Limited concurrency level task scheduler (with task priority) handling wrapped tasks

Question

I'm having a hard time finding a task scheduler on which I can schedule prioritised tasks but can also handle "wrapped" tasks. It is something like what Task.Run tries to solve, but you cannot specify a task scheduler to Task.Run. I have been using a QueuedTaskScheduler from the Parallel Extensions Extras Samples to solve the task priority requirement (also suggested by this post).

Here is my example:

class Program
{
    private static QueuedTaskScheduler queueScheduler = new QueuedTaskScheduler(targetScheduler: TaskScheduler.Default, maxConcurrencyLevel: 1);
    private static TaskScheduler ts_priority1;
    private static TaskScheduler ts_priority2;
    static void Main(string[] args)
    {
        ts_priority1 = queueScheduler.ActivateNewQueue(1);
        ts_priority2 = queueScheduler.ActivateNewQueue(2);

        QueueValue(1, ts_priority2);
        QueueValue(2, ts_priority2);
        QueueValue(3, ts_priority2);
        QueueValue(4, ts_priority1);
        QueueValue(5, ts_priority1);
        QueueValue(6, ts_priority1);

        Console.ReadLine();           
    }

    private static Task QueueTask(Func<Task> f, TaskScheduler ts)
    {
        return Task.Factory.StartNew(f, CancellationToken.None, TaskCreationOptions.HideScheduler | TaskCreationOptions.DenyChildAttach, ts);
    }

    private static Task QueueValue(int i, TaskScheduler ts)
    {
        return QueueTask(async () =>
        {
            Console.WriteLine("Start {0}", i);
            await Task.Delay(1000);
            Console.WriteLine("End {0}", i);
        }, ts);
    }
}

The typical output of the example above is:

Start 4
Start 5
Start 6
Start 1
Start 2
Start 3
End 4
End 3
End 5
End 2
End 1
End 6

What I want is:

Start 4
End 4
Start 5
End 5
Start 6
End 6
Start 1
End 1
Start 2
End 2
Start 3
End 3

EDIT:

I think I'm looking for a task scheduler, similar to QueuedTaskScheduler, that will solve this problem. But any other suggestions are welcome.

Answer 1

Unfortunately, this can't be solved with a TaskScheduler, because they always work at the Task level, and an async method almost always contains multiple Tasks.

You should use a SemaphoreSlim in conjunction with a prioritizing scheduler. Alternatively, you could use AsyncLock (which is also included in my AsyncEx library).

class Program
{
  private static QueuedTaskScheduler queueScheduler = new QueuedTaskScheduler(targetScheduler: TaskScheduler.Default, maxConcurrencyLevel: 1);
  private static TaskScheduler ts_priority1;
  private static TaskScheduler ts_priority2;
  private static SemaphoreSlim semaphore = new SemaphoreSlim(1);
  static void Main(string[] args)
  {
    ts_priority1 = queueScheduler.ActivateNewQueue(1);
    ts_priority2 = queueScheduler.ActivateNewQueue(2);

    QueueValue(1, ts_priority2);
    QueueValue(2, ts_priority2);
    QueueValue(3, ts_priority2);
    QueueValue(4, ts_priority1);
    QueueValue(5, ts_priority1);
    QueueValue(6, ts_priority1);

    Console.ReadLine();           
  }

  private static Task QueueTask(Func<Task> f, TaskScheduler ts)
  {
    return Task.Factory.StartNew(f, CancellationToken.None, TaskCreationOptions.HideScheduler | TaskCreationOptions.DenyChildAttach, ts).Unwrap();
  }

  private static Task QueueValue(int i, TaskScheduler ts)
  {
    return QueueTask(async () =>
    {
      await semaphore.WaitAsync();
      try
      {
        Console.WriteLine("Start {0}", i);
        await Task.Delay(1000);
        Console.WriteLine("End {0}", i);
      }
      finally
      {
        semaphore.Release();
      }
    }, ts);
  }
}

Answer 2

The best solution I could find is to make my own version of the QueuedTaskScheduler (original found in the Parallel Extensions Extras Samples source code).

I added a bool awaitWrappedTasks parameter to the constructors of the QueuedTaskScheduler.

public QueuedTaskScheduler(
        TaskScheduler targetScheduler,
        int maxConcurrencyLevel,
        bool awaitWrappedTasks = false)
{
    ...
    _awaitWrappedTasks = awaitWrappedTasks;
    ...
}

public QueuedTaskScheduler(
        int threadCount,
        string threadName = "",
        bool useForegroundThreads = false,
        ThreadPriority threadPriority = ThreadPriority.Normal,
        ApartmentState threadApartmentState = ApartmentState.MTA,
        int threadMaxStackSize = 0,
        Action threadInit = null,
        Action threadFinally = null,
        bool awaitWrappedTasks = false)
{
    ...
    _awaitWrappedTasks = awaitWrappedTasks;

    // code starting threads (removed here in example)
    ...
}

I then modified the ProcessPrioritizedAndBatchedTasks() method to be async

private async void ProcessPrioritizedAndBatchedTasks()

I then modified the code just after the part where the scheduled task is executed:

private async void ProcessPrioritizedAndBatchedTasks()
{
    bool continueProcessing = true;
    while (!_disposeCancellation.IsCancellationRequested && continueProcessing)
    {
        try
        {
            // Note that we're processing tasks on this thread
            _taskProcessingThread.Value = true;

