C++多线程并行计算

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C++多线程并行计算

多线程(std::thread)

多线程(std::thread)

写了一个简短的并行计算例子
存粹的利用CPU性能计算素数个数

//实现标准库头文件<numeric>中accumulate函数的并行版本
#include <iostream>
#include <thread>
#include <numeric>
#include <algorithm>
#include <vector>
#include <functional>
#include <utility>
#include <windows.h>using std::thread;
using std::vector;
using std::accumulate;
using std::cout;
using std::endl;// 被执行的函数
template <typename Iterator, typename T> class Accum
{
public:void operator() (Iterator first, Iterator last, T &sum){sum = 0;for (; first != last; ++first){// 简单的计算素数auto k = std::sqrtf(*first);int i = 0;for (i = 2; i <= k; i++){if ((*first)%i == 0){break;}}if (i > k){++sum;}}}
};// 分发任务的函数
template <typename Iterator, typename T>
T ParallelAccum(Iterator first, Iterator last, T &sum)
{//计算迭代器中包含的元素数量const unsigned int len = std::distance(first, last);//若迭代器中没有元素则直接返回if (!len){return sum;}//获取机器支持的并发线程数const unsigned int num_thread = thread::hardware_concurrency();//最终实际上每个线程处理的元素个数const unsigned int block_size = len / num_thread;//保存每个线程累加的结果vector<T> results(num_thread);//启动比num_thread - 1个线程，因为main函数本身已开启一个线程vector<thread> threads(num_thread - 1);//开始并行计算Iterator block_begin = first;for (unsigned int i = 0; i < (num_thread - 1); ++i){Iterator block_end = block_begin;//将迭代器向前推进一个块,到达当前块的末尾位置std::advance(block_end, block_size);//传递参数,通常情况下thread的构造函数将复制所提供的参数,需要将模板参数转为引用threads[i] = thread(Accum<Iterator, T>(), block_begin, block_end, std::ref(results[i]));block_begin = block_end;}//处理最后一个线程,由于block_size = len / num_thread得到的结果不一定为整数,该线程处理剩余的所有元素Accum<Iterator, T>()(block_begin, last, results[num_thread - 1]);//对threads中所有线程调用join()std::for_each(threads.begin(), threads.end(), std::mem_fn(&thread::join));//return accumulate(results.begin(), results.end(), sum);
}// 计时结构
LARGE_INTEGER t1, t2, tc;void time_begin()
{QueryPerformanceFrequency(&tc);QueryPerformanceCounter(&t1);
}float time_end()
{QueryPerformanceCounter(&t2);return ((t2.QuadPart - t1.QuadPart)*1.0 / tc.QuadPart) * 1000;
}int main()
{// 装备任务队列vector<int> i_vec;int sum = 0;for (int i = 1; i != 501000; ++i){i_vec.push_back(i);}// 并行计算示例time_begin();sum = ParallelAccum(i_vec.cbegin(), i_vec.cend(), sum);cout << "time: " << time_end() << ", sum = " << sum << endl;// 一般计算示例time_begin();Accum<vector<int>::iterator, int> a;a(i_vec.begin(), i_vec.end(), sum);cout << "time: " << time_end() << ", sum = " << sum << endl;return 0;
}