OpenCL入门：(三：GPU内存结构和性能优化)

如果我们需要优化kernel程序，我们必须知道一些GPU的底层知识，本文简单介绍一下GPU内存相关和线程调度知识，并且用一个小示例演示如何简单根据内存结构优化。

一、GPU总线寻址和合并内存访问

假设X指向一个32位整数数组的指针，数组首地址是0x00001232，那么一个线程需要访问第0个成员时是也许是如下访问的：

int tmp = X[0]

假设内存总线宽度是256位，内存访问时必须和总线宽度对齐，所以内存只能访问0x00000020,0x00000040这种地址(0x20=256位)，如果要访问0x00001232，那么内存必须同时获取0x00001220-0x0000123f的数据，一次获取了32字节的数据，但是我们有用的只有4字节，这就造成了28个字节的浪费。

事实上，GPU为了利用总线带宽，它会合并内存访问，尽量将多个线程读取内存合并到一起进行访问，例如我们有16个线程，每个线程访问4字节，总共需要访问0x00001232-0x00001272，如果不合并内存访问，那么他需要访问内存16次，每次浪费28字节空间；如果合并内存访问，它第一次访问0x00001220-0x0000123f，第二次访问0x00001240-0x0000125f，第三次访问0x00001260-0x0000133f，总共只需要访问三次，这样可以大大减少内存访问次数。优化kernel的性能。

二、性能优化

考虑一个矩阵相乘的问题，一个MXP的矩阵A，和一个P*N的矩阵B相乘得到MXN的C矩阵，在CPU中计算的代码入下：

#define M 1024
#define P 512
#define N 2048

void RunAsCpu(
    const float *A,
    const float *B,
    float* C)
{
    for (int i = 0; i < M; i++)
    {
        for (int j = 0; j < N; j++)
        {
            C[i*N + j] = 0.0;
            for (int k = 0; k < P; k++)
            {
                C[i*N + j] += A[i*P + k] * B[k*N + j];
            }
        }
    }
}

如果使用GPU运行，那么通过降维操作，创建M*N个线程，第一个维度大小的M，第二个维度大小为N，kernel中代码可能如下：

__kernel void RunAsGpu_1(
    __global  float *A,
    __global  float *B,
    int M,
    int N,
    int P,
    __global float* C)
{
    int x = get_global_id(0);
    int y = get_global_id(1);
    float sum = 0;
    for(int i = 0;i<P;i++)
    {
        sum += A[x*P + i]*B[i*N + y];
    }
    C[x*N + y] = sum;
}

此时，如果思考一下，可能会发现，还有第二种方案，即第一个维度大小的N，第二个维度大小为M

__kernel void RunAsGpu_2(
    __global  float *A,
    __global  float *B,
    int M,
    int N,
    int P,
    __global float* C)
{
    int x = get_global_id(0);
    int y = get_global_id(1);
    float sum = 0;
    for(int i = 0;i<P;i++)
    {
        sum += A[y*P + i]*B[i*N + x];
    }
    C[y*N + x] = sum;
}

这两个kernel运行结果是一样的，那运行效率有什么不同呢？host文件用如下代码，然后运行一下看看效果：

#include <iostream>
#include <CL/cl.h>
#include <cassert>
#include <windows.h>
#include <ctime>
using namespace std;

#define M 1024
#define P 512
#define N 2048

void RunAsCpu(
    const float *A,
    const float *B,
    float* C)
{
    for (int i = 0; i < M; i++)
    {
        for (int j = 0; j < N; j++)
        {
            C[i*N + j] = 0.0;
            for (int k = 0; k < P; k++)
            {
                C[i*N + j] += A[i*P + k] * B[k*N + j];
            }
        }
    }
}

//计时函数
double time_stamp()
{
    LARGE_INTEGER curclock;
    LARGE_INTEGER freq;
    if (
        !QueryPerformanceCounter(&curclock) ||
        !QueryPerformanceFrequency(&freq)
        )
    {
        return -1;
    }

    return double(curclock.QuadPart) / freq.QuadPart;
}
#define OPENCL_CHECK_ERRORS(ERR)        \
    if(ERR != CL_SUCCESS)                  \
    {                                      \
    cerr                                   \
    << "OpenCL error with code " << ERR    \
    << " happened in file " << __FILE__    \
    << " at line " << __LINE__             \
    << ". Exiting...\n";                   \
    exit(1);                               \
    }
int main(int argc, const char** argv)
{
    cl_int error = 0;   // Used to handle error codes
    cl_context context;
    cl_command_queue queue;
    cl_device_id device;

    // 遍历系统中所有OpenCL平台
    cl_uint num_of_platforms = 0;
    // 得到平台数目
    error = clGetPlatformIDs(0, 0, &num_of_platforms);
    OPENCL_CHECK_ERRORS(error);
    cout << "可用平台数: " << num_of_platforms << endl;

    cl_platform_id* platforms = new cl_platform_id[num_of_platforms];
    // 得到所有平台的ID
    error = clGetPlatformIDs(num_of_platforms, platforms, 0);
    OPENCL_CHECK_ERRORS(error);
    //遍历平台，选择一个Intel平台的
    cl_uint selected_platform_index = num_of_platforms;
    for (cl_uint i = 0; i < num_of_platforms; ++i)
    {
        size_t platform_name_length = 0;
        error = clGetPlatformInfo(
            platforms[i],
            CL_PLATFORM_NAME,
            0,
            0,
            &platform_name_length
        );
        OPENCL_CHECK_ERRORS(error);

