Parallel Programming: Task Schedulers and Synchronization Context

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Thanks to everyone who provided feedback on my previous post Parallel Programming in .NET Framework 4: Getting Started. As promised, I am continuing the series. This time, let’s go a little bit deeper and talk about task schedulers, synchronization context, tasks that return values, and some other cool features of the Task Parallel Library (TPL).

Here's the full list of posts in this series:

This time I’m not going to add any new functionality to my sample application, but rather I will show how you can communicate with the UI thread a little easier and make your application less dependent on a particular UI framework.

Here is the final code from my last post:

using System;  
using System.Collections.Generic;  
using System.Windows;  
using System.Windows.Documents;  
using System.Threading.Tasks;  
using System.Diagnostics;  
  
namespace ParallelApplication  
{  
    public partial class MainWindow : Window  
    {  
        public MainWindow()  
        {  
            InitializeComponent();  
        }  
        public static double SumRootN(int root)  
        {  
            double result = 0;  
            for (int i = 1; i < 10000000; i++)  
            {  
                result += Math.Exp(Math.Log(i) / root);  
            }  
            return result;  
        }  
        private void start_Click(object sender, RoutedEventArgs e)  
        {  
            textBlock1.Text = "";  
            label1.Content = "Milliseconds: ";  
  
            var watch = Stopwatch.StartNew();  
            List<Task> tasks = new List<Task>();  
            for (int i = 2; i < 20; i++)  
            {  
                int j = i;  
                var t = Task.Factory.StartNew(() =>  
                {  
                    var result = SumRootN(j);  
                    this.Dispatcher.BeginInvoke(new Action(() =>  
                         textBlock1.Text += "root " + j.ToString() + " " +  
                                             result.ToString() +  
                                             Environment.NewLine)  
                    , null);  
                });  
                tasks.Add(t);  
            }  
            Task.Factory.ContinueWhenAll(tasks.ToArray(),  
                  result =>  
                  {  
                      var time = watch.ElapsedMilliseconds;  
                      this.Dispatcher.BeginInvoke(new Action(() =>  
                          label1.Content += time.ToString()));  
                  });  
        }  
    }  
}

Yes, it does everything that I wanted, but it uses the WPF API extensively. Although it is perfectly fine to use WPF as shown above, with the TPL you can make your code much less dependent on the particular UI framework. Right now, if you copy and paste the button event handler and the SumRootN method to a Windows Forms application with almost identical UI, you would need to do a lot of work, because Windows Forms doesn’t have the Dispatcher object and uses different API for managing interactions with the UI thread.

Let’s go back to the previous post and remember why I added the Dispatcher object in the first place. Well, I needed it to communicate with the UI thread, because all the computation results were in background threads created by the TPL. However, the TPL provides a different way of handling interactions between threads. It has task schedulers: very useful objects that are responsible for queuing and executing tasks.

My application already uses a default task scheduler, because this is how the TPL works with the ThreadPool. The TPL has other schedulers in addition to the default one and also allows you to create custom schedulers. One of the schedulers that TPL provides is based on the current synchronization context, and it can be used to ensure that my task executes on the UI thread. For example, let’s take a look at this code:

Task.Factory.ContinueWhenAll(tasks.ToArray(),  
      result =>  
      {  
          var time = watch.ElapsedMilliseconds;  
          this.Dispatcher.BeginInvoke(new Action(() =>  
              label1.Content += time.ToString()));  
      });

The time computation is very simple and fast, so it doesn’t require a background thread. The next line is about displaying the result, which is pure UI work.

I need to somehow get a reference to the UI thread, so I can run a task on it. In this case, it’s quite easy. The code above is from the button event handler, so before I start the task I am in fact operating on the UI thread. I just need to remember the current context and then pass it to the TaskFactory.ContinueWhenAll method.

Here is how I can do this (plus I need to add System.Threading to the list of namespaces):

var ui = TaskScheduler.FromCurrentSynchronizationContext();  
Task.Factory.ContinueWhenAll(tasks.ToArray(),  
    result =>  
    {  
        var time = watch.ElapsedMilliseconds;  
        label1.Content += time.ToString();  
    }, CancellationToken.None, TaskContinuationOptions.None, ui);

The TaskScheduler.FromCurrentSynchronizationContext method returns a task scheduler for the current context, in this case, the UI thread. The ContinueWhenAll method has an overload that accepts the task scheduler parameter. This overload requires some other parameters as well, but since I don’t need them, I use corresponding None properties. I got rid of the inner delegate, so it’s much easier to see what the task is doing and there is no Dispatcher any more.

