【开源项目】POCO线程池分析

一、接口

    ThreadPool(int minCapacity = 2,
        int maxCapacity = 16,
        int idleTime = 60,
        int stackSize = POCO_THREAD_STACK_SIZE);
        /// Creates a thread pool with minCapacity threads.
        /// If required, up to maxCapacity threads are created
        /// a NoThreadAvailableException exception is thrown.
        /// If a thread is running idle for more than idleTime seconds,
        /// and more than minCapacity threads are running, the thread
        /// is killed. Threads are created with given stack size.

• 创建个minCapacity线程的线程池，线程池最多拥有maxCapacity个线程，在基本的minCapacity线程外的线程空闲时间超过idleTimes，将自动被杀死。

二、创建过程

ThreadPool::ThreadPool(int minCapacity,
    int maxCapacity,
    int idleTime,
    int stackSize): 
    _minCapacity(minCapacity), 
    _maxCapacity(maxCapacity), 
    _idleTime(idleTime),
    _serial(0),
    _age(0),
    _stackSize(stackSize)
{
    poco_assert (minCapacity >= 1 && maxCapacity >= minCapacity && idleTime > 0);

    for (int i = 0; i < _minCapacity; i++)
    {
        PooledThread* pThread = createThread();
        _threads.push_back(pThread);
        pThread->start();
    }
}

• 先创建minCapacity个PooledThread对象并保存到对象容器中，并同时执行start()函数：

void PooledThread::start()
{
    _thread.start(*this);
    _started.wait();
}

• _thread是Poco::Thread对象，具体构造函数：

Thread::Thread(): 
    _id(uniqueId()), 
    _name(makeName()), 
    _pTLS(0)
{
}

• Thread类继承于Poco里面ThreadImpl类，而这个ThreadImpl类是对线程创建等相关函数的封装。看看Thread::strart()接口：

void Thread::start(Runnable& target)
{
    startImpl(target);
}

• 这里的startImpl调用的就是：

void ThreadImpl::startImpl(Runnable& target)
{
    if (isRunningImpl())
        throw SystemException("thread already running");

    _pRunnableTarget = ⌖

    createImpl(runnableEntry, this);
}

• 我们最后看下，createImpl接口：

void ThreadImpl::createImpl(Entry ent, void* pData)
{
#if defined(_DLL)
    _thread = CreateThread(NULL, _stackSize, ent, pData, 0, &_threadId);
#else
    unsigned threadId;
    _thread = (HANDLE) _beginthreadex(NULL, _stackSize, ent, this, 0, &threadId);
    _threadId = static_cast(threadId);
#endif
    if (!_thread)
        throw SystemException("cannot create thread");
    if (_prio != PRIO_NORMAL_IMPL && !SetThreadPriority(_thread, _prio))
        throw SystemException("cannot set thread priority");
}

• 经过一连串的调用，我们终于看到最底层的创建线程方法，和我们普通写应用层线程创建一致，采用_beginthreadex方法。
• 我们可以很轻易的创建一个线程池：ThreadPool pool(2, 3, 3);

三、它是如何动态管理所创建的线程？

• 我们先使用掉一个线程，调用方法：

RunnableAdapter ra(*this, &CThreadPoolTest::Count);
pool.start(ra); //!<用掉一个

• 我们使用线程方法，是采用ThreadPool::start()方法：

void ThreadPool::start(Runnable& target)
{
    getThread()->start(Thread::PRIO_NORMAL, target);
}

• 关键的函数出现了getThread()，获取创建好的线程，执行start()方法。

PooledThread* ThreadPool::getThread()
{
    FastMutex::ScopedLock lock(_mutex);

    if (++_age == 32)
        housekeep();

    PooledThread* pThread = 0;
    for (ThreadVec::iterator it = _threads.begin(); !pThread && it != _threads.end(); ++it)
    {
        if ((*it)->idle()) pThread = *it;
    }
    if (!pThread)
    {
        if (_threads.size() < _maxCapacity)
        {
            pThread = createThread();
            try
            {
                pThread->start();
                _threads.push_back(pThread);
            }
            catch (...)
            {
                delete pThread;
                throw;
            }
        }
        else throw NoThreadAvailableException();
    }
    pThread->activate();
    return pThread;
}

• 这个函数很简单，首先遍历之前创建并存储到容器的线程，判断哪个线程是处理空闲状态，如果有则直接取出对象赋值。如果不存在空闲的线程，则判断当前已创建线程的总数量，是否超过预设的最大值，如果没有则创建新的线程。
• 最后直接激活pThread->activate();就是将空闲状态置为false

void PooledThread::activate()
{
    FastMutex::ScopedLock lock(_mutex);

    poco_assert (_idle);
    _idle = false;
    _targetCompleted.reset();
}

• 整体分析下来，Poco采用了更加严谨的逻辑处理线程池的分配。那如何增大一开始设置的最大线程数呢，我们采用pool.addCapacity(1);的方法，将最大线程数+1。

void ThreadPool::addCapacity(int n)
{
    FastMutex::ScopedLock lock(_mutex);

    poco_assert (_maxCapacity + n >= _minCapacity);
    _maxCapacity += n;
    housekeep();
}

在我们增加最大线程数的同时，还执行了housekeep()函数：重新整理已有线程池，这边就涉及到自动清除空闲太久的线程方法：

void ThreadPool::housekeep()
{
    _age = 0;
    if (_threads.size() <= _minCapacity)
        return;

