Producer-Consumer with a variation - How to synchronize with thread signal/wait?

Question

While working on a large project I realized I was making a lot of calls to be scheduled in the future. Since these were fairly light-weight, I thought it might be better to use a separate scheduler.

ThreadPool.QueueUserWorkItem (() => 
{
    Thread.Sleep (5000);
    Foo (); // Call is to be executed after sometime
});

So I created a separate scheduler class that runs on its own thread and executes these events. I have 2 functions that access a shared queue from separate threads. I'd use a lock, but since one of the threads needs to sleep-wait, I wasn't sure how to release the lock.

class Scheduler
{
    SortedDictionary <DateTime, Action> _queue;
    EventWaitHandle _sync;

    // Runs on its own thread
    void Run ()
    {
        while (true)
        {
            // Calculate time till first event
            // If queue empty, use pre-defined value
            TimeSpan timeDiff = _queue.First().Key - DateTime.Now;

            // Execute action if in the next 100ms
            if (timeDiff < 100ms)
                ...
            // Wait on event handle for time
            else
                _sync.WaitOne (timeDiff);
        }
    }

    // Can be called by any thread
    void ScheduleEvent (Action action, DataTime time)
    {
        _queue.Add (time, action);
        // Signal thread to wake up and check again
        _sync.Set ();
    }
}

The trouble is, I'm not sure how to synchronize access to the queue between the 2 functions. I can't use a monitor or mutex, because Run() will sleep-wait, thus causing a deadlock. What is the right synchronization mechanism to use here? (If there a mechanism to atomically start the sleep-wait process and immediately release the lock, that might solve my problem)

How can I verify there is no race-condition?

Is this a variation of the producer consumer problem, or is there a more relevant synchronization problem-description?

While this is somewhat geared towards C#, I'd be happy to hear a general solution to this. Thanks!

Answer 1

OK, take 2 with Monitor/Pulse.

    void Run ()    
    {
        while (true)
        {
            Action doit = null;

            lock(_queueLock)
            {
                while (_queue.IsEmpty())
                    Monitor.Wait(_queueLock);

                TimeSpan timeDiff = _queue.First().Key - DateTime.Now;
                if (timeDiff < 100ms)
                    doit = _queue.Dequeue();
            }

            if (doit != null)
                ; //execute doit
            else
             _sync.WaitOne (timeDiff);  
        }
    }


void ScheduleEvent (Action action, DataTime time)
{
    lock (_queueLock)
    {
        _queue.Add(time, action);
        // Signal thread to wake up and check again
        _sync.Set ();
        if (_queue.Count == 1)
             Monitor.Pulse(_queuLock);
    }
}

Answer 2

The problem is easily solved, make sure the WaitOne is outside the lock.

  //untested
  while (true)
  {
      Action doit = null;

      // Calculate time till first event
      // If queue empty, use pre-defined value
      lock(_queueLock)
      {
         TimeSpan timeDiff = _queue.First().Key - DateTime.Now;
         if (timeDiff < 100ms)
            doit = _queue.Dequeue();
      }
      if (doit != null)
        // execute it
      else
         _sync.WaitOne (timeDiff);
  }

_queueLock is a private helper object.

Answer 3

Since your goal is to schedule a task after a particular period of time, why not just use the System.Threading.Timer? It doesn't require dedicating a thread for the scheduling and takes advantage of the OS to wake up a worker thread. I've used this (removed some comments and other timer service functionality):

public sealed class TimerService : ITimerService
{
    public void WhenElapsed(TimeSpan duration, Action callback)
    {
        if (callback == null) throw new ArgumentNullException("callback");

        //Set up state to allow cleanup after timer completes
        var timerState = new TimerState(callback);
        var timer = new Timer(OnTimerElapsed, timerState, Timeout.Infinite, Timeout.Infinite);
        timerState.Timer = timer;

        //Start the timer
        timer.Change((int) duration.TotalMilliseconds, Timeout.Infinite);
    }

    private void OnTimerElapsed(Object state)
    {
        var timerState = (TimerState)state;
        timerState.Timer.Dispose();
        timerState.Callback();
    }

    private class TimerState
    {
        public Timer Timer { get; set; }

        public Action Callback { get; private set; }

        public TimerState(Action callback)
        {
            Callback = callback;
        }
    }
}

Answer 4

The monitores were created for this kind of situation, simple problems that can cost mutch for the application, i present my solution to this very simple and if u want to make a shutdown easy to implement:

    void Run()
    {
      while(true)
         lock(this)
         {
            int timeToSleep = getTimeToSleep() //check your list and return a value
            if(timeToSleep <= 100) 
                action...
            else
            {

               int currTime = Datetime.Now;
               int currCount = yourList.Count;
               try{
               do{
                 Monitor.Wait(this,timeToSleep);

                 if(Datetime.now >= (tomeToSleep + currtime))
                      break; //time passed

                 else if(yourList.Count != currCount)
                    break; //new element added go check it
                 currTime = Datetime.Now;
               }while(true);
            }
            }catch(ThreadInterruptedException e)
            {
                //do cleanup code or check for shutdown notification
            }
         }
      }
    }

void ScheduleEvent (Action action, DataTime time)
{
    lock(this)
    {
       yourlist.add ...
       Monitor.Pulse(this);

} }