OpenMP 简单的规约

▶ 简单的计算和规约

● 第一种方法，将全局和的指针传入工作函数中进行加和，使用 critical 来控制临界区的访问

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

 const int thread = ;

 void work(int* globalSum)
 {
     #pragma omp critical        // 使用 critical 来控制临界区的访问
     *globalSum += omp_get_thread_num();
     return;
 }

 int main(int argc, char* argv[])
 {
     int threadReal, globalSum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     globalSum = ;              // 初始化全局和

     #pragma omp parallel num_threads(threadReal)
     work(&globalSum);           // 将全局和的指针传给每个线程进行计算

     printf("\nglobalSum = %d\n", globalSum);
     getchar();
     return ;
 }

● 输出结果，下面几种方法的输出结果均相同，不再重复罗列

globalSum = 

● 第二种方法，使用局部变量，在主函数中进行加和

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

 const int thread = ;

 int work()
 {
     return omp_get_thread_num();
 }

 int main(int argc, char* argv[])
 {
     int threadReal, globalSum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     globalSum = ;             

     #pragma omp parallel num_threads(threadReal)
     {
         int localSum = work();  // 使用局部变量
         #pragma omp critical
         globalSum += localSum;  // 将局部和加到全局和中
     }

     printf("\nglobalSum = %d\n", globalSum);
     getchar();
     return ;
 }

● 第三种方法，使用规约子句

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

 const int thread = ;

 int work()
 {
     return omp_get_thread_num();
 }

 int main(int argc, char* argv[])
 {
     int threadReal, globalSum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     globalSum = ;            

     #pragma omp parallel num_threads(threadReal) reduction(+ : globalSum)// 要求对 globalSum 使用加法规约
     globalSum += work();

     printf("\nglobalSum = %d\n", globalSum);
     getchar();
     return ;
 }

● 第四种方法，使用 parallel for 循环

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

 const int thread = ;

 int work()
 {
     return omp_get_thread_num();
 }

 int main(int argc, char* argv[])
 {
     int i, threadReal, globalSum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     globalSum = ;

     #pragma omp parallel for num_threads(threadReal) reduction(+:globalSum)// 联用了规约子句和 parallel for 循环
     for (i = ; i < threadReal; i++)
         globalSum += work();

     printf("\nglobalSum = %d\n", globalSum);
     getchar();
     return ;
 }

▶ 单独的 parallel for 用法

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

 const int thread = ;

 int main(int argc, char* argv[])
 {
     int i, threadReal, a[thread];
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;

     #pragma omp parallel for num_threads(threadReal)
     for (i = ; i < threadReal; i++)// 注意，把下面的  a[i] = i 写到括号里来会报错 “OpenMP“for”语句中的增量格式不正确”
         a[i] = i;

     for (i = ; i < thread; i++)
         printf("a[%2d] = %2d\n", i, a[i]);
     getchar();
     return ;
 }

● 输出结果

a[ ] =
a[ ] =
a[ ] =
a[ ] =
a[ ] =
a[ ] =
a[ ] =
a[ ] =  

▶ 使用私有变量估计 π 的值

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

 const int thread = , count = ;

 int main0(int argc, char* argv[])// 一般方法，使用局部变量
 {
     int i, threadReal;
     double sum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     sum = 0.0;

     #pragma omp parallel for num_threads(threadReal) reduction(+ : sum)
     for (i = ; i < count; i++)
     {
         double sign = (i %  ? -1.0 : 1.0);
         sum += sign / ( * i + );
     }

     printf("π = %.10f\n", sum * );
     getchar();
     return ;
 }

 int main1(int argc, char* argv[])// 将已经声明了的 sign 作为私有变量
 {
     int i, threadReal;
     double sign, sum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     sum = 0.0;

     #pragma omp parallel for num_threads(threadReal) reduction(+ : sum) private(sign)
     for (i = ; i < count; i++)
     {
         sign = (i %  ? -1.0 : 1.0);
         sum += sign / ( * i + );
     }

     printf("π = %.10f\n", sum * );
     getchar();
     return ;
 }

 int main2(int argc, char* argv[])// 使用 default(none) 子句，然后明确规定每个变量的作用域
 {
     int i, threadReal;
     double sign, sum;
     if (argc >  && *argv[] > ''&& *argv[] < '')
         threadReal = atoi(argv[]);
     else
         threadReal = thread;
     sum = 0.0;

     #pragma omp parallel for num_threads(threadReal) reduction(+ : sum) default(none) private(i, sign) shared(count)// 多个变量用逗号隔开
     for (i = ; i < count; i++)
     {
         sign = (i %  ? -1.0 : 1.0);
         sum += sign / ( * i + );
     }

     printf("π = %.10f\n", sum * );
     getchar();
     return ;
 }

● 输出结果

π = 3.1415826536