Question

I’m having a bit of trouble understanding how to send a 2D array to Cuda. I have a program that parses a large file with a 30 data points on each line. I read about 10 rows at a time and then create a matrix for each line and items(so in my example of 10 rows with 30 data points, it would be int list[10][30]; My goal is to send this array to my kernal and have each block process a row(I have gotten this to work perfectly in normal C, but Cuda has been a bit more challenging).

Here’s what I’m doing so far but no luck(note: sizeofbucket = rows, and sizeOfBucketsHoldings = items in row…I know I should win a award for odd variable names):

    int list[sizeOfBuckets][sizeOfBucketsHoldings]; //this is created at the start of the file and I can confirmed its filled with the correct data
#define sizeOfBuckets 10 //size of buckets before sending to process list
#define sizeOfBucketsHoldings  30
    //Cuda part
                //define device variables
                int *dev_current_list[sizeOfBuckets][sizeOfBucketsHoldings];
                //time to malloc the 2D array on device
                size_t pitch;
                cudaMallocPitch((int**)&dev_current_list,  (size_t *)&pitch, sizeOfBucketsHoldings * sizeof(int), sizeOfBuckets);

                //copy data from host to device
                cudaMemcpy2D( dev_current_list, pitch, list, sizeOfBuckets * sizeof(int), sizeOfBuckets * sizeof(int), sizeOfBucketsHoldings * sizeof(int),cudaMemcpyHostToDevice );

                process_list<<<count,1>>> (sizeOfBuckets, sizeOfBucketsHoldings, dev_current_list, pitch);
                //free memory of device
                cudaFree( dev_current_list );


    __global__ void process_list(int sizeOfBuckets, int sizeOfBucketsHoldings, int *current_list, int pitch) {
        int tid = blockIdx.x;
        for (int r = 0; r < sizeOfBuckets; ++r) {
            int* row = (int*)((char*)current_list + r * pitch);
            for (int c = 0; c < sizeOfBucketsHoldings; ++c) {
                 int element = row[c];
            }
        }

The error I’m getting is:

main.cu(266): error: argument of type "int *(*)[30]" is incompatible with parameter of type "int *"
1 error detected in the compilation of "/tmp/tmpxft_00003f32_00000000-4_main.cpp1.ii".

line 266 is the kernel call process_list<<<count,1>>> (count, countListItem, dev_current_list, pitch); I think the problem is I am trying to create my array in my function as int * but how else can I create it? In my pure C code, I use int current_list[num_of_rows][num_items_in_row] which works but I can’t get the same outcome to work in Cuda.

My end goal is simple I just want to get each block to process each row(sizeOfBuckets) and then have it loop through all items in that row(sizeOfBucketHoldings). I orginally just did a normal cudamalloc and cudaMemcpy but it wasn’t working so I looked around and found out about MallocPitch and 2dcopy(both of which were not in my cuda by example book) and I have been trying to study examples but they seem to be giving me the same error(I’m currently reading the CUDA_C programming guide found this idea on page22 but still no luck). Any ideas? or suggestions of where to look?

Edit:
To test this, I just want to add the value of each row together(I copied the logic from the cuda by example array addition example).
My kernel:

__global__ void process_list(int sizeOfBuckets, int sizeOfBucketsHoldings, int *current_list, size_t pitch, int *total) {
    //TODO: we need to flip the list as well
    int tid = blockIdx.x;
    for (int c = 0; c < sizeOfBucketsHoldings; ++c) {
        total[tid] = total + current_list[tid][c];
    }
}

Here’s how I declare the total array in my main:

int *dev_total;
cudaMalloc( (void**)&dev_total, sizeOfBuckets * sizeof(int) );

Answer 1

You have some mistakes in your code.

• Then you copy host array to device you should pass one dimensional host pointer.See the function signature.
• You don’t need to allocate static 2D array for device memory. It creates static array in host memory then you recreate it as device array. Keep in mind it must be one dimensional array, too. See this function signature.

This example should help you with memory allocation:

__global__ void process_list(int sizeOfBucketsHoldings, int* total, int* current_list, int pitch)
{
    int tid = blockIdx.x;
    total[tid] = 0;
    for (int c = 0; c < sizeOfBucketsHoldings; ++c)
    {
        total[tid] += *((int*)((char*)current_list + tid * pitch) + c);
    }
}

int main()
{
    size_t sizeOfBuckets         = 10;
    size_t sizeOfBucketsHoldings = 30;

    size_t width = sizeOfBucketsHoldings * sizeof(int);//ned to be in bytes
    size_t height = sizeOfBuckets;

    int* list = new int [sizeOfBuckets * sizeOfBucketsHoldings];// one dimensional
    for (int i = 0; i < sizeOfBuckets; i++)
        for (int j = 0; j < sizeOfBucketsHoldings; j++)
            list[i *sizeOfBucketsHoldings + j] = i;

    size_t pitch_h = sizeOfBucketsHoldings * sizeof(int);// always in bytes

    int* dev_current_list;
    size_t pitch_d;
    cudaMallocPitch((int**)&dev_current_list, &pitch_d, width, height);

    int *test;
    cudaMalloc((void**)&test, sizeOfBuckets * sizeof(int));
    int* h_test = new int[sizeOfBuckets];

    cudaMemcpy2D(dev_current_list, pitch_d, list, pitch_h, width, height, cudaMemcpyHostToDevice);

    process_list<<<10, 1>>>(sizeOfBucketsHoldings, test, dev_current_list, pitch_d);
    cudaDeviceSynchronize();

    cudaMemcpy(h_test, test, sizeOfBuckets * sizeof(int), cudaMemcpyDeviceToHost);

    for (int i = 0; i < sizeOfBuckets; i++)
        printf("%d %d\n", i , h_test[i]);
    return 0;
}

To access your 2D array in kernel you should use pattern base_addr + y * pitch_d + x.
WARNING: the pitvh allways in bytes. You need to cast your pointer to byte*.