c++ - 根据硬件并发度将任务均分给线程

Question

我正在尝试运行这个简单的程序，想知道为什么输出会出错。代码查询硬件并发性，然后尝试启动该数量的线程并执行一些任务。为了完成该任务，我正在写入已调整大小的 vector 的各个元素，但结果仍然出错 -

#include <iostream>
#include <thread>
#include <vector>
#include <functional>

void myMethod(std::vector<int> &v, int threadNumber) {
    for(int i = threadNumber - 1; i < v.size(); i+= threadNumber) {
        v[i] = threadNumber;
    }
}

int main() {
    const auto numThread = std::thread::hardware_concurrency();
    std::vector<int> vec;
    vec.resize(100, 0);

    if(numThread < 2) {
        std::cout << "Not running\n";
        return 0;
    }

    std::vector<std::thread> vT;
    for(int i = 1; i <= numThread; ++i) {
        vT.emplace_back(myMethod, std::ref(vec), i);
    }
    for(auto &t: vT) { t.join(); }


    for(const auto &i: vec) {
        std::cout << i << ' ';
    }
    std::cout << std::endl;
}

输出为 -

1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4 1 3 1 4 3 2 1 4 1 2 3 4

但我期待 - 1 2 3 4 1 2 3 4 ...

Answer 1

在您的 myMethod 函数中，您增加了错误数量的变量 i -

void myMethod(std::vector<int> &v, int threadNumber) {
    for(int i = threadNumber - 1; i < v.size(); i+= threadNumber) {
        v[i] = threadNumber;
    }
}

实际上应该是-

void myMethod(std::vector<int> &v, int threadNumber) {
    const auto numThread = std::thread::hardware_concurrency();
    for(int i = threadNumber - 1; i < v.size(); i+= numThread) {
        v[i] = threadNumber;
    }
}