        // 调用两次，第一次是得到名称的长度
        char* platform_name = new char[platform_name_length];
        error = clGetPlatformInfo(
            platforms[i],
            CL_PLATFORM_NAME,
            platform_name_length,
            platform_name,
            0
        );
        OPENCL_CHECK_ERRORS(error);

        cout << "    [" << i << "] " << platform_name;

        if (
            strstr(platform_name, "Intel") &&
            selected_platform_index == num_of_platforms // have not selected yet
            )
        {
            cout << " [Selected]";
            selected_platform_index = i;
        }

        cout << endl;
        delete[] platform_name;
    }
    if (selected_platform_index == num_of_platforms)
    {
        cerr
            << "没有找到Intel平台\n";
        return 1;
    }
    // Device
    cl_platform_id platform = platforms[selected_platform_index];
    error = clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, &device, NULL);
    OPENCL_CHECK_ERRORS(error)

        // Context
        context = clCreateContext(0, 1, &device, NULL, NULL, &error);
    OPENCL_CHECK_ERRORS(error)

        // Command-queue CL_QUEUE_PROFILING_ENABLE开启才能计时
        queue = clCreateCommandQueue(context, device, CL_QUEUE_PROFILING_ENABLE, &error);
    OPENCL_CHECK_ERRORS(error)

        //下面初始化测试数据(主机数据)
    float* A_h = new float[M*P];
    float* B_h = new float[P*N];
    float* C_h = new float[M*N];
    //srand((unsigned)time(NULL));
    srand(100);
    for (int i = 0; i < M*P; i++)
        A_h[i] = rand() % 50;

    for (int i = 0; i < P*N; i++)
        B_h[i] = rand() % 50;
    //初始化设备数据
    // 标志位表示数据只读，并且从nums1_h和nums2_h复制数据
    cl_mem A_d = clCreateBuffer(context, CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR, sizeof(float)*M*P, A_h, &error);
    OPENCL_CHECK_ERRORS(error)
        cl_mem B_d = clCreateBuffer(context, CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR, sizeof(float)*P*N, B_h, &error);
    OPENCL_CHECK_ERRORS(error)
        cl_mem C_d = clCreateBuffer(context, CL_MEM_WRITE_ONLY, sizeof(float)*M*N, NULL, &error);
    OPENCL_CHECK_ERRORS(error)

    cout << "CPU 运行开始:" << time_stamp() << endl;
    RunAsCpu(A_h, B_h, C_h);
    cout << "CPU 运行结束:" << time_stamp() << endl;

        //读取OpenCLSum.cl文件内容

    FILE* fp = fopen("OpenCLMulMatrix.cl", "rb");
    fseek(fp, 0, SEEK_END);
    size_t src_size = ftell(fp);
    fseek(fp, 0, SEEK_SET);
    const char* source = new char[src_size];
    fread((void*)source, 1, src_size, fp);
    fclose(fp);

    //创建编译运行kernel函数
    cl_program program = clCreateProgramWithSource(context, 1, &source, &src_size, &error);
    OPENCL_CHECK_ERRORS(error)
        delete[] source;

    // Builds the program
    error = clBuildProgram(program, 1, &device, NULL, NULL, NULL);
    OPENCL_CHECK_ERRORS(error)

        // Shows the log
        char* build_log;
    size_t log_size;
    // First call to know the proper size
    clGetProgramBuildInfo(program, device, CL_PROGRAM_BUILD_LOG, 0, NULL, &log_size);
    build_log = new char[log_size + 1];
    // Second call to get the log
    clGetProgramBuildInfo(program, device, CL_PROGRAM_BUILD_LOG, log_size, build_log, NULL);
    build_log[log_size] = '\0';
    cout << build_log << endl;
    delete[] build_log;

    // Extracting the kernel
    cl_kernel run_as_gpu_1 = clCreateKernel(program, "RunAsGpu_1", &error);
    OPENCL_CHECK_ERRORS(error)
    //设置kernel参数
    cl_int M_d = M;
    cl_int P_d = P;
    cl_int N_d = N;
    error = clSetKernelArg(run_as_gpu_1, 0, sizeof(cl_mem), &A_d);
    error |= clSetKernelArg(run_as_gpu_1, 1, sizeof(cl_mem), &B_d);
    error |= clSetKernelArg(run_as_gpu_1, 2, sizeof(int), &M_d);
    error |= clSetKernelArg(run_as_gpu_1, 3, sizeof(int), &N_d);
    error |= clSetKernelArg(run_as_gpu_1, 4, sizeof(int), &P_d);
    error |= clSetKernelArg(run_as_gpu_1, 5, sizeof(cl_mem), &C_d);
    OPENCL_CHECK_ERRORS(error)

