cuda 原子操作案例

让10000 去增加同一个数组元素

#include 

#define NUM_THREADS 1000000
#define ARRAY_SIZE  100

#define BLOCK_WIDTH 1000

void print_array(int *array, int size)
{
     
    printf("{ ");
    for (int i = 0; i < size; i++)  {
      printf("%d ", array[i]); }
    printf("}\n");
}

__global__ void increment_naive(int *g)
{
     
	int i = blockIdx.x * blockDim.x + threadIdx.x; 

	i = i % ARRAY_SIZE;  
	g[i] = g[i] + 1;
}

__global__ void increment_atomic(int *g)
{
     
	int i = blockIdx.x * blockDim.x + threadIdx.x; 

	i = i % ARRAY_SIZE;  
	atomicAdd(& g[i], 1);
}

int main(int argc,char **argv)
{
        
    GpuTimer timer;
    printf("%d total threads in %d blocks writing into %d array elements\n",
           NUM_THREADS, NUM_THREADS / BLOCK_WIDTH, ARRAY_SIZE);

    int h_array[ARRAY_SIZE];
    const int ARRAY_BYTES = ARRAY_SIZE * sizeof(int);
 
    int * d_array;
    cudaMalloc((void **) &d_array, ARRAY_BYTES);
    cudaMemset((void *) d_array, 0, ARRAY_BYTES); 

    timer.Start();
    increment_naive<<<NUM_THREADS/BLOCK_WIDTH, BLOCK_WIDTH>>>(d_array);
    timer.Stop();
    
    cudaMemcpy(h_array, d_array, ARRAY_BYTES, cudaMemcpyDeviceToHost);
    print_array(h_array, ARRAY_SIZE);
    printf("Time elapsed = %g ms\n", timer.Elapsed());
 
    cudaFree(d_array);
    return 0;
}

运行无原子操作的increment_naive核函数

运行有原子操作的increment_atomic核函数

显而易见，在没有原子操作时，线程不等待之前的线程完成工作，g[i]被后启动的线程错误访问
导致最后的加和结果不正确