To be fair when comparing Rx to C# 5.0 Async...

By Admin at August 03, 2011 20:45 Tags: , , , , ,

After reading the 900th morning brew, one article by mike taulty about comparing Rx, TPL and async caught my attention.

Mike tries to explain the history, differences and similarities between all these frameworks, and kudos, that's not an easy thing to do.

Asynchrony, (and I'm not talking parallelism), is a complex topic that fools even the best of us.

 

Comparing Rx and Async

Rx and Async and much more similar in that regard, because going off to an other thread is not mandatory, and most operators use either the CurrentThread (timebase priority queue) scheduler or the Immediate (passive wait) scheduler.

This means that the code you are writing is doing some cooperative multi-threading, or fibers-like processing. Everything happens on the same thread, except that work is optionally being queued up, and that thread works as long as there's work left to do, then waits for outstanding operations. These oustanding operations can be I/O completion port bound, like Stream.BeginRead/EndRead.

But back to the comparison, mike is trying to do buffer reading of content from a web response stream, and doing so using the Async CTP's ReadAsync and WriteAsync eases a lot the writing of that kind of code. (Also, the Rx example does not work correctly, but I'll talk about that later in this post.)

These two functions are not tied to the complexity of the BeginRead/EndRead, and behave very much like an IObservable would. Call ReadAsync, you get a Task and wait on it.

Let's jump to the end result and we can get this with Rx based composed operators, using methods symilar to the ReadAsync and GetResponseAsync :

string url = "http://www.microsoft.com";

var webRequest = WebRequest.Create(url);

webRequest.ToResponse()
          .SelectMany(wr => wr.GetResponseStream().ToBytes())
          .ForEach(
            b => Console.WriteLine(Encoding.Default.GetString(b))
          );

That way, it is a lot easier to read. ToResponse maps to GetResponseAsync and ToBytes maps to ReadAsync.

I'll concede the complexity of the SelectMany operator that is related to the fact that IObservable deals with sequences (the duality with IEnumerable). What we would need is more like a IObservableValue that returns only one value. At that point, an appropriate operator would be something like SelectOne, but that's an other topic I'll discuss soon.

 

The ToResponse operator

This one is easy, and is pretty much an encapsulation of the code provided in Mike's Rx example :

public static IObservable ToResponse(this WebRequest request)
{
    var asyncGetResponse = Observable.FromAsyncPattern(
                            request.BeginGetResponse, request.EndGetResponse);

    return Observable.Defer(asyncGetResponse);
}

The use of defer allows the execution of the actual call to GetResponse when someone is subscribing to the deffered observable.

 

The ToBytes operator

That one is a bit tricker :

public static IObservable ToBytes(this Stream stream)
{
    return 
        Observable.Create(
            observer =>
            {
                byte[] buffer = new byte[24];

                var obsReadFactory = Observable.Defer(() => stream.AsReader()(buffer, 0, buffer.Length));

                return Observable
                         .Repeat(obsReadFactory)
                         .Select(i => buffer.Take(i).ToArray())

                         // Subscribe on the thread pool, otherwise the repeat operator will operate during the 
                         // call to subscribe, preventing the whole expression to complete properly
                         .SubscribeOn(Scheduler.ThreadPool)

                         .Subscribe(
                             _ =>
                             {
                                 if (_.Length > 0)
                                 {
                                     observer.OnNext(_);
                                 }
                                 else
                                 {
                                     observer.OnCompleted();
                                 }
                             },
                             observer.OnError,
                             observer.OnCompleted
                         );
            }
        );
}

and needs a bit of explaining.

The ToBytes extension is creating a Observable that will be able to control finely the OnNext/OnCompleted events, especially because of the loopy nature of the BeginRead API. Loopy means that in a synchronous mode, you needs to call Read through a loop until you get all you need. The BeginRead/EndRead still expose this loopy nature, but in an asynchronous way.

With Rx, that loop can be introduced with the use of Repeat, the same way a while(true) would do.

The Select operator is pretty straightforward, even if it may not be as fast as a Buffer.BlockCopy, it's pretty conscise.

The SubscribeOn is the tricky part of this method, and is very important for the OnCompleted events to get through. If this operator is not present, the call to the ToBytes method blocks in the Subscribe of the SelectMany operator in the final example. This means that events like OnCompleted get buffered and not interpreted, and the repeat operator will continue indefinitely to turns into loops getting nothing, because noone's unsubscribing. This would be CPU consuming, and memory consuming because the observable expression could not get dispose.

Then in the subscribe, we notify the observer that either there's a new buffer, or that we're done because the EndRead method returned 0 (or an exception).

 

Continuing the comparison

All this to say that the .NET 5.0 (or whatever it will be called) has a Task friendly BCL that makes it easy to write asynchronous code.

I'm definitely not saying that Rx is as easy as Async will be, but, with a minimum of understanding and abstraction, it can be as powerful and even more powerful because of its ability to compose observables.

Now, for the issues in Mike's Rx sample :

  • The TakeWhile operator only completes when the source observable completes, and the source is a Repeat observable, which never ends, meaning that the whole subscription will never get disposed
  • The Observable.Repeat operator runs on the Immediate scheduler, meaning that the OnNext method will be called in the thread context of the original Subscribe, and the expression will not get disposed.

The sample actually shows something, but the CPU will stay at 100% until the process ends.

