Cuda性能测量 - 经过的时间返回零

Question

我编写了一些内核函数，并想知道处理这些函数需要多少毫秒。

using namespace std;
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#define N 8000

void fillArray(int *data, int count) {
    for (int i = 0; i < count; i++)
        data[i] = rand() % 100;
}

__global__ void add(int* a, int *b) {
    int add = 0;

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

__global__ void subtract(int* a, int *b) {
    int subtract = 0;

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        subtract = a[tid] - b[tid];
    }
}

__global__ void multiply(int* a, int *b) {
    int multiply = 0;

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        multiply = a[tid] * b[tid];
    }
}

__global__ void divide(int* a, int *b) {
    int divide = 0;

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        divide = a[tid] / b[tid];
    }
}

__global__ void modu(int* a, int *b) {
    int modulus = 0;

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        modulus = a[tid] % b[tid];
    }
}

__global__ void neg(int *data) {

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        data[tid] = -data[tid];
    }
}

float duration(int *devA, int *devB, int blocksPerGrid, int threadsPerBlock) {

    cudaEvent_t start, stop;
    float elapsedTime;

    cudaEventCreate(&start);
    cudaEventCreate(&stop);
    cudaEventRecord(start, 0);

    add<<<blocksPerGrid, threadsPerBlock>>>(devA, devB);
    subtract<<<blocksPerGrid, threadsPerBlock>>>(devA, devB);
    multiply<<<blocksPerGrid, threadsPerBlock>>>(devA, devB);
    divide<<<blocksPerGrid, threadsPerBlock>>>(devA, devB);
    modu<<<blocksPerGrid, threadsPerBlock>>>(devA, devB);
    neg<<<blocksPerGrid, threadsPerBlock>>>(devA);
    neg<<<blocksPerGrid, threadsPerBlock>>>(devB);

    cudaEventRecord(stop, 0);
    cudaEventSynchronize(stop);
    cudaEventElapsedTime(&elapsedTime, start, stop);

    cudaEventDestroy(start);
    cudaEventDestroy(stop);

    return elapsedTime;
}

int main(void) {

    int a[N], b[N];
    float dur = 0;



    int *devA, *devB;

    cudaMalloc((void**) &devA, N * sizeof(int));
    cudaMalloc((void**) &devB, N * sizeof(int));

    fillArray(a, N);
    fillArray(b, N);

    cudaMemcpy(devA, a, N * sizeof(int), cudaMemcpyHostToDevice);
    cudaMemcpy(devA, b, N * sizeof(int), cudaMemcpyHostToDevice);



    dur = duration(a, b, N, 1);

    cout << "Global memory version:\n";
    cout << "Process completed in " << dur;
    cout << " for a data set of " << N << " integers.";

    return 0;
}

Milisecond总是返回零。为什么？我在这里缺少什么？如果我从持续时间持续时间函数中删除了neg函数。它返回0.15687毫秒。我认为处理这些功能的数量很少。该计划有什么问题？

编辑后，我这样做了：

using namespace std;
#include <iostream>
#include <stdio.h>
#include <stdlib.h>

const int N = 8000;

void fillArray(int *data, int count) {
    for (int i = 0; i < count; i++)
        data[i] = rand() % 100;
}

__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];
    }
}

__global__ void subtract(int* a, int *b, int *c) {

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        c[tid] = a[tid] - b[tid];
    }
}

__global__ void multiply(int* a, int *b, int *c) {

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

__global__ void divide(int* a, int *b, int *c) {

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        c[tid] = a[tid] / b[tid];
    }
}

__global__ void modu(int* a, int *b, int *c) {

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        c[tid] = a[tid] % b[tid];
    }
}

