目录

• 基于c++11实现线程池的工作原理.
  • 简介
  • 线程池的组成
  • 线程池工作的四种情况.
  • 实现
  • 测试程序
    • addtask()、prioritytaskqueue
  • 源码下载

基于c++11实现线程池的工作原理.

---

不久前写过一篇线程池，那时候刚用c++写东西不久，很多c++标准库里面的东西没怎么用，今天基于c++11重新实现了一个线程池。

简介

线程池（thread pool）：一种线程的使用模式，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。

线程池的组成

1、线程池管理器

创建一定数量的线程，启动线程，调配任务，管理着线程池。
本篇线程池目前只需要启动(start())，停止方法(stop())，及任务添加方法(addtask).
start()创建一定数量的线程池,进行线程循环.
stop()停止所有线程循环，回收所有资源.
addtask()添加任务.

2、工作线程

线程池中线程，在线程池中等待并执行分配的任务.
本篇选用条件变量实现等待与通知机制.

3、任务接口，

添加任务的接口,以供工作线程调度任务的执行。

4、任务队列

用于存放没有处理的任务。提供一种缓冲机制
同时任务队列具有调度功能，高优先级的任务放在任务队列前面;本篇选用priority_queue 与pair的结合用作任务优先队列的结构.

线程池工作的四种情况.

假设我们的线程池大小为3，任务队列目前不做大小限制.

1、主程序当前没有任务要执行，线程池中的任务队列为空闲状态.

此情况下所有工作线程处于空闲的等待状态，任务缓冲队列为空.

2、主程序添加小于等于线程池中线程数量的任务.

此情况基于情形1，所有工作线程已处在等待状态，主线程开始添加三个任务，添加后通知(notif())唤醒线程池中的线程开始取(take())任务执行. 此时的任务缓冲队列还是空。

3、主程序添加任务数量大于当前线程池中线程数量的任务.

此情况发生情形2后面，所有工作线程都在工作中，主线程开始添加第四个任务，添加后发现现在线程池中的线程用完了,于是存入任务缓冲队列。工作线程空闲后主动从任务队列取任务执行.

4、主程序添加任务数量大于当前线程池中线程数量的任务,且任务缓冲队列已满.

此情况发生情形3且设置了任务缓冲队列大小后面，主程序添加第n个任务，添加后发现池子中的线程用完了，任务缓冲队列也满了，于是进入等待状态、等待任务缓冲队列中的任务腾空通知。
但是要注意这种情形会阻塞主线程,本篇暂不限制任务队列大小,必要时再来优化.

实现

等待通知机制通过条件变量实现，logger和currentthread,用于调试，可以无视.

#ifndef _threadpool_hh
#define _threadpool_hh

#include <vector>
#include <utility>
#include <queue>
#include <thread>
#include <functional>
#include <mutex>

#include "condition.hh"

class threadpool{
public:
  static const int kinitthreadssize = 3;
  enum taskprioritye { level0, level1, level2, };
  typedef std::function<void()> task;
  typedef std::pair<taskprioritye, task> taskpair;

  threadpool();
  ~threadpool();

  void start();
  void stop();
  void addtask(const task&);
  void addtask(const taskpair&);

private:
  threadpool(const threadpool&);//禁止复制拷贝.
  const threadpool& operator=(const threadpool&);

  struct taskprioritycmp
  {
    bool operator()(const threadpool::taskpair p1, const threadpool::taskpair p2)
    {
        return p1.first > p2.first; //first的小值优先
    }
  };

  void threadloop();
  task take();

  typedef std::vector<std::thread*> threads;
  typedef std::priority_queue<taskpair, std::vector<taskpair>, taskprioritycmp> tasks;

  threads m_threads;
  tasks m_tasks;

  std::mutex m_mutex;
  condition m_cond;
  bool m_isstarted;
};