 

A word on Asynchrony

Asynchrony is a complex topic, very easy to get wrong, even with the best intentions. Parallelism is even more complex (and don't get me started on the lock() keyword).

I'm expecting that People are going to have a hard time grasping it, and I'm worried that Async will make it too easy to make parallelism (not asynchrony this time) mistakes, because it is will be easy to introduce mutating states in the loop, hence hard to reproduce transitory states bugs. Calling "new Thread()" was scary, and for good reasons, but using await will not, but with mostly the same bad consequences.

I'd rather have better support for immutable structures method or class purity and some more functional concepts baked into C#, where the language makes it harder to make mistakes, than trying to bend (or abstract) asynchrony to make it work with the current state of C#.

Then again, I'm not saying Rx is better, it's trying to work around the fact that the BCL and C# 3.0 don't have asynchrony baked in, so the complexity argument still stands.

 

On the other side, the more developers use Async, the more developers will need async savvy consultants as firemen :)

 

 

When declarativeness goes away for performance

By jay at July 25, 2011 20:35 Tags: , , ,

TLDR: The creation a C# expression tree is not cached, and using it to simulate a methodof keyword is terribly slow. This article is about reconsidering the use of this technique when performance is a concern.

 

During the development of my last projects, and it's been like that for a while, I've been used to look for ways to express programs in a more declarative or functional way.

LINQ is a pretty good tool to acheive that, as well as fluent interfaces, lamdbas and all those neat language features and tricks.

 

When the language is against you

But there are some times when the language is not there to support patterns, like with the use of the IPropertyChanged interface. The language (and the CLR for that matter) does not publicly support intercepting calls to properties or methods. That can actually be done through Transparent Proxies or dynamic proxy generation, but these are not supported on Windows Phone 7. The latter somehow will on WP7.1.

Anyway, using that interface requires raising an event with a string containing the name of the property that has changed.

The use of metadata in the form of strings is unfortunately not refactoring friendly, and if you change your property name, you've got a bug on your hands.

Since there is no methodof keyword in C#, you could say that the language is against you since there is no direct way to get the name of a property or method at compile time.

This can still be done through reflection with plumbing code that uses Expression Trees to work around it, and it works pretty fine.

That way you can easily write nice wrappers like this one :

public int MyProperty 
{
    get { return GetValue(() => MyProperty); }
    set { SetValue(() => MyProperty, value); }
}

That way, you get both the type safety and the refactoring friendly use of INotifyPropertyChanged.

 

Where declarativeness does not shine

If you crack open the assembly for a property like this one with a disassembler, you get this :

public int MyProperty
{
    get
    {
        return this.GetValue<int>(
            Expression.Lambda<Func<int>>(
               Expression.Property(
                  Expression.Constant(this, typeof(MainPage)
               ),
               (MethodInfo) methodof(MainPage.get_MyProperty)
            ),
        new ParameterExpression[0]));
    }
    set
    {
        this.SetValue<int>(
           Expression.Lambda<Func<int>>(
              Expression.Property(
                 Expression.Constant(this, typeof(MainPage)),
                 (MethodInfo) methodof(MainPage.get_MyProperty)
              ),
           new ParameterExpression[0]),
        value);
    }
}

That's a lot of code !

And worse, that's not the end of it, because you'll have to traverse the expression just to get the "methodof", and then call the "Name" property to get the string. All this for a string that will never change after you've compiled your assembly.

 

When declarativeness goes away for performance

But that would not be that bad if you executed that code once, or ran it on a desktop computer (or you don't care about performance). For desktop and server applications, where the cost of executing that kind of code is (almost) neglectible, you do not care much about that.

But if you execute that code a few million times, or run it on a Windows Phone 7 on the UI thread, you've got a problem. The expression is not cached, neither in a static variable or an instance variable, depending on the context. Sure, you could store it in an expression typed variable, and cache it manually that way, but you'd lose a bit declarativeness.

To make a small comparison, it takes 13 seconds on my Samsung Focus to execute 10,000 gets of the property using expressions, and takes 0.2 milliseconds to do the same using a simple string.

Pretty easy to choose, isn't it ?

That's where you lose the declarativeness away for performance, and cringe a little bit about it when you know that you'll have to chase a bug in the future because of a lazy rename. Still, you can have debug-time only code that walks up the stack and checks that the property actually exists, but you have to execute that code to find the bug.

 

Mitigating

Hopefully, there are tools like Postsharp that help in that regard, where a post processing step does the reflection once and for all, and injects that missing string. That's direction I'd rather not take, but since there's still no out of the box solution, that can be a good fit.

 

We're so used to techniques that avoid us to write boring code, but when performance is a concern, it is necessary to reconsider all coding reflexes and think twice before using them.

Why using a timer may not be the best idea

By jay at July 25, 2011 00:00 Tags: , , , , , ,

TLDR: The Reactive Extensions provide a TestScheduler class that abstracts time and allows for a precise control of a virtual time base. Using the Rx Schedulers mecanism instead of real timers enables the creation of fast running unit tests that validate time related algorithms without the inconvients of the actual time.

 

Granted, the title is a bit provocative, but nonetheless it's still a bad idea to use timer classes like System.Threading.Timer.

