Cuda 全局函数的工作速度比主机慢

Question

我有两个空，它们在做同样的事情：向量乘以数字。 一种是简单的 c void，另一种是全局的。 但是在测量时间后，我发现常规 c 函数的运行速度比全局函数快得多。 他们来了：

#define N 1000
__global__ void VectorOnNumber(double *vector1, double number, double *resultVector){
    int tid = threadIdx.x + blockIdx.x*blockDim.x;
    if(tid < N){
        resultVector[tid] = vector1[tid]*number;
    }
}

void von(double vec[N], double n, double res[N]){
    for(int i = 0; i < N; i++){
        res[i] = vec[i]*n;
    }
}

int main(){
    double x[N], u[N];
    double *pointerToVector = (double *)x, *pointerToU = (double *)u;
    
    for(int i = 0; i < N; i++){
        x[i] = i*i;
        v[i] = i+i;
    }

    clock_t start = clock();
    von(x, 10, u);
    clock_t end = clock();
    float seconds = (float)(end - start) / CLOCKS_PER_SEC;
    printf("%f\n", seconds); // 0.03 ms
    
    cudaMalloc(&pointerToVector, N*sizeof(double));
    cudaMalloc(&pointerToU, N*sizeof(double));
    cudaMemcpy(pointerToVector, x, N*sizeof(double), cudaMemcpyHostToDevice);
    cudaMemcpy(pointerToU, u, N*sizeof(double), cudaMemcpyHostToDevice);

    cudaEvent_t start, stop;
    float elapsedTime;
    cudaEventCreate(&start);
    cudaEventRecord(start,0);
    VectorOnNumber<<<N, 1>>>(pointerToVector, 10, pointerToVector);
    cudaEventCreate(&stop);
    cudaEventRecord(stop,0);
    cudaEventSynchronize(stop);
    cudaEventElapsedTime(&elapsedTime, start,stop);
    printf(%f ms\n" ,elapsedTime);//gives 0.08ms
     

    cudaFree(pointerToVector);
    cudaFree(pointerToU);
    return 0;
}

为什么会发生？ 先感谢您。

Answer 1

有很多缺陷：不使用pointerToU ，如v ，你不使用dim3 grid,threads 。 无论如何，我知道只有一个线程有N个块，因此您的内核无法从合并内存访问中受益，这可能是 cuda 版本比 cpu 版本慢的主要原因尝试

VectorOnNumber<<<N/32+1,32>>>(pointerToVector, 10, pointerToVector);

Answer 2

这是我的代码：GPU 内核：

void VectorOnNumber(double *vector1, double number, double *resultVector,int N)
{
    dim3 grid(N/256+1),threads(256);

    VectorOnNumber_K<<<grid,threads>>>(vector1, number, resultVector,N);
}

__global__
void VectorOnNumber_K(double *vector1, double number, double *resultVector,int N)
{
    int tid = threadIdx.x + blockIdx.x*blockDim.x;
    if(tid < N){
        resultVector[tid] = vector1[tid]*number;
    }
}


void VectorOnNumberf(float *vector1, float number, float *resultVector,int N)
{
    dim3 grid(N/256+1),threads(256);

    VectorOnNumberf_K<<<grid,threads>>>(vector1, number, resultVector,N);
}

__global__
void VectorOnNumberf_K(float *vector1, float number, float *resultVector,int N)
{
    int tid = threadIdx.x + blockIdx.x*blockDim.x;
    if(tid < N){
        resultVector[tid] = vector1[tid]*number;
    }
}

中央处理器 ：

void Stack_von(double *vec, double n, double *res,int N)
{
int i;
    for(i = 0; i < N; i++){
        res[i] = vec[i]*n;
    }
}
void Stack_vonf(float *vec, float n, float *res,int N)
{
int i;
    for(i = 0; i < N; i++){
        res[i] = vec[i]*n;
    }
}

全面测试：

void Stack()
{
int i,N;
double *x,*u,*dx,*du;
float  *fx,*fu,*dfx,*dfu;

    N=1000000;
    x=new double[N];
    u=new double[N];
    fx=new float[N];
    fu=new float[N];

    
    for(i = 0; i < N; i++){
        x[i] = i*i;
        fx[i] = i*i;
    }

    // cpu
    printf("start\n");
    clock_t start = clock();
    for(int k=0; k < 1000; k++) Stack_von(x, 10, u,N);
    clock_t end = clock();
    float seconds = (float)(end - start) / CLOCKS_PER_SEC;
    printf("host double %f ms\n", seconds); 

    start = clock();
    for(int k=0; k < 1000; k++) Stack_vonf(fx, 10, fu,N);
    end = clock();
    seconds = (float)(end - start) / CLOCKS_PER_SEC;
    printf("host float  %f ms\n", seconds); // 0.03 ms
    
    // gpu
    cudaMalloc(&dfx, N*sizeof(float));
    cudaMalloc(&dfu, N*sizeof(float));
    cudaMemcpy(dfx, fx, N*sizeof(float), cudaMemcpyHostToDevice);
    cudaMalloc(&dx, N*sizeof(double));
    cudaMalloc(&du, N*sizeof(double));
    cudaMemcpy(dx, x, N*sizeof(double), cudaMemcpyHostToDevice);

    cudaEvent_t dstart, dstop;
    float elapsedTime;
    cudaEventCreate(&dstart);
    cudaEventCreate(&dstop);

    cudaEventRecord(dstart,0);
    VectorOnNumber(dx, 10, du,N);
    cudaEventRecord(dstop,0);
    cudaEventSynchronize(dstop);
    cudaEventElapsedTime(&elapsedTime, dstart,dstop);
    printf("device double %f ms\n" ,elapsedTime);

    cudaEventRecord(dstart,0);
    VectorOnNumberf(dfx, 10, dfu,N);
    cudaEventRecord(dstop,0);
    cudaEventSynchronize(dstop);
    cudaEventElapsedTime(&elapsedTime, dstart,dstop);
    printf("device float  %f ms\n" ,elapsedTime);
     

    cudaFree(dx);
    cudaFree(du);
    cudaFree(dfx);
    cudaFree(dfu);
    delete [] x;
    delete [] u;
    delete [] fx;
    delete [] fu;
}

结果主机双:1.35ms 浮点 0.45ms 设备双 0.067ms 浮点 0.037ms

设备（GTX1080）比主机（XEON 3.50GHz 8 核）快 10 倍我将 N 设置为 10^6 以使其可测量

但是如果每个块只有一个线程，设备上的时间为 1.37 毫秒！