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、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 pool
using 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;
};
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;
}
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: 0
How many products sell: 1
How many products sell: 2
How many products sell: 3
How many products sell: 4
How many products sell: 5
How many products sell: 6
How many products sell: 7
How many products sell: 8
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How many products sell: 149
相关的代码，上传在Github，欢迎使用并提出意见。
