2.4_runtime.cpp

//
// Created by light on 19-11-5.
//
#include <iostream>
#include <algorithm>
#include <thread>
#include <vector>
#include <numeric>

using namespace std;

//使得每个线程具有最小数目的元素以避免过多的线程开销
template<typename Iterator, typename T>
struct accumulate_block {
    void operator()(Iterator first, Iterator last, T &result) {
        result = std::accumulate(first, last, result);
    }
};

template<typename Iterator, typename T>
T parallel_accumlate(Iterator first, Iterator last, T init) {
    unsigned long const length = std::distance(first, last);

    if (!length)
        return init;

    unsigned long const min_per_thread = 25;
    unsigned long const max_threads = (length + min_per_thread - 1) / min_per_thread;
    cout<<max_threads<<endl;
    unsigned long const hardware_threads = std::thread::hardware_concurrency();
    cout<<hardware_threads<<endl;
    unsigned long const num_threads = std::min(hardware_threads != 0 ? hardware_threads : 2, max_threads);
    cout<<num_threads<<endl;
    unsigned long const block_size = length / num_threads;
    cout<<block_size<<endl;

    std::vector<T> results(num_threads);
    std::vector<std::thread> threads(num_threads - 1);

    Iterator block_start = first;
    for (unsigned long i = 0; i < (num_threads - 1); ++i) {
        Iterator block_end = block_start;
        std::advance(block_end, block_size);
        threads[i] = std::thread(accumulate_block<Iterator, T>(), block_start, block_end, std::ref(results[i]));
        block_start = block_end;
    }
    accumulate_block<Iterator, T>()(block_start, last, results[num_threads - 1]);
    std::for_each(threads.begin(), threads.end(), std::mem_fn(&std::thread::join));

    return std::accumulate(results.begin(), results.end(), init);
}


int main() {

    vector<int> v{3,4,5,6};
    int res=0;
    cout<<parallel_accumlate(v.begin(),v.end(),res);
    return 0;
}