#endif

//cpp

#include <assert.h>

#include "logger.hh" // debug
#include "currentthread.hh" // debug
#include "threadpool.hh"

threadpool::threadpool()
  :m_mutex(),
  m_cond(m_mutex),
  m_isstarted(false)
{

}

threadpool::~threadpool()
{
  if(m_isstarted)
  {
    stop();
  }
}

void threadpool::start()
{
  assert(m_threads.empty());
  m_isstarted = true;
  m_threads.reserve(kinitthreadssize);
  for (int i = 0; i < kinitthreadssize; ++i)
  {
    m_threads.push_back(new std::thread(std::bind(&threadpool::threadloop, this)));
  }

}

void threadpool::stop()
{
  log_trace << "threadpool::stop() stop.";
  {
    std::unique_lock<std::mutex> lock(m_mutex);
    m_isstarted = false;
    m_cond.notifyall();
    log_trace << "threadpool::stop() notifyall().";
  }

  for (threads::iterator it = m_threads.begin(); it != m_threads.end() ; ++it)
  {
    (*it)->join();
    delete *it;
  }
  m_threads.clear();
}


void threadpool::threadloop()
{
  log_trace << "threadpool::threadloop() tid : " << currentthread::tid() << " start.";
  while(m_isstarted)
  {
    task task = take();
    if(task)
    {
      task();
    }
  }
  log_trace << "threadpool::threadloop() tid : " << currentthread::tid() << " exit.";
}

void threadpool::addtask(const task& task)
{
  std::unique_lock<std::mutex> lock(m_mutex);
  /*while(m_tasks.isfull())
    {//when m_tasks have maxsize
      cond2.wait();
    }
  */
  taskpair taskpair(level2, task);
  m_tasks.push(taskpair);
  m_cond.notify();
}

void threadpool::addtask(const taskpair& taskpair)
{
  std::unique_lock<std::mutex> lock(m_mutex);
  /*while(m_tasks.isfull())
    {//when m_tasks have maxsize
      cond2.wait();
    }
  */
  m_tasks.push(taskpair);
  m_cond.notify();
}

threadpool::task threadpool::take()
{
  std::unique_lock<std::mutex> lock(m_mutex);
  //always use a while-loop, due to spurious wakeup
  while(m_tasks.empty() && m_isstarted)
  {
    log_trace << "threadpool::take() tid : " << currentthread::tid() << " wait.";
    m_cond.wait(lock);
  }

  log_trace << "threadpool::take() tid : " << currentthread::tid() << " wakeup.";

  task task;
  tasks::size_type size = m_tasks.size();
  if(!m_tasks.empty() && m_isstarted)
  {
    task = m_tasks.top().second;
    m_tasks.pop();
    assert(size - 1 == m_tasks.size());
    /*if (taskqueuesize_ > 0)
    {
      cond2.notify();
    }*/
  }

  return task;

}

测试程序

start() 、stop()

测试线程池基本的创建退出工作，及检测资源是否正常回收.

int main()
{
  {
  threadpool threadpool;
  threadpool.start();

  getchar();
  }

  getchar();

  return 0;
}

./test.out 
2018-11-25 16:50:36.054805 [trace] [threadpool.cpp:53] [threadloop] threadpool::threadloop() tid : 3680 start.
2018-11-25 16:50:36.054855 [trace] [threadpool.cpp:72] [take] threadpool::take() tid : 3680 wait.
2018-11-25 16:50:36.055633 [trace] [threadpool.cpp:53] [threadloop] threadpool::threadloop() tid : 3679 start.
2018-11-25 16:50:36.055676 [trace] [threadpool.cpp:72] [take] threadpool::take() tid : 3679 wait.
2018-11-25 16:50:36.055641 [trace] [threadpool.cpp:53] [threadloop] threadpool::threadloop() tid : 3681 start.
2018-11-25 16:50:36.055701 [trace] [threadpool.cpp:72] [take] threadpool::take() tid : 3681 wait.
2018-11-25 16:50:36.055736 [trace] [threadpool.cpp:53] [threadloop] threadpool::threadloop() tid : 3682 start.
2018-11-25 16:50:36.055746 [trace] [threadpool.cpp:72] [take] threadpool::take() tid : 3682 wait.

