximum memory pitch: 2147483647 bytes

Texture alignment: 512 bytes

Answer 1

Actually the answer =could not fit in long as well so after using long double for datatypes this issue was resolved. Thanks all!

Answer 2

One problem in your code is that your last cudaMemcpy is not set up correctly:

cudaMemcpy(output, dev_output, totalBlocks * sizeof(int), cudaMemcpyDeviceToHost);

All of your data is long data so you should be copying using sizeof(long) not sizeof(int)

Another problem in your code is using the wrong printf format identifier for a long datatype:

printf("\n %d \n",output[i]);

use something like this instead:

printf("\n %ld \n",output[i]);

You may also have a problem with a large block count if you are not compiling for sm_30 architecture. In that case, proper cuda error checking would identify the problem.

Answer 3

You don't check for errors after sumKernel<<<totalBlocks, BLOCK_SIZE>>>(dev_input, dev_output);. Normally, if you would check for the last occured error it should give the error invalid configuration argument. Try adding the following after the sumKernel line.

cudaStatus = cudaGetLastError();
if (cudaStatus != cudaSuccess) {
    printf(stderr, "sumKernel failed: %s\n", cudaGetErrorString(cudaStatus));
    goto Error;
}

See this question for more information about the error.