Why is that ? Because classes like this one are based on the actual time, and that makes it a problem because it is non-deterministic. This means that each time you want to test a piece of code that depends on time, you'll be having a somehow different result, and particularly if you're using very long delays, like a few hours, you probably will not want to wait that long to make sure your code works as expected.

What you want actually, to avoid the side effect of "real" time passing by, is virtual time.

 

Abstracting Time with the Reactive Extensions

The reactive extensions are pretty good at abstracting time, with the IScheduler interface and TestScheduler class.

Most operators in the Rx framework have an optional scheduler they can use, like the Dispatcher or the ThreadPool schedulers. These schedulers are used to change the context of execution of the OnNext, OnCompleted and OnError events.

But for the case of time, the point is to freeze the time and make it "advance" to the point in time we need, and most importantly when we need it.

Let's say that we have a view model with a command that performs a lengthy action on the network, and that we need that action to timeout after a few seconds.

Consider this service contract :

public interface IRemoteService
{
    /// 
    /// Gets the data from the server,
    /// returns an observable call to the server 
    /// that will provide either no values, one
    /// value or an error.
    /// 
    IObservable<string> GetData(string url);
}

This implementation is exposing an observable based API, where the consumer of this contract must take into account the fact that getting data must be performed asynchronously, because it may not provide any value for a long time or even not return anything at all.

Next, a method of a view model that is using it :

public void GetServerDataCommand()
{
    IsQueryRunning = true;
    ShowError = false;

    Service.GetData(_url)
           .Timeout(TimeSpan.FromSeconds(15))
           .Finally(() => IsQueryRunning = false)
           .Subscribe(
               s => OnQueryCompleted(s),
               e => ShowError = true
           );
}

And we can test it using Moq like this :

var remoteService = new Mock<IRemoteService>();

// Returns far in the future
remoteService.Setup(s => s.GetData(It.IsAny()))
             .Returns(
                Observable.Return("the result")
                          .Delay(TimeSpan.FromDays(1))
             );

var model = new ViewModel(remoteService.Object);

// Call the fake server
model.GetServerDataCommand();
Assert.IsTrue(model.IsQueryRunning);
Assert.IsFalse(model.ShowError);

// Sleep for a while, before the timeout occurs
Thread.Sleep(TimeSpan.FromSeconds(5));
Assert.IsTrue(model.IsQueryRunning);
Assert.IsFalse(model.ShowError);

// Sleep beyond the timeout
Thread.Sleep(TimeSpan.FromSeconds(11));

// Do we have an error ?
Assert.IsFalse(model.IsQueryRunning);
Assert.IsTrue(model.ShowError);

The problem with this test is that it depends on actual time, meaning that it takes at least 16 seconds to complete. This is not acceptable in a automated tests scenario, where you want your tests to run as fast as possible.

 

Adding the IScheduler in the loop

We can introduce the use of an injected IScheduler instance into the view model, like this :

.Timeout(TimeSpan.FromSeconds(15), _scheduler)

Meaning that the both Start and Timeout will get executed on the scheduler we provide, for which the time is controlled.

We can update the test like this :

var remoteService = new Mock<IRemoteService>();
var scheduler = new TestScheduler();

// Never returns
remoteService.Setup(s => s.GetData(It.IsAny<string>()))
             .Returns(
                Observable.Return("the result", scheduler)
                          .Delay(TimeSpan.FromDays(1), scheduler)
            );

var model = new ViewModel(remoteService.Object, scheduler);

// Call the fake server
model.OnGetServerData2();
Assert.IsTrue(model.IsQueryRunning);
Assert.IsFalse(model.ShowError);

// Go before the failure point
scheduler.AdvanceTo(TimeSpan.FromSeconds(5).Ticks);
Assert.IsTrue(model.IsQueryRunning);
Assert.IsFalse(model.ShowError);

// Go beyond the failure point
scheduler.AdvanceTo(TimeSpan.FromSeconds(16).Ticks);

// Do we have an error ?
Assert.IsFalse(model.IsQueryRunning);
Assert.IsTrue(model.ShowError);

When the scheduler is created, it holds of all the scheduled operations until AdvanceTo is called. Then the scheduled actions are executed according to the virtual current time.

That way, your tests run at full speed, and you can test properly your time depedent code.

[Rx] Using the ObserveOn and SubscribeOn operators

By jay at July 24, 2011 20:04 Tags: , , , , ,

TLDR: This post talks about how the Reactive Extensions ObserveOn operator changes the execution context (the thread) of the IObservable OnNext/OnComplete/OnError methods, whereas the SubscribeOn operator changes the execution context of the implementation of the Subscribe method in the chain of observers. Both methods can be useful to improve the performance of an application's UI by putting work on background threads.

 

When developing asynchronous code, or consuming asynchronous APIs, you find yourself forced to use specific methods on a specific thread.

The most prominent examples being WPF/Silverlight and Winforms, where you cannot use UI bound types outside of the UI Thread. In the context of WPF, you'll find yourself forced to use the Dispatcher.Invoke method to manipulate the UI in the proper context.

However, you don't want to execute everything on the UI Thread, because the UI performance relies on it. Doing too much on the UI thread can lead to a very bad percieved performance, and angry users...

 

Rx framework's ObserveOn operator

I've discussed a few times the use of ObserveOn in the context of WP7, where it is critical to leave the UI thread alone and avoid choppy animations, for instance.