__global__ void neg(int *data, int *c) {

    int tid = threadIdx.x + blockIdx.x * blockDim.x;
    if (tid < N) {
        c[tid] = -data[tid];
    }
}

float duration(int *devA, int *devB, int *devC, int blocksPerGrid, int threadsPerBlock) {

    cudaEvent_t start, stop;
    float elapsedTime;

    cudaEventCreate(&start);
    cudaEventCreate(&stop);
    cudaEventRecord(start, 0);

    double hArrayC[N];

    add<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    subtract<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    multiply<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    divide<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    modu<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    neg<<<blocksPerGrid, threadsPerBlock>>>(devA,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    neg<<<blocksPerGrid, threadsPerBlock>>>(devB,devC);
    cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);

    cudaEventRecord(stop, 0);
    cudaEventSynchronize(stop);
    cudaEventElapsedTime(&elapsedTime, start, stop);

    cudaEventDestroy(start);
    cudaEventDestroy(stop);

    return elapsedTime;
}

int main(void) {

    int a[N], b[N],c[N];
    float dur = 0;

    int *devA, *devB,*devC;

    cudaMalloc((void**) &devA, N * sizeof(int));
    cudaMalloc((void**) &devB, N * sizeof(int));
    cudaMalloc((void**) &devC, N * sizeof(int));

    fillArray(a, N);
    fillArray(b, N);

    cudaMemcpy(devA, a, N * sizeof(int), cudaMemcpyHostToDevice);
    cudaMemcpy(devB, b, N * sizeof(int), cudaMemcpyHostToDevice);
    cudaMemcpy(devC, c, N * sizeof(int), cudaMemcpyHostToDevice);




    dur = duration(devA, devB, devC,N, 1);

    cout << "Global memory version:\n";
    cout << "Process completed in " << dur;
    cout << " for a data set of " << N << " integers.";



    cudaFree(devA);
    cudaFree(devB);
    return 0;
}

Answer 1

Cuda任务正在设备上运行而不会阻塞CPU线程。因此，cuda调用仅在您尝试从设备内存获取计算数据时才会阻止，并且尚未准备就绪。或者，当您使用cudaDeviceSyncronize()调用显式地将CPU线程与GPU同步时。如果要测量计算时间，则需要在停止计时器之前进行同步。

如果您对计算内存复制时间感兴趣，则需要在计算开始后和复制计时器开始之前进行同步，否则计算时间将显示为复制时间。

您可以使用cuda SDK中包含的探查器来衡量所有cuda次来电的时间。

Answer 2

您的内核没有做任何事情，因为您只将结果存储在寄存器中。编译时，会收到一些警告：

  

kernel.cu（13）：警告：变量“add”已设置但从未使用过

此外，如果您想看一些更好的时间，请使用NVIDIA的分析器：nvprof（CLI）或nvvp（GUI）。

  

$ nvprof ./kernel

======== NVPROF is profiling kernel...
======== Command: kernel
Global memory version: Process completed in 0 for a data set of 8000 integers.
======== Profiling result:
  Time(%)     Time   Calls       Avg       Min       Max  Name
  100.00   18.46us       2    9.23us    6.02us   12.45us  [CUDA memcpy HtoD]
    0.00       0ns       1       0ns       0ns       0ns  multiply(int*, int*)
    0.00       0ns       1       0ns       0ns       0ns  add(int*, int*)
    0.00       0ns       1       0ns       0ns       0ns  modu(int*, int*)
    0.00       0ns       2       0ns       0ns       0ns  neg(int*)
    0.00       0ns       1       0ns       0ns       0ns  subtract(int*, int*)
    0.00       0ns       1       0ns       0ns       0ns  divide(int*, int*)

您还在每个网格使用N个块，每个块使用1个线程。您应该考虑阅读this question的答案。

更新

关于向量添加（以及其他简单操作）本身，您应该研究CUDA SDK的vectorAdd sample，或使用Thrust。第一个选项将教您如何使用CUDA，第二个选项将向您展示您可以使用Thrust执行的高级操作。如果我是你，我会做两件事。

Answer 3

尝试使用float（或double）变量和数组而不是int来存储所有算术变量和操作。有时候时间间隔太小，整数值总是会舍入到零。