2018-11-25 16:51:01.411792 [trace] [threadpool.cpp:36] [stop] threadpool::stop() stop.
2018-11-25 16:51:01.411863 [trace] [threadpool.cpp:39] [stop] threadpool::stop() notifyall().
2018-11-25 16:51:01.411877 [trace] [threadpool.cpp:76] [take] threadpool::take() tid : 3680 wakeup.
2018-11-25 16:51:01.411883 [trace] [threadpool.cpp:62] [threadloop] threadpool::threadloop() tid : 3680 exit.
2018-11-25 16:51:01.412062 [trace] [threadpool.cpp:76] [take] threadpool::take() tid : 3682 wakeup.
2018-11-25 16:51:01.412110 [trace] [threadpool.cpp:62] [threadloop] threadpool::threadloop() tid : 3682 exit.
2018-11-25 16:51:01.413052 [trace] [threadpool.cpp:76] [take] threadpool::take() tid : 3679 wakeup.
2018-11-25 16:51:01.413098 [trace] [threadpool.cpp:62] [threadloop] threadpool::threadloop() tid : 3679 exit.
2018-11-25 16:51:01.413112 [trace] [threadpool.cpp:76] [take] threadpool::take() tid : 3681 wakeup.
2018-11-25 16:51:01.413141 [trace] [threadpool.cpp:62] [threadloop] threadpool::threadloop() tid : 3681 exit.

addtask()、prioritytaskqueue

测试添加任务接口，及优先任务队列.

主线程首先添加了5个普通任务、 1s后添加一个高优先级任务，当前3个线程中的最先一个空闲后，会最先执行后面添加的priorityfunc().

std::mutex g_mutex;

void priorityfunc()
{
  for (int i = 1; i < 4; ++i)
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
      std::lock_guard<std::mutex> lock(g_mutex);
      log_debug << "priorityfunc() [" << i << "at thread [ " << currentthread::tid() << "] output";// << std::endl;
  }

}

void testfunc()
{
  // loop to print character after a random period of time
  for (int i = 1; i < 4; ++i)
  {
      std::this_thread::sleep_for(std::chrono::seconds(1));
      std::lock_guard<std::mutex> lock(g_mutex);
      log_debug << "testfunc() [" << i << "] at thread [ " << currentthread::tid() << "] output";// << std::endl;
  }

}


int main()
{
  threadpool threadpool;
  threadpool.start();

  for(int i = 0; i < 5 ; i++)
    threadpool.addtask(testfunc);

  std::this_thread::sleep_for(std::chrono::seconds(1));

  threadpool.addtask(threadpool::taskpair(threadpool::level0, priorityfunc));