The ObserveOn operator changes the context of execution (scheduler) of a chain of operators until an other operator changes it.

To be able to demonstrate this, let's write our own scheduler :

public class MyScheduler : IScheduler
{
    // Warning: ThreadStatic is not supported on Windows Phone 7.0 and 7.1
    // This code will not work properly on this platform.
    [ThreadStatic]
    public static int? SchedulerId;

    private int _schedulerId;
    private IScheduler _source;
        
    public MyScheduler(IScheduler source, int schedulerId)
    {
        _source = source;
        _schedulerId = schedulerId;
    }

    public DateTimeOffset Now { get { return _source.Now; } }

    public IDisposable Schedule<TState>(
              TState state, 
              Func<IScheduler, TState, IDisposable> action
           )
    {
        return _source.Schedule(state, WrapAction(action));
    }

    private Func<IScheduler, TState, IDisposable> WrapAction<TState>(
              Func<IScheduler, TState, IDisposable> action)
    {
        return (scheduler, state) => {

            // Set the TLS with the proper ID
            SchedulerId = _schedulerId;

            return action(_source, state);
        };
    }
}

This scheduler's purpose is to intercept calls to the ISchedule methods (You'll fill the missing Schedule methods by yourself) and flag them with a custom thread ID. That way, we'll know which scheduler is executing our code.

Note that this code will not work properly on Windows Phone 7, since ThreadStaticAttribute is not supported. And it's still not supported on 7.1... Seems like not enough people are using ThreadStatic to make its way to the WP7 CLR...

Anyway, now if we write the following Rx expression :

Observable.Timer(TimeSpan.FromSeconds(1), new MyScheduler(Scheduler.ThreadPool, 42))
          .Do(_ => Console.WriteLine(MyScheduler.SchedulerId))
          .First();

We force the timer to raise OnNext on the ThreadPool through our scheduler, and we'll get the following :

42

Which means that the lambda passed as a parameter to the Do operator got executed in the context of the Scheduler used when declaring the Timer operator.

If we go a bit farther :

Observable.Timer(TimeSpan.FromSeconds(1), new MyScheduler(Scheduler.ThreadPool, 42))
          .Do(_ => Console.WriteLine("Do(1): " + MyScheduler.SchedulerId))
          .ObserveOn(new MyScheduler(Scheduler.ThreadPool, 43))
          .Do(_ => Console.WriteLine("Do(2): " + MyScheduler.SchedulerId))
          .ObserveOn(new MyScheduler(Scheduler.ThreadPool, 44))
          .Do(_ => Console.WriteLine("Do(3): " + MyScheduler.SchedulerId))
          .Do(_ => Console.WriteLine("Do(4): " + MyScheduler.SchedulerId))
          .First();

We'll get the following :

Do(1): 42
Do(2): 43
Do(3): 44
Do(4): 44

Each time a scheduler was specified, the following operators OnNext delegates were executed on that scheduler.

In this case, we're using the Do operator which does not take a scheduler as a parameter. There some operators though, like Delay, that implicitly use a scheduler that changes the context.

Using this operator is particularly useful when the OnNext delegate is performing a context sensitive operation, like manipulating the UI, or when the source scheduler is the UI and the OnNext delegate is not related to the UI and can be executed on an other thread.

You'll find that operator handy with the WebClient or GeoCoordinateWatcher classes, which both execute their handlers on the UI thread. Watchout for Windows Phone 7.1 (mango) though, this may have changed a bit.

 

An Rx Expression's life cycle

Using an Rx expression is performed in a least 5 stages :

  • The construction of the expression,
  • The subscription to the expression,
  • The optional execution of the OnNext delegates passed as parameters (whether it be observers or explicit OnNext delegates),
  • The observer chain gets disposed either explicitly or implicitly,
  • The observers can optionally get collected by the GC.

The third part's execution context is covered by ObserveOn. But for the first two, this is different.

The expression is constructed like this : 

var o = Observable.Timer(TimeSpan.FromSeconds(1));

Almost nothing's been executed here, just the creation of the observers for the entire expression, in a similar way IEnumerable expressions work. Until you call the IEnumerator.MoveNext, nothing is performed. In Rx expressions, until the Subscribe method is called, nothing is happening.

Then you can subscribe to the expression :

var d = o.Subscribe(_ => Console.WriteLine(_));

At this point, the whole chain of operators get their Subscribe method called, meaning they can start sending OnNext/OnError/OnComplete messages.

 

The case of Observable.Return and SubscribeOn

Then you meet that kind of expressions :

Observable
   .Return(42L)
   // Merge both enumerables into one, whichever the order of appearance
   .Merge(
      Observable.Timer(
         TimeSpan.FromSeconds(1), 
         new MyScheduler(Scheduler.ThreadPool, 42)
      )
   )
   .Subscribe(_ => Console.WriteLine("Do(1): " + MyScheduler.SchedulerId));

Console.WriteLine("Subscribed !");

This expression will merge the two observables into one that will provide two values, one from Return and one from the timer.

And this is the output :

Do(1):
Subscribed !
Do(1): 42

The Observable.Return OnNext was executed during the call to Subscribe, and has that thread has no SchedulerId, meaning that a whole lot of code has been executed in the context of the caller of Subscribe. You can imagine that if that expression is complex, and that the caller is the UI Thread, that can become a performance issue.

