0_Simple__asyncAPI

▶ CPU - GPU 异步操作

▶ 源代码

 #include <stdio.h>
 #include <cuda_runtime.h>
 #include "device_launch_parameters.h"
 #include <helper_cuda.h>
 #include <helper_functions.h>

 __global__ void increment_kernel(int *g_data, int inc_value)
 {
     int idx = blockIdx.x * blockDim.x + threadIdx.x;
     g_data[idx] = g_data[idx] + inc_value;
 }

 bool correct_output(int *data, const int n, const int x)
 {
     for (int i = ; i < n; i++)
     {
         if (data[i] != x)
         {
             printf("Error! data[%d] = %d, ref = %d\n", i, data[i], x);
             return false;
         }
     }
     return true;
 }

 int main(int argc, char *argv[])
 {
     printf("Start.\n");
     int devID = findCudaDevice(argc, (const char **)argv);  // 通过命令行参数选择设备，可以为空
     cudaDeviceProp deviceProps;
     cudaGetDeviceProperties(&deviceProps, devID);
     printf("CUDA device [%s]\n", deviceProps.name);

     const int n =  *  * ;
     const int nbytes = n * sizeof(int);
     const int value = ;

     int *a, *d_a;
     cudaMallocHost((void **)&a, nbytes);
     cudaMalloc((void **)&d_a, nbytes);
     memset(a, , nbytes);
     cudaMemset(d_a, , nbytes);

     cudaEvent_t start, stop;                // GPU 端计时器
     cudaEventCreate(&start);
     cudaEventCreate(&stop);

     StopWatchInterface *timer = NULL;       // CPU 端计时器
     sdkCreateTimer(&timer);
     sdkResetTimer(&timer);

     dim3 threads = dim3(, , );
     dim3 blocks = dim3(n / threads.x, , );

     sdkStartTimer(&timer);                  // 注意 GPU 计时器是夹在 CPU 计时器内的，但是 GPU 函数都是异步的
     cudaEventRecord(start, );
     cudaMemcpyAsync(d_a, a, nbytes, cudaMemcpyHostToDevice, );
     increment_kernel << <blocks, threads, ,  >> > (d_a, value);
     cudaMemcpyAsync(a, d_a, nbytes, cudaMemcpyDeviceToHost, );
     cudaEventRecord(stop, );
     sdkStopTimer(&timer);

     unsigned long int counter = ;          // 记录 GPU 运行完成以前 CPU 运行了多少次 while 的循环
     while (cudaEventQuery(stop) == cudaErrorNotReady)
         counter++;

     float gpu_time = 0.0f;                  // 此时保证 GPU 运行完成，才能记录时间
     cudaEventElapsedTime(&gpu_time, start, stop);

     printf("time spent by GPU: %.2f\n", gpu_time);
     printf("time spent by CPU: %.2f\n", sdkGetTimerValue(&timer));
     printf("CPU executed %lu iterations while waiting for GPU to finish\n", counter);
     printf("\n\tFinish: %s.", correct_output(a, n, value) ? "Pass" : "Fail");

     cudaEventDestroy(start);
     cudaEventDestroy(stop);
     cudaFreeHost(a);
     cudaFree(d_a);
     getchar();
     return ;
 }

● 输出结果：

GPU Device : "GeForce GTX 1070" with compute capability 6.1

CUDA device [GeForce GTX ]
time spent by GPU: 11.50
time spent by CPU: 0.05
CPU executed  iterations while waiting for GPU to finish

        Finish!

▶ 新姿势：

● 调用主函数时的第0个参数作为程序名字符串，可以用于输出。

 int main(int argc, char *argv[])
 ...
 printf("%s", argv[]);

● 在没有附加 flag 的情况下申请主机内存，注意使用cudaFreeHost释放

 int *a, nbytes = n * sizeof(int);
 cudaMallocHost((void **)&a, nbytes);
 ...
 cudaFreeHost(a);

● 记录 CPU 调用 CUDA 所用的时间

 StopWatchInterface *timer = NULL;
 sdkCreateTimer(&timer);
 sdkResetTimer(&timer);
 sdkStartTimer(&timer);

 ...// 核函数调用

 sdkStopTimer(&timer);
 printf("%.2f ms", sdkGetTimerValue(&timer));

● 查看GPU队列状态的函数

extern __host__ cudaError_t CUDARTAPI cudaEventQuery(cudaEvent_t event);

■ stop为放置到流中的一个事件，cudaEventQuery(stop)返回该事件的状态，等于cudaSuccess（值等于0）表示已经发生；等于cudaErrorNotReady（值等于35）表示尚未发生。源代码中利用这段时间让CPU空转，记录了迭代次数。

while (cudaEventQuery(stop) == cudaErrorNotReady) counter++;

● stdlib.h 中关于返回成功和失败的宏

 #define EXIT_SUCCESS 0
 #define EXIT_FAILURE  1

● 示例文件中的错误检查函数（定义在helper_cuda.h中），报告出错文件、行号、函数名，并且重启cudaDevice。

 #define checkCudaErrors(val)  check((val), #val, __FILE__, __LINE__)

 template< typename T >
 void check(T result, char const *const func, const char *const file, int const line)
 {
     if (result)
     {
         fprintf(stderr, "CUDA error at %s:%d code=%d(%s) \"%s\" \n",
             file, line, static_cast<unsigned int>(result), _cudaGetErrorEnum(result), func);
         DEVICE_RESET// Make sure we call CUDA Device Reset before exiting
         exit(EXIT_FAILURE);
     }
 }

 #define DEVICE_RESET  cudaDeviceReset();