OpenMP 循环调度 + 计时

▶ 使用子句 schedule() 来调度循环，对于循环中每次迭代时间不相等的情况较为有效

● 代码

 #include <stdio.h>
 #include <stdlib.h>
 #include <math.h>
 #include <omp.h>
 #include <time.h>

 const int thread = , dataSize = ;

 double f(int i)// 工作函数每次运行的时间与输入值 i 有关
 {
     const int first = i * (i + ) / , last = first + i;
     int j;
     double sum;
     for (j = first, sum = 0.0; j <= last; j++)
         sum += sin(j);
     return sum;
 }
 int main(int argc, char* argv[])
 {
     int i, threadReal;
     clock_t time;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;

     time = clock();
     #pragma omp parallel for num_threads(thread) schedule(static, 1)// 静态调度，每个线程循环分配 1 个迭代块，默认调度相当于 schedule(static, dataSize / thread)
     for (i = ; i < dataSize; i++)                                  // 新的 OpenMP 还支持 auto （系统自选）和 runtime （从环境变量 OMP_SCHEDULE 中选定，值为 static，dynamic，guided 之一）
         f(i);
     time = clock() - time;
     printf("finished, time = %d ms\n", time);
     getchar();
     return ;
 }

● 输出结果

 finished, time =  ms    // 单线程
 finished, time =  ms     // 8 线程，默认调度
 finished, time =  ms     // 8 线程，schedule(static, 1)
 finished, time =  ms     // 8 线程，schedule(dynamic)
 finished, time =  ms     // 8 线程，schedule(guided)

▶ OpenMP 的计时工具

 #include <stdio.h>
 #include <stdlib.h>
 #include <windows.h>
 #include <omp.h>

 int main(int argc, char* argv[])
 {
     double start = omp_get_wtime();     // 以毫秒为单位的计时器
     Sleep();
     double end = omp_get_wtime();
     double wtick = omp_get_wtick();     // 时钟分辨率，单位是秒
     printf_s("start = %.16g\nend = %.16g\ndiff = %.16g\n", start, end, end - start);
     printf_s("wtick = %.16g\n1/wtick = %.16g\n", wtick, 1.0 / wtick);

     getchar();
     return ;
 }

● 输出结果：

start = 31852.98727121565
end = 31853.99453063095
diff = 1.007259415295266
wtick = 2.438090779872098e-07
/wtick =