This is where the SubscribeOn operator becomes handy :

Observable
   .Return(42L)
   // Merge both enumerables into one, whichever the order of appearance
   .Merge(
      Observable.Timer(
         TimeSpan.FromSeconds(1), 
         new MyScheduler(Scheduler.ThreadPool, 42)
      )
   )
   .SubscribeOn(new MyScheduler(Scheduler.ThreadPool, 43))
   .Subscribe(_ => Console.WriteLine("Do(1): " + MyScheduler.SchedulerId));

Console.WriteLine("Subscribed !");

You then get this :

Subscribed !
Do(1): 43
Do(1): 42

The first OnNext is now executed under of a different scheduler, making subscribe a whole lot faster from the caller's point of view.

 

Why not always Subscribe on an other thread ?

That might come in handy, but you may not want that as an opt-out because of this scenario :

Observable.FromEventPattern(textBox, "TextChanged")
          .SubscribeOn(new MyScheduler(Scheduler.ThreadPool, 43))
          .Subscribe(_ => { });

Console.WriteLine("Subscribed !");

You'd get an "Invalid cross-thread access." System.UnauthorizedAccessException, because yo would try to add an event handler to a UI element from a different thread. 

Interestingly though, this code does not work on WP7 but does on WPF 4.

An other scenario may be one where delaying the subscription may loose messages, so you need to make sure you're completely subscribed before raising events.

 

So there you have it :) I hope this helps you understand a bit better those two operators.

Team Build and Windows Phone 7

By jay at May 01, 2011 00:00 Tags: , , ,

Building Windows Phone 7 applications in an agile way encourages the use of Continuous Integration, and that can be done using Team System 2010.

There are a few pitfalls to avoid to get there, but this can be acheived quite easily with great results.

I won't cover the goodness of automated builds, this has already been covered a lot.

 

Adding Unit Tests

Along with the continous integration to create hopefully successful builds out of every check-in of source code, you'll also find the automated execution of unit tests. Team System has the ability to provide nice code coverage and unit tests success rates in Reporting Services reports, where statistics can be viewed, which gives good health indicators of the project.

Unfortunately for us, at this point there are no ways to automatically execute tests using the WP7 .NET runtime. But if you successfuly use an MVVM approach, your view models and non UI code can be compiled for multiple platforms, because they do not rely on the UI components that may be specific to the WP7 platform. That way, we are still able to test our code using the .NET 4.0 runtime with MSTest and Visual Studio 2010 test projects.

To avoid repeating code, multi-targeted files can be integrated into single target projects either by :

  • Using the Project Linker tool and link multiple projects,
  • Creating project files in the same folder and use the "include file" command when showing all files in the solution explorer. Make sure to change the output assembly name to something like "MyAssembly.Phone.dll" to avoid conflicts.

Multi-targeted files are using the #if directive and the WINDOWS_PHONE define, or the lack thereof, to compile code for the current runtime target.

There is also the option of creating projects with the Portable Library add-in, but there are some caveats on that side, and there are a few constraints when using this method. You may need to externalize code that is not supported, like UrlDecode.

Testing with MSTest ensures that your code runs successfully on .NET 4.0 runtime, but this does not test on the WP7 runtime. So to be sure that your code is successfully running on it, and since Windows Phone 7 is build on Silverlight 3, tools like the SL3 Unit Test framework can be used to manually run tests in the emulator. This cannot be integrated into the build for now, unfortunately; you'll have to place that in your QA tests.

 

TeamBuild with the WP7 toolkit

To be able to buid WP7 applications on your build machine, you need to install the SDK on your build machine, and a Team Build agent and/or controller.

Creating a build definition is done the same way as for any other build definition, except for one detail. You need to set the MSBuild platform to x86 instead of Auto if your build machine is running on a 64 Bits Windows. This forces the MSBuild runtime to use the 32 bits runtime, and not 64 bits, where the SDK does not work properly.

If you don't, when building your WP7 application, you'll find that intriguing message :

Could not load file or assembly 'System.Windows, Version=2.0.5.0'

Which is particularly odd considering that you've already installed the SDK, and that dll is definitely available.

You may also find that if you install that DLL from the SDK in the GAC, you'll get that other nice message :

Common Language Runtime detected an invalid program.

Which is most commonly found when mixing 32 bits and 64 bits assemblies, for which the architecture has been explicitly specified instead of "Any CPU". So don't install that DLL in the GAC and set the MSBuild architecture to x86.

 

That's it  for now, and Happy WP7 building !

[WP7] HttpWebRequest and the Flickr app "Black Screen" issue

By jay at April 22, 2011 14:54 Tags: , , , , ,

TL;DR: While trying to fix the "Black Screen" issue of the Windows Phone 7 flickr app 1.3, I found out that HttpWebRequest is internally making a synchronous call to the UI Thread, making a network call negatively impact the UI. The entire building of an asynchronous web query is performed on the UI thread, and you can't do anything about it.

Edit: This post was formerly named "About the UI Thread performance and HttpWebRequest", but was in fact about Yahoo's Flickr application and was enhanced accordingly.

When programming on Windows Phone 7, you'll hear often that to improve the perceived performance, you'll need to get off of the UI Thread (i.e. the dispatcher) to perform non UI related operations. By good perceived performance, I mean having the UI respond immediately, not stall when some background processing is done.

