基於C++11實現線程池的工做原理

時間  2019-11-12
標籤 基於 c++11 實現 線程 原理 欄目 C&C++ 简体版
原文   原文鏈接

目錄git

基於C++11實現線程池的工做原理.

不久前寫過一篇線程池，那時候剛用C++寫東西不久，不少C++標準庫裏面的東西沒怎麼用，今天基於C++11從新實現了一個線程池。

簡介

線程池（thread pool）：一種線程的使用模式，線程過多會帶來調度開銷，進而影響緩存局部性和總體性能。而線程池維護着多個線程，等待着監督管理者分配可併發執行的任務。這避免了在處理短期任務時建立與銷燬線程的代價。線程池不只可以保證內核的充分利用，還能防止過度調度。可用線程數量應該取決於可用的併發處理器、處理器內核、內存、網絡sockets等的數量。github

線程池的組成

一、線程池管理器

建立必定數量的線程，啓動線程，調配任務，管理着線程池。
本篇線程池目前只須要啓動(start())，中止方法(stop())，及任務添加方法(addTask).
start()建立必定數量的線程池,進行線程循環.
stop()中止全部線程循環，回收全部資源.
addTask()添加任務.緩存

二、工做線程

線程池中線程，在線程池中等待並執行分配的任務.
本篇選用條件變量實現等待與通知機制.網絡

三、任務接口，

添加任務的接口,以供工做線程調度任務的執行。併發

四、任務隊列

用於存放沒有處理的任務。提供一種緩衝機制
同時任務隊列具備調度功能，高優先級的任務放在任務隊列前面;本篇選用priority_queue 與pair的結合用做任務優先隊列的結構.dom

線程池工做的四種狀況.

假設咱們的線程池大小爲3，任務隊列目前不作大小限制.socket

一、主程序當前沒有任務要執行，線程池中的任務隊列爲空閒狀態.

此狀況下全部工做線程處於空閒的等待狀態，任務緩衝隊列爲空.oop

二、主程序添加小於等於線程池中線程數量的任務.

此狀況基於情形1，全部工做線程已處在等待狀態，主線程開始添加三個任務，添加後通知(notif())喚醒線程池中的線程開始取(take())任務執行. 此時的任務緩衝隊列仍是空。性能

三、主程序添加任務數量大於當前線程池中線程數量的任務.

此狀況發生情形2後面，全部工做線程都在工做中，主線程開始添加第四個任務，添加後發現如今線程池中的線程用完了,因而存入任務緩衝隊列。工做線程空閒後主動從任務隊列取任務執行.
測試

四、主程序添加任務數量大於當前線程池中線程數量的任務,且任務緩衝隊列已滿.

此狀況發生情形3且設置了任務緩衝隊列大小後面，主程序添加第N個任務，添加後發現池子中的線程用完了，任務緩衝隊列也滿了，因而進入等待狀態、等待任務緩衝隊列中的任務騰空通知。
可是要注意這種情形會阻塞主線程,本篇暫不限制任務隊列大小,必要時再來優化.

實現

等待通知機制經過條件變量實現，Logger和CurrentThread,用於調試，能夠無視.

#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&);//禁止複製拷貝.
  const ThreadPool& operator=(const ThreadPool&);

  struct TaskPriorityCmp
  {
    bool operator()(const ThreadPool::TaskPair p1, const ThreadPool::TaskPair p2)
    {
        return p1.first > p2.first; //first的小值優先
    }
  };

  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;

}

測試程序

start() 、stop()

測試線程池基本的建立退出工做，及檢測資源是否正常回收.

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

測試添加任務接口，及優先任務隊列.

主線程首先添加了5個普通任務、 1s後添加一個高優先級任務，當前3個線程中的最早一個空閒後，會最早執行後面添加的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.

源碼下載

若是有須要，能夠訪問個人GitHub進行下載: https://github.com/BethlyRoseDaisley/ThreadPool

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