C++ 线程池

2023-03-06
上海
本文字数：5023 字
阅读完需：约 16 分钟

在做 C++开发的时候，线程池是一个经常需要使用的技术。通过它，可以充分利用多线程带来的好处，同时避免频繁创建和、销毁线程所带来的消耗。

借用一下Wiki的定义：

线程池（英语：thread pool）：一种线程使用模式。线程过多会带来调度开销，进而影响缓存局部性和整体性能。而线程池维护着多个线程，等待着监督管理者分配可并发执行的任务。这避免了在处理短时间任务时创建与销毁线程的代价。线程池不仅能够保证内核的充分利用，还能防止过分调度。可用线程数量应该取决于可用的并发处理器、处理器内核、内存、网络 sockets 等的数量。例如，对于计算密集型任务，线程数一般取 cpu 数量+2 比较合适，线程数过多会导致额外的线程切换开销。

但是，在标准 C++库里，并未提供线程池的实现，于是，这世上就有了 n 种实现方式。

更糟糕的是，我又贡献了第 n+1 种。

期待以后的 C++ 2x 能统一实现一个版本。

Linux 老大 Linus Torvalds 说："Talk is cheap. Show me the code."

所以，先贴代码为敬。

// ThreadPool with C++ std::thread// Author: Yuchuan Wang// yuchuan.wang@gmail.com// 
#include <iostream>#include <atomic>#include <thread>#include <chrono>#include <mutex>#include <condition_variable>#include <functional>#include <vector>#include <queue>
// Function will be running inside thread poolusing ThreadTask = std::function<void()>;
class ThreadPool{public:    // If threads_num is 0, it will use the same number of CPU cores    // If tasks_num is -1, the number of tasks will be unlimited    ThreadPool(int threads_num = 0, int tasks_num = -1)    {        if(threads_num == 0)        {            max_threads = std::thread::hardware_concurrency();        }        else        {            max_threads = threads_num;        }        max_tasks = tasks_num;        is_running = false;    }
    ~ThreadPool()    {        WaitForStop();    }
    // Add task to queue    bool AddTask(ThreadTask task)    {        // Scope for lock        {            std::unique_lock<std::mutex> lock(tasks_guard);            if(max_tasks == -1)            {                // Unlimited                tasks.push(task);            }            else            {                if(tasks.size() >= max_tasks)                {                    return false;                }                else                {                    tasks.push(task);                }            }        }
        // Notify thread        tasks_event.notify_one();
        return true;     }
    // Start threads    bool Start()    {        if(is_running)        {            // Running already            return false;        }
        is_running = true;        if(threads.empty())        {            CreateThreads();        }
        return true;    }
    void WaitForStop()    {        if(!is_running)        {            // I am not running            return;        }
        is_running = false;         tasks_event.notify_all();        for(auto &t : threads)        {            // Wait for all threads to exit            t.join();        }        threads.clear();    }
private:    void CreateThreads()    {        for(int i = 0; i < max_threads; i++)        {            threads.push_back(std::thread(&ThreadPool::ThreadRoutine, this));        }    }
    // Thread worker function    // Take task from queue, and run it    static void ThreadRoutine(ThreadPool* ptr)    {        if(ptr == nullptr)        {            return;         }
        while(ptr->is_running || !ptr->tasks.empty())        {            ThreadTask task;             // Scope for lock            {                // Get task to run                std::unique_lock<std::mutex> lock(ptr->tasks_guard);                while(ptr->tasks.empty())                {                    // Wait until task is ready                    ptr->tasks_event.wait(lock);                }                                // OK, now there is a task ready to run                task = ptr->tasks.front();                ptr->tasks.pop();            }            // Run it            task();        }    }
private:    // Max threads allowed    int max_threads;    // Max tasks inside queue    int max_tasks;     // Vector of threads    std::vector<std::thread> threads;     // Queue of tasks    std::queue<ThreadTask> tasks;     // Flag of runnin status    bool is_running;    // Mutex to protect the tasks queue    std::mutex tasks_guard;     // Condition of tasks event    std::condition_variable tasks_event; };

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大概解释几个点：

using ThreadTask = std::function<void()>;

类似于以前的 typedef 用法，定义了一个 ThreadTask 的类型，代表 C++的函数对象。它会被放进线程池的任务队列里面。后续的各个线程，会从任务队列里面取出具体的任务(函数对象)，然后执行这个任务。

ThreadPool(int threads_num = 0, int tasks_num = -1)