To acheive this, you'll need to use the common asynchrony techniques like queueing in the ThreadPool, create a new thread, or use the Begin/End pattern.

All of this is very true, and one very good example of bad UI Thread use is the processing of the body of a web request, particularly when using the WebClient where the raised events are in the context of the dispatcher. From a beginner's perspective, not having to care about changing contexts when developing a simple app that updates the UI, provides a particularly good and simple experience.

But that has the annoying effect of degrading the perceived performance of the application, because many parts of the application tend to run on the UI thread.

 

HttpWebRequest to the rescue ?

You'll find that the HttpWebRequest is a better choice in that regard. It uses the Begin/End pattern and the execution of the AsyncCallback is performed in the context of ThreadPool. This performs the execution of the code in that callback in a way that does not impact the perceived performance of the application.

Using the Reactive Extensions, this can be written like this :

var request = WebRequest.Create("http://www.google.com");

var queryBuilder = Observable.FromAsyncPattern(
                                (h, o) => request.BeginGetResponse(h, o),
                                ar => request.EndGetResponse(ar));

queryBuilder()
                /* Perform the expensive work in the context of the AsyncCall back */
                /* from the WebRequest. This will be the ThreadPool. */
                .Select(response => DoSomeExpensiveWork(response))

                /* Go back to the UI Thread to execute the OnNext method on the subscriber */
                .ObserveOnDispatcher()
                .Subscribe(result => DisplayResult(result));

That way, you'll get most of your code to execute out of the UI thread, where that does not impact the perceived performance of the application.

 

Why would it not be to the rescue then ?

Actually, it will always be (as of Windows Phone NoDo), but there's a catch. And that's a big deal, from a performance perspective.

Consider this code :

 public App()
 {
  /* some application initialization code */


  ManualResetEvent ev = new ManualResetEvent(false);

     ThreadPool.QueueUserWorkItem(
  d =>
  {
      var r = WebRequest.Create("http://www.google.com");
      r.BeginGetResponse((r2) => { }, null);

      ev.Set();
  }
     );

     ev.WaitOne();
 }

This code is basically beginning a request on the thread pool, while blocking the UI thread in the App.xaml.cs file. This makes the construction (but not the actual call on the network) of the WebRequest synchronous, and makes the application wait for the request to begin before showing any page to the user.

While this code is definitely not a best practice, there was a code path in the Flickr 1.3 application that was doing something remotely similar, in a more convoluted way. And if you try it for yourself, you'll find that the application hangs in a deadlock during the startup of the application, meaning that our event is never set.

 

What's happening ?

If you dig a bit, you'll find that the stack trace for a thread in the thread pool is the following :

  mscorlib.dll!System.PInvoke.PAL.Threading_Event_Wait() 
  mscorlib.dll!System.Threading.EventWaitHandle.WaitOne() 
  System.Windows.dll!System.Windows.Threading.Dispatcher.FastInvoke(...) 
  System.Windows.dll!System.Net.Browser.AsyncHelper.BeginOnUI(...)
  System.Windows.dll!System.Net.Browser.ClientHttpWebRequest.BeginGetResponse(...) 
  WindowsPhoneApplication2.dll!WindowsPhoneApplication2.App..ctor.AnonymousMethod__0(...)

The BeginGetResponse method is trying to execute something on the UI thread. And in our example, since the UI thread is blocked by the manual reset event, the application hangs in a deadlock between a resource in the dispatcher and our manual reset event.

This is also the case for the EndGetResponse method.

But if you dig even deeper, you'll find in the version of the System.Windows.dll assembly in the WP7 emulator (the one in the SDK is a stub for all public types), that the BeginGetResponse method is doing all the work of actually building the web query on the UI thread !

That is particularly disturbing. I'm still wondering why that network-only code would need to be executed to UI Thread.

 

What's the impact then ?

The impact is fairly simple : The more web requests you make, the less your UI will be responsive, both for processing the beginning and the end of a web request. Each call to the methods BeginGetResponse and EndGetResponse implicitly goes to the UI thread.

In the case of Remote Control applications like mine that are trying to have remote mouse control, all are affected by the same lagging behavior of the mouse. That's partially because the UI thread is particularly busy processing Manipulation events, this explains a lot about the performance issues of the web requests performed at the same time, even by using HttpWebRequest instead of WebClient. This also explains why until the user stops touching the screen, the web requests will be strongly slowed down.

 

The Flickr 1.3 "Black Screen" issue

In the Flickr application for which I've been able to work on, a lot of people were reporting a "black screen" issue, where the application stopped working after a few days.

The application was actually trying to update a resource from the application startup in an asynchronous fashion using the HttpWebRequest. Because of a race condition with an other lock in the application and UI Thread that was waiting in the app's initialization, this resulted in an infinite "Black Screen" that could only be bypassed by reinstalling the application.

Interestingly enough, at this point in the application's initialization, in the App's class constructor, the application is not killed after 10 seconds if it is not showing a page to the user. However, if the application stalls in the constructor of the first page, the application is automatically killed by the OS after something like 10 seconds.

Regarding the use of the UI Thread inside the HttpWebRequest code, applications that are network intensive to get a lot of small web resources like images, this is has a negative impact on the performance. The UI thread is constantly interrupted to process network resources query and responses.

