OpenMP 之 sections 求素数、求和、求积分圆周率（星星笔记）

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概述

sections 的主要功能是将一个任务分成独立的几个section，每个section由不同的线程来处理。换句话说，section语句用在sections语句里，用来将sections语句里的代码划分成几个不同的段，每段都并行执行吗，sections指令和for指令一样，必须与parallel联合使用才会并行化，否则串行执行。

例子1.求素数的个数，程序如下：

// sushuo1.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <omp.h>
#include <stdio.h>
#include <time.h>
#include <math.h>
#include <Windows.h>
#define NUM_THREADS 2

int _tmain(int argc, _TCHAR* argv[])
{
	long start = 3,end = 10000000;
	bool prime,prime1;
	clock_t t1,t2;
	int sum1 = 0,sum = 0;
	t1 =clock();
	omp_set_num_threads(NUM_THREADS);
    #pragma omp parallel sections reduction(+:sum)
	{
     #pragma omp section
		{
			for (int m= start; m<= end; m = m+4)
			{
				prime = true;
			    int	k= (int)sqrt((double)m);
				for(int i= 2; i<=k;i ++)
					if ( m%i == 0)
					{
						prime = false;
						break;
					}
					if (prime)
					{
					//	printf("++++++++++++:::   %dn",omp_get_thread_num());
					//	printf("parallel 素数 = %dn",m);
						sum ++;
					}
			}
		} 
      #pragma omp section
		{
			for (int m= start + 2; m<= end; m = m+4)
			{
				prime1 = true;
			    int	k = (int)sqrt((double)m);
				for(int i= 2; i<=k; i++)
					if ( m % i == 0)
					{
						prime1 =false;
						break;
					}
					if (prime1)
					{
					//	printf("---------------   %dn",omp_get_thread_num());
					//  printf("parallel 素数 = %dn",m);
						sum ++;
					}
			}
		}
	}
	t2 = clock();
	printf("parallel 素数sum = %dn",sum);
	printf("parallel time=%dn",(t2-t1));
	
//---------------------------------------------------------
	int i,k;
	sum = 0;
	t1 = clock();
	for ( int m= start; m<= end; m = m+2)
	{
		prime = true;
		k= (int)sqrt((double)m);
		for(i= 2;i<=k;i++)
			if (m%i ==0)
			{
				prime =false;
				break;
			}
		if (prime)
		{
		//	printf("serial 素数 = %dn",m);
			sum ++;
		}
	}
	t2 = clock();
	printf("serial 素数sum = %dn",sum);
	printf("serial time=%dn",(t2-t1));
	
	system("pause");
	return 0;
}

运行结果如下：

例子2.求和运算，程序如下：

// SectionsAdd.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include "windows.h"
#include <time.h>
#include "omp.h"
#define NUM_THREADS 2

int _tmain(int argc, _TCHAR* argv[])
{
	omp_set_num_threads(NUM_THREADS);
	long long sum = 0;

	long long temp = 0l;
	clock_t t1 = clock();
    #pragma omp parallel sections reduction(+:sum)
	{
       #pragma omp section
		{
			for (long i = 0; i<=1000000000; i= i+NUM_THREADS)
			{
				sum+=i;
			}
	     }
       #pragma omp section
		{
			for (long i = 1; i<=1000000000; i= i+NUM_THREADS)
			{
				sum +=i;
			}
		}
	}
	clock_t t2 = clock();
	printf("sum=%lldn",sum);
	printf("parallel time=%dn",(t2-t1));

	sum = 0;
	t1 = clock();
	for (long i=1; i<= 1000000000;i++)
	{
		sum = sum + i;
	}
	t2 = clock();
	printf("sum=%lldn",sum);
	printf("serial time=%dn",(t2-t1));
	system("pause");
	return 0;
}

运行结果如下：

例子3.求积分（圆周率）运算，程序如下：

// sectionspi.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include "windows.h"
#include <time.h>
#include "omp.h"

static long num_steps = 100000000;
double step;
#define NUM_THREADS 2

int _tmain(int argc, _TCHAR* argv[])
{
	int i;
	clock_t t1,t2;
	double sum = 0 ,pi = 0.0;
	step = 1.0/(double) num_steps;
	omp_set_num_threads(NUM_THREADS);
	t1 = clock();
    #pragma omp parallel sections reduction(+:sum)
	{
		
        #pragma omp section
		{
			double x;
			for (int i=omp_get_thread_num();i <num_steps;i+=NUM_THREADS)
			{
				x=(i+0.5)*step;
				sum = sum + 4.0/(1.0 + x*x);
			}
		}
        #pragma omp section
		{
			double x;
			for (int i=omp_get_thread_num();i <num_steps;i+=NUM_THREADS)
			{
				x=(i+0.5)*step;
				sum = sum + 4.0/(1.0 + x*x);
			}
		}
	}
	pi = step * sum;
	t2 = clock();
	printf("pi = %.15fn",pi);
	printf("parallel time=%dn",(t2-t1));
	t1 = clock();
	sum =0;
	double x;
	for (int i=0;i <num_steps;i++)
	{
		x=(i+0.5)*step;
		sum = sum + 4.0/(1.0 + x*x);
	}
	pi = step * sum;
	t2 = clock();
	printf("pi = %.15fn",pi);
	printf("serial time=%dn",(t2-t1));
	printf("星星笔记n");
	system("pause");
	return 0;

}

运行结果如下：