c++ - 线程等待父级

Question

我正在为我的 ubuntu 服务器(为我的多客户端匿名聊天程序)实现一个简单的线程池机制，我需要让我的工作线程休眠直到一个作业(以函数指针和参数的形式)需要被执行。

我当前的系统正在崩溃。我(工作线程正在)询问经理是否有工作可用，以及是否有 5 毫秒没有 sleep 。如果有，将作业添加到工作队列并运行该函数。可悲的周期浪费。

我喜欢做的是制作一个简单的类似事件的系统。我正在考虑拥有一个互斥量 vector (每个工作人员一个)，并在创建时将互斥量的句柄作为参数传入。然后在我的经理类(负责并分发作业)中，每当创建线程时，锁定互斥锁。当需要执行作业时解锁下一个互斥量，等待它被锁定和解锁，然后重新锁定它。不过，我想知道是否有更好的方法来达到这一目的。

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tldr; 所以我的问题是这样的。让线程等待来自管理类的作业的最有效、最有效和最安全的方法是什么？轮询是一种我什至应该考虑的技术(一次超过 1000 个客户端)，互斥锁是否合适？还是有其他技巧？

Answer 1

你需要的是条件变量。
所有工作线程调用 wait() 将暂停它们。

然后，父线程将一个工作项放入队列中，并在条件变量上调用信号。这将唤醒一个正在休眠的线程。它可以从队列中删除作业并执行作业，然后在条件变量上调用 wait 以返回休眠状态。

尝试:

#include <pthread.h>
#include <memory>
#include <list>

// Use RAII to do the lock/unlock
struct MutexLock
{
    MutexLock(pthread_mutex_t& m) : mutex(m)    { pthread_mutex_lock(&mutex); }
    ~MutexLock()                                { pthread_mutex_unlock(&mutex); }
    private:
        pthread_mutex_t&    mutex;
};

// The base class of all work we want to do.
struct Job
{
    virtual void doWork()  = 0;
};

// pthreads is a C library the call back must be a C function.
extern "C" void* threadPoolThreadStart(void*);

// The very basre minimal part of a thread pool
// It does not create the workers. You need to create the work threads
// then make them call workerStart(). I leave that as an exercise for you.
class ThreadPool
{

    public:
         ThreadPool(unsigned int threadCount=1);
        ~ThreadPool();

        void addWork(std::auto_ptr<Job> job);
    private:

        friend void* threadPoolThreadStart(void*);
        void workerStart();

        std::auto_ptr<Job>  getJob();

        bool                finished;   // Threads will re-wait while this is true.
        pthread_mutex_t     mutex;      // A lock so that we can sequence accesses.
        pthread_cond_t      cond;       // The condition variable that is used to hold worker threads.
        std::list<Job*>     workQueue;  // A queue of jobs.
        std::vector<pthread_t>threads;
};

// Create the thread pool
ThreadPool::ThreadPool(int unsigned threadCount)
    : finished(false)
    , threads(threadCount)
{
    // If we fail creating either pthread object than throw a fit.
    if (pthread_mutex_init(&mutex, NULL) != 0)
    {   throw int(1);
    }

    if (pthread_cond_init(&cond, NULL) != 0)
    {
        pthread_mutex_destroy(&mutex);
        throw int(2);
    }
    for(unsigned int loop=0; loop < threadCount;++loop)
    {
       if (pthread_create(threads[loop], NULL, threadPoolThreadStart, this) != 0)
       {
            // One thread failed: clean up
            for(unsigned int kill = loop -1; kill < loop /*unsigned will wrap*/;--kill)
            {
                pthread_kill(threads[kill], 9);
            }
            throw int(3);
       }
    }
}

// Cleanup any left overs.
// Note. This does not deal with worker threads.
//       You need to add a method to flush all worker threads
//       out of this pobject before you let the destructor destroy it.
ThreadPool::~ThreadPool()
{
    finished = true;
    for(std::vector<pthread_t>::iterator loop = threads.begin();loop != threads.end(); ++loop)
    {
        // Send enough signals to free all threads.
        pthread_cond_signal(&cond);
    }
    for(std::vector<pthread_t>::iterator loop = threads.begin();loop != threads.end(); ++loop)
    {
        // Wait for all threads to exit (they will as finished is true and
        //                               we sent enough signals to make sure
        //                               they are running).
        void*  result;
        pthread_join(*loop, &result);
    }
    // Destroy the pthread objects.
    pthread_cond_destroy(&cond);
    pthread_mutex_destroy(&mutex);

    // Delete all re-maining jobs.
    // Notice how we took ownership of the jobs.
    for(std::list<Job*>::const_iterator loop = workQueue.begin(); loop != workQueue.end();++loop)
    {
        delete *loop;
    }
}

// Add a new job to the queue
// Signal the condition variable. This will flush a waiting worker
// otherwise the job will wait for a worker to finish processing its current job.
void ThreadPool::addWork(std::auto_ptr<Job> job)
{
    MutexLock  lock(mutex);

    workQueue.push_back(job.release());
    pthread_cond_signal(&cond);
}

// Start a thread.
// Make sure no exceptions escape as that is bad.
void* threadPoolThreadStart(void* data)
{
    ThreadPool* pool = reinterpret_cast<ThreadPool*>(workerStart);
    try
    {
        pool->workerStart();
    }
    catch(...){}
    return NULL;
}

// This is the main worker loop.
void ThreadPool::workerStart()
{
    while(!finished)
    {
        std::auto_ptr<Job>    job    = getJob();
        if (job.get() != NULL)
        {
            job->doWork();
        }
    }
}

// The workers come here to get a job.
// If there are non in the queue they are suspended waiting on cond
// until a new job is added above.
std::auto_ptr<Job> ThreadPool::getJob()
{
    MutexLock  lock(mutex);

    while((workQueue.empty()) && (!finished))
    {
        pthread_cond_wait(&cond, &mutex);
        // The wait releases the mutex lock and suspends the thread (until a signal).
        // When a thread wakes up it is help until it can acquire the mutex so when we
        // get here the mutex is again locked.
        //
        // Note: You must use while() here. This is because of the situation.
        //   Two workers:  Worker A processing job A.
        //                 Worker B suspended on condition variable.
        //   Parent adds a new job and calls signal.
        //   This wakes up thread B. But it is possible for Worker A to finish its
        //   work and lock the mutex before the Worker B is released from the above call.
        //
        //   If that happens then Worker A will see that the queue is not empty
        //   and grab the work item in the queue and start processing. Worker B will
        //   then lock the mutext and proceed here. If the above is not a while then
        //   it would try and remove an item from an empty queue. With a while it sees
        //   that the queue is empty and re-suspends on the condition variable above.
    }
    std::auto_ptr<Job>  result;
    if (!finished)
    {    result.reset(workQueue.front());
         workQueue.pop_front();
    }

    return result;
}