    ThreadVec idleThreads;
    ThreadVec expiredThreads;
    ThreadVec activeThreads;
    idleThreads.reserve(_threads.size());
    activeThreads.reserve(_threads.size());

    for (ThreadVec::iterator it = _threads.begin(); it != _threads.end(); ++it)
    {
        if ((*it)->idle())
        {
            if ((*it)->idleTime() < _idleTime)
                idleThreads.push_back(*it);
            else 
                expiredThreads.push_back(*it);  
        }
        else activeThreads.push_back(*it);
    }
    int n = (int) activeThreads.size();
    int limit = (int) idleThreads.size() + n;
    if (limit < _minCapacity) limit = _minCapacity;
    idleThreads.insert(idleThreads.end(), expiredThreads.begin(), expiredThreads.end());
    _threads.clear();
    for (ThreadVec::iterator it = idleThreads.begin(); it != idleThreads.end(); ++it)
    {
        if (n < limit)
        {
            _threads.push_back(*it);
            ++n;
        }
        else (*it)->release();
    }
    _threads.insert(_threads.end(), activeThreads.begin(), activeThreads.end());
}

• 直接遍历已创建的线程，空闲线程则比对空闲的时间是否超过设置的时间，如果超过则添加到另一个容器中expiredThreads，非空闲线程都加入activeThreads中。这里面有个设计，可能大伙看的不是很明白，为什么都区分超时和非超时的线程，最后还要将超时的expiredThreads内容插入到，非超时的idleThreads里面呢？其实，是为了防止已激活的和未超时的总数达不到limit值，并且我们是将expiredThreads插入到的是idleThreads的尾部，即使后面遍历的时候，也是先遍历未超时的。
• 最后遍历idleThreads，当总数小于limit时，我们先插入空闲的线程，总数达到limit后，剩下的空闲线程直接执行release()释放。最后再将已激活的线程加入到_threads尾部。着就是线程池的重新整理。
  …
  五、调用实例

#pragma once
#include <assert.h>
#include "OutputDebugString.h"
#include "Poco/ThreadPool.h"
#include "Poco/RunnableAdapter.h"
#include "Poco/Exception.h"
#include "Poco/Thread.h"
#include "Poco/Event.h"
#include "Poco/Mutex.h"

using Poco::ThreadPool;
using Poco::RunnableAdapter;
using Poco::Thread;

class CThreadPoolTest
{
public:
    CThreadPoolTest(): _count(0), _event(false){};
    ~CThreadPoolTest(){};

    void Count()
    {
        _event.wait();
        odprintf("Test run.....");
        for (int i = 0; i < 10000; ++i)
        {
            _mutex.lock();
            ++_count;
            //odprintf("values:%d", _count);
            printf("values:%d\n", _count);
            _mutex.unlock();
        }
    };

    void Test()
    {
        ThreadPool pool(2, 3, 3); //!<创建个2线程的线程池，线程池最多拥有3个线程，在基本的2线程外的线程空闲时间超过3s，将被杀死
        pool.setStackSize(1);

        assert (pool.allocated() == 2); //!<判断当前是不是只有2线程
        assert (pool.used() == 0); //!<判断当前是否线程在工作
        assert (pool.capacity() == 3); //!<判断最大的线程数
        assert (pool.available() == 3);
        pool.addCapacity(1); 
        assert (pool.allocated() == 2);
        assert (pool.used() == 0);
        assert (pool.capacity() == 4);
        assert (pool.available() == 4);

        RunnableAdapter ra(*this, &CThreadPoolTest::Count);
        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 2);
        assert (pool.used() == 1);
        assert (pool.capacity() == 4);
        assert (pool.available() == 3);

        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 2);
        assert (pool.used() == 2);
        assert (pool.capacity() == 4);
        assert (pool.available() == 2);

        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 3);
        assert (pool.used() == 3);
        assert (pool.capacity() == 4);
        assert (pool.available() == 1);

        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 4);
        assert (pool.used() == 4);
        assert (pool.capacity() == 4);
        assert (pool.available() == 0);

        try
        {
            pool.start(ra); //!<用掉一个，此时没有可用将发生异常
            odprintf("thread pool exhausted - must throw exception");
        }   
        catch (Poco::NoThreadAvailableException&)
        {
            odprintf("NoThreadAvailable exception thrown");
        }
        catch (...)
        {
            odprintf("wrong exception thrown");
        }

        _event.set(); // go!!! Run 开始执行循环
        pool.joinAll(); //!<等待所有线程完成

        assert (_count == 40000); //!<4个线程在累加，总的值为4W

        //!<所有线程全部退出了
        assert (pool.allocated() == 4);
        assert (pool.used() == 0);
        assert (pool.capacity() == 4);
        assert (pool.available() == 4);

        Thread::sleep(4000);

        pool.collect(); //!<开始回收了，最终只会剩下2个线程
        assert (pool.allocated() == 2);
        assert (pool.used() == 0);
        assert (pool.capacity() == 4);
        assert (pool.available() == 4);

        _count = 0;
        _event.reset(); //!
        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 2);
        assert (pool.used() == 1);
        assert (pool.capacity() == 4);
        assert (pool.available() == 3);

        pool.start(ra); //!<用掉一个
        assert (pool.allocated() == 2);
        assert (pool.used() == 2);
        assert (pool.capacity() == 4);
        assert (pool.available() == 2);
        _event.set(); // go!!!
        pool.joinAll();

        assert (_count == 20000);

        assert (pool.allocated() == 2);
        assert (pool.used() == 0);
        assert (pool.capacity() == 4);
        assert (pool.available() == 4);
    };

private:
    Poco::Event _event;
    Poco::FastMutex _mutex;
    int _count;
};