인프런 커뮤니티 질문&답변

오타가 없는 것 같은데 무슨 이유로 다른 값이 나오는지 알수가 없네요. 도움부탁드립니다.

아래는 코드 전문입니다.

#include <iostream>
#include <thread>
#include <mutex>
#include <utility>
#include <future>
#include <vector>
#include <atomic>
#include <chrono>
#include <numeric>
#include <random>
#include <execution>

using namespace std;

auto dotProductFuture(const vector<int>& v0, const vector<int>& v1,
    const unsigned i_start, const unsigned i_end)
{
    int sum = 0;
    for(unsigned i = i_start; i < i_end; ++i)
    {
       sum += v0[i] * v1[i];
    }
    return sum;
}

auto dotProductNaive(const vector<int>& v0, const vector<int>& v1,
    const unsigned i_start, const unsigned i_end, unsigned long long & sum)
{

    for(unsigned i = i_start; i < i_end; ++i)
        sum += v0[i] * v1[i]; 
    return sum;   
}

mutex mtx; 
auto dotProductLock(const vector<int>& v0, const vector<int>& v1,
    const unsigned i_start, const unsigned i_end, unsigned long long & sum)
{
    for(unsigned i = i_start; i < i_end;++i)
    {
        std::scoped_lock lock(mtx); //c++17
        sum += v0[i] * v1[i];
    }
    
}



int main()
{
    const long long n_data = 100'000'000;
    const unsigned n_threads = 4;

    //init vectors
    std::vector<int> v0, v1;
    v0.reserve(n_data);
    v1.reserve(n_data);

    random_device seed;
    mt19937 engine(seed());

    uniform_int_distribution<> uniformDist(1, 10);

    for(long long i = 0; i < n_data; ++i)
    {
        v0.push_back(uniformDist(engine));
        v1.push_back(uniformDist(engine));
    }


    cout << "std::inner_product" << endl;
    {
        const auto sta = chrono::steady_clock::now();

        const auto sum = std::inner_product(v0.begin(), v0.end(), v1.begin(), 0ull);//unsigned long long

        const chrono::duration<double> dur = chrono::steady_clock::now() - sta;

        cout << dur.count() << endl;
        cout << sum << endl;
        cout << endl;
    }


    cout << "Naive" << endl;
    {
        const auto sta = std::chrono::steady_clock::now();

        std::vector<thread> threads;
        threads.resize(n_threads);

        const unsigned n_per_thread = n_data / n_threads;

        unsigned long long sum = 0;
        for(unsigned t = 0; t < n_threads; ++t)
        {
            threads[t] = std::thread(dotProductNaive, std::ref(v0), 
                            std::ref(v1), t*n_per_thread, (t+1)*n_per_thread, std::ref(sum));
        }

        for(unsigned t = 0; t < n_threads; ++t)
            threads[t].join();
        const auto dur = std::chrono::steady_clock::now() - sta;

        cout << dur.count() << endl;
        cout << sum << endl;
        cout << endl;

    }

    cout << "Lock Guard" << endl;
    {
        const auto sta = std::chrono::steady_clock::now();
        
        unsigned long long sum = 0; 
        std::vector<thread> threads;
        threads.resize(n_threads);

        const unsigned n_per_thread = n_data / n_threads;

        
        for(unsigned t = 0; t < n_threads; ++t)
        {
            threads[t] = std::thread(dotProductLock, std::ref(v0), 
                            std::ref(v1), t*n_per_thread, (t+1)*n_per_thread, std::ref(sum));
        }

        // for(unsigned t = 0; t < n_threads; ++t)
        //     threads[t].join();
        const chrono::duration<double> dur = std::chrono::steady_clock::now() - sta;

        cout << dur.count() << endl;
        cout << sum << endl;
        cout << endl;
    }




    return 0; 
}