        // 启动kernel
    size_t globalws_1[2] = { M,N };
    cl_event ev;
    error = clEnqueueNDRangeKernel(queue, run_as_gpu_1, 2, NULL, globalws_1, NULL, 0, NULL, &ev);
    clFinish(queue);
    OPENCL_CHECK_ERRORS(error)
        //计算kerenl执行时间
    cl_ulong startTime, endTime;
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_START,
        sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_END,
        sizeof(cl_ulong), &endTime, NULL);
    cl_ulong kernelExecTimeNs = endTime - startTime;
    printf("Gpu_1运行时间 :%8.6f ms\n", kernelExecTimeNs*1e-6);

        //取得kernel返回值
    float* gpu_C_1 = new float[M*N];
    clEnqueueReadBuffer(queue, C_d, CL_TRUE, 0, M*N*sizeof(float), gpu_C_1, 0, NULL, NULL);
    assert(memcmp(C_h, gpu_C_1, M*N * sizeof(float)) == 0);

    // Extracting the kernel
    cl_kernel run_as_gpu_2 = clCreateKernel(program, "RunAsGpu_2", &error);
    OPENCL_CHECK_ERRORS(error)
        //设置kernel参数
    error = clSetKernelArg(run_as_gpu_2, 0, sizeof(cl_mem), &A_d);
    error |= clSetKernelArg(run_as_gpu_2, 1, sizeof(cl_mem), &B_d);
    error |= clSetKernelArg(run_as_gpu_2, 2, sizeof(int), &M_d);
    error |= clSetKernelArg(run_as_gpu_2, 3, sizeof(int), &N_d);
    error |= clSetKernelArg(run_as_gpu_2, 4, sizeof(int), &P_d);
    error |= clSetKernelArg(run_as_gpu_2, 5, sizeof(cl_mem), &C_d);
    OPENCL_CHECK_ERRORS(error)

        // 启动kernel
        size_t globalws_2[2] = { N,M };
    error = clEnqueueNDRangeKernel(queue, run_as_gpu_2, 2, NULL, globalws_2, NULL, 0, NULL, &ev);
    clFinish(queue);
    OPENCL_CHECK_ERRORS(error)
        //计算kerenl执行时间
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_START,
        sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_END,
        sizeof(cl_ulong), &endTime, NULL);
    kernelExecTimeNs = endTime - startTime;
    printf("Gpu_2运行时间 :%8.6f ms\n", kernelExecTimeNs*1e-6);
        //取得kernel返回值
    float* gpu_C_2 = new float[M*N];
    clEnqueueReadBuffer(queue, C_d, CL_TRUE, 0, M*N * sizeof(float), gpu_C_2, 0, NULL, NULL);

    assert(memcmp(C_h, gpu_C_2, M*N * sizeof(float)) == 0);

    error = clEnqueueNDRangeKernel(queue, run_as_gpu_1, 2, NULL, globalws_1, NULL, 0, NULL, &ev);
    clFinish(queue);
    OPENCL_CHECK_ERRORS(error)
        //计算kerenl执行时间
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_START,
        sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(ev, CL_PROFILING_COMMAND_END,
        sizeof(cl_ulong), &endTime, NULL);
     kernelExecTimeNs = endTime - startTime;
    printf("Gpu_1运行时间 :%8.6f ms\n", kernelExecTimeNs*1e-6);

    delete[] A_h;
    delete[] B_h;
    delete[] C_h;
    delete[] gpu_C_1;
    delete[] gpu_C_2;
    delete[] platforms;
    clReleaseKernel(run_as_gpu_1);
    clReleaseKernel(run_as_gpu_2);
    clReleaseCommandQueue(queue);
    clReleaseContext(context);
    clReleaseMemObject(A_d);
    clReleaseMemObject(B_d);
    clReleaseMemObject(C_d);
    return 0;
}

三、运行结果

这里可以看出，两个方案虽然结果一样，但是效率是有很大差别的，原因是什么呢？上面说到，GPU会合并内存访问来优化性能，多维情况下，内存空间是按照行主序的方式储存的，如下图，一个5列的二维数组内存排列方式如下：

而在GPU执行过程中，他是先执行第一个纬度，再执行第二个纬度。所以，在第一种情况下，第一维是M，第二维是N，此时，B和C的内存无法合并访问(访问顺序是00 10 20 30 40 01 11 21 …)

在第二种情况下，B和C的内存可以合并访问（访问顺序是00 01 02 03 04 11 12 13 …）

合并访问会减小内存请求，优化性能。

四、其他示例

试试添加一个kernel函数，测试它的运行时间。

__kernel void RunAsGpu_3(
    __global  float *A,
    __global  float *B,
    int M,
    int N,
    int P,
    __global float* C)
{
    int x = get_global_id(0);
    int y = get_global_id(1);
    C[x*N + y] = 0;
    for(int i = 0;i<P;i++)
    {
        C[x*N + y] += A[x*P + i]*B[i*N + y];
    }
}

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