            // Until there are no more tasks to process
            while (!_disposeCancellation.IsCancellationRequested)
            {
                // Try to get the next task.  If there aren't any more, we're done.
                Task targetTask;
                lock (_nonthreadsafeTaskQueue)
                {
                    if (_nonthreadsafeTaskQueue.Count == 0) break;
                    targetTask = _nonthreadsafeTaskQueue.Dequeue();
                }

                // If the task is null, it's a placeholder for a task in the round-robin queues.
                // Find the next one that should be processed.
                QueuedTaskSchedulerQueue queueForTargetTask = null;
                if (targetTask == null)
                {
                    lock (_queueGroups) FindNextTask_NeedsLock(out targetTask, out queueForTargetTask);
                }

                // Now if we finally have a task, run it.  If the task
                // was associated with one of the round-robin schedulers, we need to use it
                // as a thunk to execute its task.
                if (targetTask != null)
                {
                    if (queueForTargetTask != null) queueForTargetTask.ExecuteTask(targetTask);
                    else TryExecuteTask(targetTask);

                    // ***** MODIFIED CODE START ****
                    if (_awaitWrappedTasks)
                    {
                        var targetTaskType = targetTask.GetType();
                        if (targetTaskType.IsConstructedGenericType && typeof(Task).IsAssignableFrom(targetTaskType.GetGenericArguments()[0]))
                        {
                            dynamic targetTaskDynamic = targetTask;
                            // Here we await the completion of the proxy task.
                            // We do not await the proxy task directly, because that would result in that await will throw the exception of the wrapped task (if one existed)
                            // In the continuation we then simply return the value of the exception object so that the exception (stored in the proxy task) does not go totally unobserved (that could cause the process to crash)
                            await TaskExtensions.Unwrap(targetTaskDynamic).ContinueWith((Func<Task, Exception>)(t => t.Exception), TaskContinuationOptions.ExecuteSynchronously);
                        }
                    }
                    // ***** MODIFIED CODE END ****
                }
            }
        }
        finally
        {
            // Now that we think we're done, verify that there really is
            // no more work to do.  If there's not, highlight
            // that we're now less parallel than we were a moment ago.
            lock (_nonthreadsafeTaskQueue)
            {
                if (_nonthreadsafeTaskQueue.Count == 0)
                {
                    _delegatesQueuedOrRunning--;
                    continueProcessing = false;
                    _taskProcessingThread.Value = false;
                }
            }
        }
    }
}

The change of method ThreadBasedDispatchLoop was a bit different, in that we cannot use the async keyword or else we will break the functionality of executing scheduled tasks in the dedicated thread(s). So here is the modified version of ThreadBasedDispatchLoop

private void ThreadBasedDispatchLoop(Action threadInit, Action threadFinally)
{
    _taskProcessingThread.Value = true;
    if (threadInit != null) threadInit();
    try
    {
        // If the scheduler is disposed, the cancellation token will be set and
        // we'll receive an OperationCanceledException.  That OCE should not crash the process.
        try
        {
            // If a thread abort occurs, we'll try to reset it and continue running.
            while (true)
            {
                try
                {
                    // For each task queued to the scheduler, try to execute it.
                    foreach (var task in _blockingTaskQueue.GetConsumingEnumerable(_disposeCancellation.Token))
                    {
                        Task targetTask = task;
                        // If the task is not null, that means it was queued to this scheduler directly.
                        // Run it.
                        if (targetTask != null)
                        {
                            TryExecuteTask(targetTask);
                        }
                        // If the task is null, that means it's just a placeholder for a task
                        // queued to one of the subschedulers.  Find the next task based on
                        // priority and fairness and run it.
                        else
                        {
                            // Find the next task based on our ordering rules...                                    
                            QueuedTaskSchedulerQueue queueForTargetTask;
                            lock (_queueGroups) FindNextTask_NeedsLock(out targetTask, out queueForTargetTask);

                            // ... and if we found one, run it
                            if (targetTask != null) queueForTargetTask.ExecuteTask(targetTask);
                        }

                        if (_awaitWrappedTasks)
                        {
                            var targetTaskType = targetTask.GetType();
                            if (targetTaskType.IsConstructedGenericType && typeof(Task).IsAssignableFrom(targetTaskType.GetGenericArguments()[0]))
                            {
                                dynamic targetTaskDynamic = targetTask;
                                // Here we wait for the completion of the proxy task.
                                // We do not wait for the proxy task directly, because that would result in that Wait() will throw the exception of the wrapped task (if one existed)
                                // In the continuation we then simply return the value of the exception object so that the exception (stored in the proxy task) does not go totally unobserved (that could cause the process to crash)
                                TaskExtensions.Unwrap(targetTaskDynamic).ContinueWith((Func<Task, Exception>)(t => t.Exception), TaskContinuationOptions.ExecuteSynchronously).Wait();
                            }
                        }
                    }
                }
                catch (ThreadAbortException)
                {
                    // If we received a thread abort, and that thread abort was due to shutting down
                    // or unloading, let it pass through.  Otherwise, reset the abort so we can
                    // continue processing work items.
                    if (!Environment.HasShutdownStarted && !AppDomain.CurrentDomain.IsFinalizingForUnload())
                    {
                        Thread.ResetAbort();
                    }
                }
            }
        }
        catch (OperationCanceledException) { }
    }
    finally
    {
        // Run a cleanup routine if there was one
        if (threadFinally != null) threadFinally();
        _taskProcessingThread.Value = false;
    }
}

I have tested this and it gives the desired output. This technique could also be used for any other scheduler. E.g. LimitedConcurrencyLevelTaskScheduler and OrderedTaskScheduler