Question

I’m trying to learn CUDA and the following code works OK for the values N<= 16384, but fails for the greater values(Summation check at the end of the code fails, c values are always 0 for the index value of i>=16384).

#include<iostream>
#include"cuda_runtime.h"
#include"../cuda_be/book.h"

#define N (16384)

__global__ void add(int *a,int *b,int *c)
{
    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if(tid<N)
    {
        c[tid] = a[tid] + b[tid];
        tid += blockDim.x * gridDim.x;
    }
}

int main()
{
    int a[N],b[N],c[N];
    int *dev_a,*dev_b,*dev_c;

    //allocate mem on gpu
    HANDLE_ERROR(cudaMalloc((void**)&dev_a,N*sizeof(int)));
    HANDLE_ERROR(cudaMalloc((void**)&dev_b,N*sizeof(int)));
    HANDLE_ERROR(cudaMalloc((void**)&dev_c,N*sizeof(int)));

    for(int i=0;i<N;i++)
    {
        a[i] = -i;
        b[i] = i*i;
    }

    HANDLE_ERROR(cudaMemcpy(dev_a,a,N*sizeof(int),cudaMemcpyHostToDevice));
    HANDLE_ERROR(cudaMemcpy(dev_b,b,N*sizeof(int),cudaMemcpyHostToDevice));
    system("PAUSE");
    add<<<128,128>>>(dev_a,dev_b,dev_c);

    //copy the array 'c' back from the gpu to the cpu

    HANDLE_ERROR( cudaMemcpy(c,dev_c,N*sizeof(int),cudaMemcpyDeviceToHost));

    system("PAUSE");
    bool success = true;
    for(int i=0;i<N;i++)
    {
        if((a[i] + b[i]) != c[i])
        {
            printf("Error in %d: %d + %d != %d\n",i,a[i],b[i],c[i]);
            system("PAUSE");
            success = false;
        }

    }

    if(success) printf("We did it!\n");

    cudaFree(dev_a);
    cudaFree(dev_b);
    cudaFree(dev_c);

    return 0;
}

I think it’s a shared memory related problem, but I can’t come up with a good explanation(Possible lack of knowledge). Could you provide me an explanation and a workaround to run for the values of N greater than 16384. Here is the specs for my GPU:

General Info for device 0
Name: GeForce 9600M GT
Compute capability: 1.1
Clock rate: 1250000
Device copy overlap : Enabled
Kernel Execution timeout : Enabled
Mem info for device 0
Total global mem: 536870912
Total const mem: 65536
Max mem pitch: 2147483647
Texture Alignment 256
MP info about device 0
Multiproccessor count: 4
Shared mem per mp: 16384
Registers per mp: 8192
Threads in warp: 32
Max threads per block: 512
Max thread dimensions: (512,512,64)
Max grid dimensions: (65535,65535,1)

Answer 1

You probably intended to write

while(tid<N)

not

if(tid<N)