 

Can I do something about it ?

During the analysis of the emulator version of the System.Windows.dll assembly, I noticed that the BeginGetResponse is checking whether the current context is the UI Thread, and does not push the execution on the dispacther.

This means that if you can group the calls to BeginGetResponse calls in the UI thread, you'll spend less time switching between contexts. That's not the panacea, but at the very least you can gain on this side.

 

What about future versions of Windows Phone ?

On the good news side, Scott Gu annouced at the Mix 11 that the manipulation events will be moved out the the UI thread, making the UI "buttery smooth" to take his words. This will a lot of applications benefit from this change.

Anyway, let's wait for Mango, I'm guessing that will this will change is a very positive way, and allow us to have high performance apps on the Windows Phone platform.

[WP7] A nasty concurrency bug in the bundled Reactive Extensions

By Admin at April 21, 2011 20:05 Tags: , , , , ,

The Reactive Extensions have been included in Windows Phone 7, which comes out of the box, and that you can include the Microsoft.Phone.Reactive.dll assembly.

 

This is a good thing, because it participates in the democratization of the Rx framework, and aside from the fact that the namespace is not exactly the same as the desktop version, Microsoft.Phone.Reactive instead of System.Concurrency and System.Linq, there were no major bugs until recently.

A few applications I've worked on that use the Rx framework, a very interesting unhandled exception was popping-up from time to time in my exception tracker. Unlike the double tap issue I found the same way on my Remote Control application, that one was a bit more tricky, see for yourself :

Exception : System.NullReferenceException: NullReferenceException
at Microsoft.Phone.Reactive.CurrentThreadScheduler.Trampoline.Run()
at Microsoft.Phone.Reactive.CurrentThreadScheduler.EnsureTrampoline(Action action)
at Microsoft.Phone.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
at Microsoft.Phone.Reactive.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
at Microsoft.Phone.Reactive.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext)

This exception popped-up out of nowhere, without anything from the caller that would make that "Trampoline" method fail. No actual parameter passed to the Subscribe method was null, I made sure of that by using a precondition for calling Subscribe.

 

Well, it turns out that it's actually a bug that is related to the use of the infamous not-supported-but-silent ThreadStaticAttribute, for which I've had to work around to make umbrella work properly on Windows Phone 7.

The lack of a Thread Local Storage creates a concurrency issue around a priority queue that is kept by the CurrentThreadScheduler to perform delayed operations. The system wide queue was accessed by multiple threads at the same time, creating random NullReferenceExceptions.

This means that any call to the Subscribe method may have failed if an other call to that same method was being made at the same time.

 

In short, do not use the bundled version of Rx in Windows Phone 7 (as of NoDo), but prefer using the latest release from the DevLabs, which does not have this nasty bug.

[WP7Dev] Double tap when you expect only one

By jay at March 27, 2011 19:24 Tags: , , ,

I've been developing a free application to do some PC remote control on Windows Phone 7, and it's been very instructive in many ways.

To improve the quality of the software, and be notified when an unhandled exception occurs somewhere in my code, or in someone else's code executed on my behalf, I've added a small opt-in unhandled exception reporting feature. This basically sends me back information about the device, most of what's available in DeviceExtendedProperties for device aggregation of exceptions, plus some informations like the culture and, of course, the exception stacktrace and details.

 

The MarketplaceDetailTask exception

A few recurring exceptions have popped up a lot recently, and one coming often is the following :

Exception : System.InvalidOperationException: Navigation is not allowed when the task is not in the foreground. Error: -2147220989 
at Microsoft.Phone.Shell.Interop.ShellPageManagerNativeMethods.CheckHResult(Int32 hr) 
at Microsoft.Phone.Shell.Interop.ShellPageManager.NavigateToExternalPage(String pageUri, Byte[] args) 
at Microsoft.Phone.Tasks.ChooserHelper.Navigate(Uri appUri, ParameterPropertyBag ppb) 
at Microsoft.Phone.Tasks.MarketplaceLauncher.Show(MarketplaceContent content, MarketplaceOperation operation, String context) 
at Microsoft.Phone.Tasks.MarketplaceDetailTask.Show() 

This code is called when a user clicks on the purchase image located on some page of the software, and it looks like this :


void PurchaseImage_ManipulationCompleted(object sender, ManipulationCompletedEventArgs e)
{
    var details = new MarketplaceDetailTask();
    details.ContentIdentifier = "d0736804-b0f6-df11-9264-00237de2db9e";
    details.Show();
}

The call is performed directly on the image's ManipulationCompleted event.

I've been trying to reproduce it for a few times and I finally got it: The user is tapping more than once on the image.

I can see a few reasons why:

  • The ManipulationCompleted event is fairly sensitive and is raised multiple times when the user did not tap twice
  • The user did actually tap twice because the action did not answer fast enough.
  • The user tapped twice because he is used to always tap twice, as some PC users do... (You know, double clicking on hyperlinks in browsers, things like that)

 

What do we do about it ?

There may be actually more, be this actually tells me a lot.

First, I should be having some kind of visual feedback on the click of that image, to tell the user that he has done something (and also to actually follow the design guidelines)

Second, that even if the feedback is there, that there may always be two subsequent clicks, and one that may be executed after the first has called the MarketplaceDetailTask.Show(), and the application has been deactivated. I cannot do much about it, except handle the exception silently or track the actual application state and not call the method.