初始化线程池。默认情况下，创建的线程数跟你系统的 CPU 核数一样，任务队列里的任务个数无限。

bool AddTask(ThreadTask task)

把需要线程执行的任务，加入到任务队列里面。

bool Start()

创建 threads_num 个线程，并开始运行。

void WaitForStop()

等待所有的线程完成当前正在处理的任务后，退出。

具体的使用方法，可以参考如下代码：

#include "ThreadPool.h"
int product_sell = 0;void ProductCounter(std::mutex* task_protect){    //std::this_thread::sleep_for(std::chrono::seconds(1));    std::this_thread::sleep_for(std::chrono::microseconds(100));
    std::lock_guard<std::mutex> lock(*task_protect);    std::cout <<"How many products sell: " << product_sell++ << std::endl;}
int main(){    std::mutex protect_task;    ThreadPool pool(0, -1);    for(int i = 0; i < 100; i++)    {        pool.AddTask(std::bind(ProductCounter, &protect_task));    }    pool.Start();    // Do more stuff...    for(int i = 0; i < 50; i++)    {        pool.AddTask(std::bind(ProductCounter, &protect_task));    }    pool.WaitForStop();    return 0;}

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在这个小例子中，创建了一个线程池。先添加了 100 个任务，然后启动线程池。接着又添加了 50 个任务。线程池会以同样的线程个数(在我的电脑，有 8 个 core，所以默认是 8 个线程)，依次把任务完成，最后退出。

输出是这样的：

How many products sell: 0How many products sell: 1How many products sell: 2How many products sell: 3How many products sell: 4How many products sell: 5How many products sell: 6How many products sell: 7How many products sell: 8How many products sell: 9How many products sell: 10How many products sell: 11How many products sell: 12How many products sell: 13How many products sell: 14How many products sell: 15How many products sell: 16How many products sell: 17How many products sell: 18How many products sell: 19How many products sell: 20How many products sell: 21How many products sell: 22How many products sell: 23How many products sell: 24How many products sell: 25How many products sell: 26How many products sell: 27How many products sell: 28How many products sell: 29How many products sell: 30How many products sell: 31How many products sell: 32How many products sell: 33How many products sell: 34How many products sell: 35How many products sell: 36How many products sell: 37How many products sell: 38How many products sell: 39How many products sell: 40How many products sell: 41How many products sell: 42How many products sell: 43How many products sell: 44How many products sell: 45How many products sell: 46How many products sell: 47How many products sell: 48How many products sell: 49How many products sell: 50How many products sell: 51How many products sell: 52How many products sell: 53How many products sell: 54How many products sell: 55How many products sell: 56How many products sell: 57How many products sell: 58How many products sell: 59How many products sell: 60How many products sell: 61How many products sell: 62How many products sell: 63How many products sell: 64How many products sell: 65How many products sell: 66How many products sell: 67How many products sell: 68How many products sell: 69How many products sell: 70How many products sell: 71How many products sell: 72How many products sell: 73How many products sell: 74How many products sell: 75How many products sell: 76How many products sell: 77How many products sell: 78How many products sell: 79How many products sell: 80How many products sell: 81How many products sell: 82How many products sell: 83How many products sell: 84How many products sell: 85How many products sell: 86How many products sell: 87How many products sell: 88How many products sell: 89How many products sell: 90How many products sell: 91How many products sell: 92How many products sell: 93How many products sell: 94How many products sell: 95How many products sell: 96How many products sell: 97How many products sell: 98How many products sell: 99How many products sell: 100How many products sell: 101How many products sell: 102How many products sell: 103How many products sell: 104How many products sell: 105How many products sell: 106How many products sell: 107How many products sell: 108How many products sell: 109How many products sell: 110How many products sell: 111How many products sell: 112How many products sell: 113How many products sell: 114How many products sell: 115How many products sell: 116How many products sell: 117How many products sell: 118How many products sell: 119How many products sell: 120How many products sell: 121How many products sell: 122How many products sell: 123How many products sell: 124How many products sell: 125How many products sell: 126How many products sell: 127How many products sell: 128How many products sell: 129How many products sell: 130How many products sell: 131How many products sell: 132How many products sell: 133How many products sell: 134How many products sell: 135How many products sell: 136How many products sell: 137How many products sell: 138How many products sell: 139How many products sell: 140How many products sell: 141How many products sell: 142How many products sell: 143How many products sell: 144How many products sell: 145How many products sell: 146How many products sell: 147How many products sell: 148How many products sell: 149

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相关的代码，上传在Github，欢迎使用并提出意见。

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王玉川

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