线程池实现基于C++可以说是一道经典的计算机本科学生练习题。本篇文章会从一个传统实现的线程池开始讲起。

一、线程池和任务

我们看一下线程池类的基本结构。线程池本质是有一些线程在后台等待队列执行任务，我们只需要将任务存储在队列中。线程会从任务队列中获取任务执行。这里最基本的成员就是线程队列，任务队列，条件变量用于通知线程组任务事件，队列锁避免竞争，终止条件。

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class ThreadPool {
public:
    void Start();
    void QueueJob(const std::function<void()>& job);
    void Stop();
    bool busy();
 
private:
    void ThreadLoop();
 
    bool should_terminate = false;           // Tells threads to stop looking for jobs
    std::mutex queue_mutex;                  // Prevents data races to the job queue
    std::condition_variable mutex_condition; // Allows threads to wait on new jobs or termination 
    std::vector<std::thread> threads;
    std::queue<std::function<void()>> jobs;
};

同时线程组应该实现的功能有以下几个函数

1. ThreadPool::Start

创建线程池，比较有效率的办法是根据硬件并发数来创建相应数量num_threads 的线程组。这样不会有性能问题，创建不合适数量的线程组，可能用线程组比串行执行更慢。

每个线程都在等待新的task调度并执行。

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void ThreadPool::Start() {
    const uint32_t num_threads = std::thread::hardware_concurrency(); // Max # of threads the system supports
    for (uint32_t ii = 0; ii < num_threads; ++ii) {
        threads.emplace_back(std::thread(&ThreadPool::ThreadLoop,this))
    }
}

1. ThreadPool::ThreadLoop

这个是死循环主体. ，在任务队列种寻找待处理的任务。

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void ThreadPool::ThreadLoop() {
    while (true) {
        std::function<void()> job;
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            mutex_condition.wait(lock, [this] {
                return !jobs.empty() || should_terminate;
            });
            if (should_terminate) {
                return;
            }
            job = jobs.front();
            jobs.pop();
        }
        job();
    }
}

1. ThreadPool::QueueJob

加入新的任务到队列中，这里避免竞争，加入队列锁。

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void ThreadPool::QueueJob(const std::function<void()>& job) {
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        jobs.push(job);
    }
    mutex_condition.notify_one();
}

调用放传入回调函数指针，和处理内容指针

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thread_pool->QueueJob([] { /* ... */ });

1. ThreadPool::busy

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bool ThreadPool::busy() {
    bool poolbusy;
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        poolbusy = !jobs.empty();
    }
    return poolbusy;
}

busy()函数 主要用在while循环, 在整个线程组销毁之前，清理任务队列剩余未处理的任务。

1. ThreadPool::Stop

终止线程池，这里主要是各种线程的join操作.

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void ThreadPool::Stop() {
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        should_terminate = true;
    }
    mutex_condition.notify_all();
    for (std::thread& active_thread : threads) {
        active_thread.join();
    }
    threads.clear();
}

二、主要流程

以下是一个可以运行起来的代码。

我们先创建线程队列。这里使用最基础的std::thread

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    std::vector< std::thread > workers;

创建job，并把job添加到每个线程

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            [this]
            {
                for(;;)
                {
                    std::function<void()> task;
 
                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        this->condition.wait(lock,
                            [this]{ return this->stop || !this->tasks.empty(); });
                        if(this->stop && this->tasks.empty())
                            return;
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                    }
 
                    task();
                }
            }

接下来加任务到队列中，这里加完之后调用条件notify唤醒 等待任务的 线程。

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// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    using return_type = typename std::result_of<F(Args...)>::type;
 
    auto task = std::make_shared< std::packaged_task<return_type()> >(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );
        
    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
 
        // don't allow enqueueing after stopping the pool
        if(stop)
            throw std::runtime_error("enqueue on stopped ThreadPool");
 
        tasks.emplace([task](){ (*task)(); });
    }
    condition.notify_one();
    return res;
}

完整的可运行的代码如下

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#ifndef THREAD_POOL_H
#define THREAD_POOL_H
 
#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>
 
class ThreadPool {
public:
    ThreadPool(size_t);
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args) 
        -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();
private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;
    
    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};
 
// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads)
    :   stop(false)
{
    for(size_t i = 0;i<threads;++i)
        workers.emplace_back(
            [this]
            {
                for(;;)
                {
                    std::function<void()> task;
 
                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        this->condition.wait(lock,
                            [this]{ return this->stop || !this->tasks.empty(); });
                        if(this->stop && this->tasks.empty())
                            return;
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                    }
 
                    task();
                }
            }
        );
}
 
// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    using return_type = typename std::result_of<F(Args...)>::type;
 
    auto task = std::make_shared< std::packaged_task<return_type()> >(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );
        
    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
 
        // don't allow enqueueing after stopping the pool
        if(stop)
            throw std::runtime_error("enqueue on stopped ThreadPool");
 
        tasks.emplace([task](){ (*task)(); });
    }
    condition.notify_one();
    return res;
}
 
// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();
    for(std::thread &worker: workers)
        worker.join();
}
 
#endif