为什么这个包含rand（）的C++11代码使用多个线程比使用一个线程慢？

Question

我正在尝试新的C++11线程，但我的简单测试有糟糕的多核性能。作为一个简单的例子，这个程序将一些平方随机数相加。

#include <iostream>
#include <thread>
#include <vector>
#include <cstdlib>
#include <chrono>
#include <cmath>

double add_single(int N) {
    double sum=0;
    for (int i = 0; i < N; ++i){
        sum+= sqrt(1.0*rand()/RAND_MAX);
    }
    return sum/N;
}

void add_multi(int N, double& result) {
    double sum=0;
    for (int i = 0; i < N; ++i){
        sum+= sqrt(1.0*rand()/RAND_MAX);
    }
    result = sum/N;
}

int main() {
    srand (time(NULL));
    int N = 1000000;

    // single-threaded
    auto t1 = std::chrono::high_resolution_clock::now();
    double result1 = add_single(N);
    auto t2 = std::chrono::high_resolution_clock::now();
    auto time_elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(t2-t1).count();
    std::cout << "time single: " << time_elapsed << std::endl;

    // multi-threaded
    std::vector<std::thread> th;
    int nr_threads = 3;
    double partual_results[] = {0,0,0};
    t1 = std::chrono::high_resolution_clock::now();
    for (int i = 0; i < nr_threads; ++i) 
        th.push_back(std::thread(add_multi, N/nr_threads, std::ref(partual_results[i]) ));
    for(auto &a : th)
        a.join();
    double result_multicore = 0;
    for(double result:partual_results)
        result_multicore += result;
    result_multicore /= nr_threads;
    t2 = std::chrono::high_resolution_clock::now();
    time_elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(t2-t1).count();
    std::cout << "time multi: " << time_elapsed << std::endl;

    return 0;
}

在Linux和3core机器上用'g++-std=c++11-pthread test.cpp'编译，一个典型的结果是

time single: 33
time multi: 565

null

哇，我发现问题了。确实是兰德（）。我用一个C++11等价物替换了它，现在运行时可以完美地伸缩。谢谢大家！

Answer 1

在我的系统中，行为是相同的，但正如Maxim提到的，rand不是线程安全的。当我将rand更改为rand_r时，多线程代码会像预期的那样更快。

void add_multi(int N, double& result) {
double sum=0;
unsigned int seed = time(NULL);
for (int i = 0; i < N; ++i){
    sum+= sqrt(1.0*rand_r(&seed)/RAND_MAX);
}
result = sum/N;
}