C++11 实现线程池

什么是线程池

线程池（thread pool）是一种线程使用模式。线程过多或者频繁创建和销毁线程会带来调度开销，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，等待着管理器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，以及保证了线程的可复用性。线程池不仅能够保证内核的充分利用，还能防止过分调度。

线程池原理

预先创建预定数量的线程，将多个任务加入到任务队列。类似于生产者消费者，多个线程相当于消费者，一个任务队列充当生产者。当任务队列被塞入任务时，线程们就去竞争这些任务，但每次只有一个线程能够得到任务，该任务执行完成后，线程可以释放出来去承接下一个任务，这样保证多个任务可以并发地执行。

线程池的原理如图所示

一个任务队列有n个任务，可以通过线程池的调度分配到m个线程上去并发执行

线程池实现

机遇C++11 的线程库编写
thread_pool.hpp

#ifndef _THREAD_POOL_H_
#define _THREAD_POOL_H_

#include <thread>
#include <mutex>
#include <atomic>
#include <condition_variable>
#include <functional>
#include <vector>
#include <queue>

class ThreadPool
{
public:
	using Task = std::function<void()>;

	explicit ThreadPool(int num): _thread_num(num), _is_running(false)
	{}

	~ThreadPool()
	{
		if (_is_running)
			stop();
	}

	void start()
	{
		_is_running = true;

		// start threads
		for (int i = 0; i < _thread_num; i++)
			_threads.emplace_back(std::thread(&ThreadPool::work, this));
	}

	void stop()
	{
		{
			// stop thread pool, should notify all threads to wake
			std::unique_lock<std::mutex> lk(_mtx);
			_is_running = false;
			_cond.notify_all(); // must do this to avoid thread block
		}

		// terminate every thread job
		for (std::thread& t : _threads)
		{
			if (t.joinable())
				t.join();
		}
	}

	void appendTask(const Task& task)
	{
		if (_is_running)
		{
			std::unique_lock<std::mutex> lk(_mtx);
			_tasks.push(task);
			_cond.notify_one(); // wake a thread to to the task
		}
	}

private:
	void work()
	{
		printf("begin work thread: %d\n", std::this_thread::get_id());

		// every thread will compete to pick up task from the queue to do the task
		while (_is_running)
		{
			Task task;
			{
				std::unique_lock<std::mutex> lk(_mtx);
				if (!_tasks.empty())
				{
					// if tasks not empty, 
					// must finish the task whether thread pool is running or not
					task = _tasks.front();
					_tasks.pop(); // remove the task
				}
				else if (_is_running && _tasks.empty())
					_cond.wait(lk);
			}

			if (task)
				task(); // do the task
		}

		printf("end work thread: %d\n", std::this_thread::get_id());
	}

public:
	// disable copy and assign construct
	ThreadPool(const ThreadPool&) = delete;
	ThreadPool& operator=(const ThreadPool& other) = delete;

private:
	std::atomic_bool _is_running; // thread pool manager status
	std::mutex _mtx;
	std::condition_variable _cond;
	int _thread_num;
	std::vector<std::thread> _threads;
	std::queue<Task> _tasks;
};


#endif // !_THREAD_POOL_H_

main.cpp

#include <iostream>
#include <chrono>
#include "thread_pool.hpp"

void fun1()
{
	std::cout << "working in thread " << std::this_thread::get_id() << std::endl;
}

void fun2(int x)
{
	std::cout << "task " << x << " working in thread " << std::this_thread::get_id() << std::endl;
}

int main(int argc, char* argv[])
{
	ThreadPool thread_pool(3);
	thread_pool.start();
	std::this_thread::sleep_for(std::chrono::milliseconds(500));

	for (int i = 0; i < 6; i++)
	{
		//thread_pool.appendTask(fun1);
		thread_pool.appendTask(std::bind(fun2, i));
		std::this_thread::sleep_for(std::chrono::milliseconds(500));
	}

	thread_pool.stop();

	getchar();
	return 0;
}

执行结果

begin work thread: 10492
begin work thread: 15024
begin work thread: 11684
task 0 working in thread 10492
task 1 working in thread 15024
task 2 working in thread 11684
task 3 working in thread 10492
task 4 working in thread 15024
task 5 working in thread 11684
end work thread: 11684
end work thread: 10492
end work thread: 15024

实现总结：

任务调度需要加锁和互斥
每次添加一个新的任务就唤醒一个线程
线程池析构时限唤醒所有线程，然后终止线程回收资源
没有实现线程的优先级调度

来源：CSDN

作者：xupeng1644

链接：https://blog.csdn.net/xp178171640/article/details/104570896