Now let’s take a look at a more complicated case:

for (int i = 2; i < 20; i++)  
{  
    int j = i;  
    var t = Task.Factory.StartNew(() =>  
    {  
        var result = SumRootN(j);  
        this.Dispatcher.BeginInvoke(new Action(() =>  
             textBlock1.Text += "root " + j.ToString() + " " +  
                                 result.ToString() +  
                                 Environment.NewLine)  
        , null);  
    });  
    tasks.Add(t);  
}

This one requires more thorough refactoring. I can’t run all the tasks on the UI thread, because they perform long-running operations and this will make my UI freeze. Furthermore, it will cancel all parallelization benefits, because there is only one UI thread.

What I can do is to split each task into two: one will compute the results and another one will display information to the UI thread. I have already used the ContinueWhenAll method that waits for an array of tasks to finish. It’s not surprising that TPL also allows you to wait for a certain task to finish and then to perform some operation. The method that does the job is Task.ContinueWith.

var ui = TaskScheduler.FromCurrentSynchronizationContext();  
for (int i = 2; i < 20; i++)  
{  
    int j = i;  
    var compute = Task.Factory.StartNew(() =>  
    {  
        var result = SumRootN(j);  
    });  
    tasks.Add(compute);  
  
    var display = compute.ContinueWith(resultTask =>  
                     textBlock1.Text += "root " + j.ToString() + " " +  
                                         result.ToString() +  
                                         Environment.NewLine,  
                     ui);  
}

OK, the code above doesn’t compile for an obvious reason. The result variable is local for the task compute and the task display doesn’t know anything about it. How can I pass the result from one task to another one? Here is one more trick from the TPL: tasks can return values. To make the task compute return the value, all I need is to change “var result =” to “return” in the compute task.

var compute = Task.Factory.StartNew(() =>  
{  
    return SumRootN(j);  
});

This makes the compiler change the type of the compute object from Task to Task<TResult>. In my case, the type of compute is now Task<double>. Objects of the Task<TResult> type save the return value in the Result property.

var display = compute.ContinueWith(resultTask =>
                  textBlock1.Text += "root " + j.ToString() + " " +
                                      compute.Result.ToString() +
                                      Environment.NewLine,
                  ui);

That’s it. Here is my final code for the event handler:

private void start_Click(object sender, RoutedEventArgs e)
{
    textBlock1.Text = "";
    label1.Content = "Milliseconds: ";

    var watch = Stopwatch.StartNew();
    List<Task> tasks = new List<Task>();
    var ui = TaskScheduler.FromCurrentSynchronizationContext();
    for (int i = 2; i < 20; i++)
    {
        int j = i;
        var compute = Task.Factory.StartNew(() =>
        {
            return SumRootN(j);
        });
        tasks.Add(compute);

        var display = compute.ContinueWith(resultTask =>
                          textBlock1.Text += "root " + j.ToString() + " " +
                                              compute.Result.ToString() +
                                              Environment.NewLine,
                          ui);

    }

    Task.Factory.ContinueWhenAll(tasks.ToArray(),
        result =>
        {
            var time = watch.ElapsedMilliseconds;
            label1.Content += time.ToString();
        }, CancellationToken.None, TaskContinuationOptions.None, ui);

}

If you copy the code from this event handler plus the SumRootN method to the Windows Forms application, you will need to change the code only slightly, mostly because the UI elements are a little bit different. (Windows Forms applications do not have TextBlock control and their labels do not have the Content property.) Just for fun, I did it myself and highlighted the changes I had to make in the event handler.

private void start_Click(object sender, EventArgs e)
{
    label2.Text = "";
    label1.Text = "Milliseconds: ";

    var watch = Stopwatch.StartNew();
    List<Task> tasks = new List<Task>();
    var ui = TaskScheduler.FromCurrentSynchronizationContext();
    for (int i = 2; i < 20; i++)
    {
        int j = i;
        var compute = Task.Factory.StartNew(() =>
        {
            return SumRootN(j);
        });
        tasks.Add(compute);

        var display = compute.ContinueWith(resultTask =>
               label2.Text += "root " + j.ToString() + " " +
                               compute.Result.ToString() +
                               Environment.NewLine,
                ui);
    }

    Task.Factory.ContinueWhenAll(tasks.ToArray(),
         result =>
         {
             var time = watch.ElapsedMilliseconds;
             label1.Text += time.ToString();
         }, CancellationToken.None, TaskContinuationOptions.None, ui);
}

Now I have a parallel Windows Forms application, with responsive UI. The migration was really easy. So, my advice is to stick to the TPL way of managing UI thread instead of the UI framework API. It makes your code much easier to migrate and allows you to write essentially the same code, no matter what UI framework you use.

I’m going to talk about task cancellation next time. For now, if you want to know more about the features used in this post, here are some interesting links:

P.S.

Thanks to Dmitry Lomov, Michael Blome, and Danny Shih for reviewing this and providing helpful comments, to Robin Reynolds-Haertle for editing.