  getchar();
  return 0;
}

./test.out 
2018-11-25 18:24:20.886837 [trace] [threadpool.cpp:56] [threadloop] threadpool::threadloop() tid : 4121 start.
2018-11-25 18:24:20.886893 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4121 wakeup.
2018-11-25 18:24:20.887580 [trace] [threadpool.cpp:56] [threadloop] threadpool::threadloop() tid : 4120 start.
2018-11-25 18:24:20.887606 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4120 wakeup.
2018-11-25 18:24:20.887610 [trace] [threadpool.cpp:56] [threadloop] threadpool::threadloop() tid : 4122 start.
2018-11-25 18:24:20.887620 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4122 wakeup.
2018-11-25 18:24:21.887779 [debug] [main.cpp:104] [testfunc] testfunc() [1] at thread [ 4120] output
2018-11-25 18:24:21.887813 [debug] [main.cpp:104] [testfunc] testfunc() [1] at thread [ 4122] output
2018-11-25 18:24:21.888909 [debug] [main.cpp:104] [testfunc] testfunc() [1] at thread [ 4121] output
2018-11-25 18:24:22.888049 [debug] [main.cpp:104] [testfunc] testfunc() [2] at thread [ 4120] output
2018-11-25 18:24:22.888288 [debug] [main.cpp:104] [testfunc] testfunc() [2] at thread [ 4122] output
2018-11-25 18:24:22.889978 [debug] [main.cpp:104] [testfunc] testfunc() [2] at thread [ 4121] output
2018-11-25 18:24:23.888467 [debug] [main.cpp:104] [testfunc] testfunc() [3] at thread [ 4120] output
2018-11-25 18:24:23.888724 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4120 wakeup.
2018-11-25 18:24:23.888778 [debug] [main.cpp:104] [testfunc] testfunc() [3] at thread [ 4122] output
2018-11-25 18:24:23.888806 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4122 wakeup.
2018-11-25 18:24:23.890413 [debug] [main.cpp:104] [testfunc] testfunc() [3] at thread [ 4121] output
2018-11-25 18:24:23.890437 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4121 wakeup.
2018-11-25 18:24:24.889247 [debug] [main.cpp:92] [priorityfunc] priorityfunc() [1at thread [ 4120] output
2018-11-25 18:24:24.891187 [debug] [main.cpp:104] [testfunc] testfunc() [1] at thread [ 4121] output
2018-11-25 18:24:24.893163 [debug] [main.cpp:104] [testfunc] testfunc() [1] at thread [ 4122] output
2018-11-25 18:24:25.889567 [debug] [main.cpp:92] [priorityfunc] priorityfunc() [2at thread [ 4120] output
2018-11-25 18:24:25.891477 [debug] [main.cpp:104] [testfunc] testfunc() [2] at thread [ 4121] output
2018-11-25 18:24:25.893450 [debug] [main.cpp:104] [testfunc] testfunc() [2] at thread [ 4122] output
2018-11-25 18:24:26.890295 [debug] [main.cpp:92] [priorityfunc] priorityfunc() [3at thread [ 4120] output
2018-11-25 18:24:26.890335 [trace] [threadpool.cpp:99] [take] threadpool::take() tid : 4120 wait.
2018-11-25 18:24:26.892265 [debug] [main.cpp:104] [testfunc] testfunc() [3] at thread [ 4121] output
2018-11-25 18:24:26.892294 [trace] [threadpool.cpp:99] [take] threadpool::take() tid : 4121 wait.
2018-11-25 18:24:26.894274 [debug] [main.cpp:104] [testfunc] testfunc() [3] at thread [ 4122] output
2018-11-25 18:24:26.894299 [trace] [threadpool.cpp:99] [take] threadpool::take() tid : 4122 wait.

2018-11-25 18:24:35.359003 [trace] [threadpool.cpp:37] [stop] threadpool::stop() stop.
2018-11-25 18:24:35.359043 [trace] [threadpool.cpp:42] [stop] threadpool::stop() notifyall().
2018-11-25 18:24:35.359061 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4120 wakeup.
2018-11-25 18:24:35.359067 [trace] [threadpool.cpp:65] [threadloop] threadpool::threadloop() tid : 4120 exit.
2018-11-25 18:24:35.359080 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4122 wakeup.
2018-11-25 18:24:35.359090 [trace] [threadpool.cpp:65] [threadloop] threadpool::threadloop() tid : 4122 exit.
2018-11-25 18:24:35.359123 [trace] [threadpool.cpp:103] [take] threadpool::take() tid : 4121 wakeup.
2018-11-25 18:24:35.359130 [trace] [threadpool.cpp:65] [threadloop] threadpool::threadloop() tid : 4121 exit.

源码下载

如果有需要，可以访问我的github进行下载: https://github.com/bethlyrosedaisley/threadpool