I'll go with the exception handler for now as it is not a very critical peace of code, but I'd rather have some way of that tell me that the application cannot do that reliably and not have to handle an exception. The API is rather limited on that side, where the PhoneApplicationService is only raising events and does not expose the current "activation" state.

 

Any other examples of exceptions ?

I'll talk more about some other findings this opt-in exception reporting feature has brought me, with some that seem to be pretty tricky. 

[WP7Dev][Reactive] Safer Reactive Extensions

By jay at September 06, 2010 20:26 Tags: , , , ,

Cet article est disponible en français.

When developing .NET applications, unhandled exception in threads have the undesirable effect of terminating the current process.

In the following example :

    static void Main(string[] args)
    {
        var t = new Thread(ThreadMethod);
        t.Start();

        Console.ReadLine();
    }

    private static void ThreadMethod()
    {
        Thread.Sleep(1000); throw new Exception();
    }

The basic exception will invariably terminate the process, and to prevent this, the exception needs to be handled properly :

    private static void ThreadMethod()
    {
        try
        {
            Thread.Sleep(1000); throw new Exception();
        }
        catch (Exception e)
        {
            // TODO: Log and report the exception
        }
    }

This makes classes like System.Threading.Thread, System.Threading.Timer or System.Threading.ThreadPool very dangerous to use if one wants to have an always running application. It is then required that no unhandled exception gets out of the custom handlers for these classes.

Even if it is possible to be notified when an exception has been raised and not handled properly, using the AppDomain.UnhandledException event, most the time this leads to the application being terminated. This termination behavior has been introduced in .NET 2.0, to prevent unhandled exception to be silently ignored.

While this is a very appropriate default behavior, in an enterprise environment, I’m usually enforcing custom static analysis or NDepend rules to prevent the use of these classes directly. This forces new code to use wrappers that provide a very wide exception handler and logs and reports the exception, but does not terminate the process. That also implies that there is still a very valid bug to be investigated, because exceptions should not be handled that late.

 

The case of the Reactive Framework

In Silverlight for Windows Phone 7, and in any other .NET 3.5 or .NET 4.0 application that uses the Reactive Extensions, it is very easy to switch between threads.

Reactive operators like Timer, BufferWithTime, ObserveOn or SubscribeOn allow for specific Schedulers like ThreadPool, TaskPool or NewThread to be used, and if a subscriber does not handle exceptions properly, it ends up with a terminated application.

The same exemple here also terminates the application :


    static void Main(string[] args)
    {
     Observable.Timer(TimeSpan.FromSeconds(10), TimeSpan.FromSeconds(10))
                   .Subscribe(_=> ThreadMethod());

            Console.ReadLine();
    }

    private static void ThreadMethod()
    {
            throw new Exception();
    }

The Observable.Timer operator uses the System.Threading.Timer class and that makes it vulnerable to the same termination problems. Every subscriber needs to handle exceptions thrown in the OnNext delegate, or the application will terminate.

Also, do not think that the OnError delegate passed to Observable.Subscribe will handle exceptions thrown during the execution of OnNext code. OnError only notifies of errors generated by previous Reactive operators, not the current.

 

The IScheduler.AsSafe() extension method

Unfortunately, it is not possible for now to override the default schedulers used internally by the Reactive operators. The only way to handle all unhandled exceptions properly is to use the ObserveOn operator and intercept calls to IScheduler.Schedule methods. Calls can then be decorated with appropriate exception handlers to log and report the exception without terminating the process.

So, to be able to generalize this logging and reporting behavior, I created the AsSafe() extension that I place at the very top of a Reactive expression :

    Observable.Timer(TimeSpan.FromSeconds(10), TimeSpan.FromSeconds(10))
              .ObserveOn(Scheduler.ThreadPool.AsSafe())
              .Subscribe(_=> ThreadMethod());


And here is the code of this very simple extension method :


public static class SafeSchedulerExtensions
{
    public static IScheduler AsSafe(this IScheduler scheduler)
    {
        return new SafeScheduler(scheduler);
    }

    private class SafeScheduler : IScheduler
    {
        private IScheduler _source;

        public SafeScheduler(IScheduler scheduler) {
            this._source = scheduler;
        }

        public DateTimeOffset Now { get { return _source.Now; } }

        public IDisposable Schedule(Action action, TimeSpan dueTime)
        {
            return _source.Schedule(Wrap(action), dueTime);
        }

        public IDisposable Schedule(Action action)
        {
            return _source.Schedule(Wrap(action));
        }

        private Action Wrap(Action action)
        {
            return () => {
                try  {
                    action();
                }
                catch (Exception e) {
                    // Log and report the exception.
                }
            };

        }
    }
}

Using the Remote Debugger

By jay at July 22, 2010 20:05 Tags: , ,

Cet article est disponible en francais.

To continue in the same kind of articles about Visual Studio features that have been available for a while now, but are commonly under-used, I'll talk in this post about the Remote Debugger.

 

Local Debugging

Visual Studio has a debugger that allows the debugging of program when running it using F5, or "Debug / Start Debugging". Visual Studio will then start in a special mode that allows step by step execution of the program, use features like BreakPoints, TracePoint, Watches, IntelliTrace, create MiniDumps and many more.

The debugger runs the program on the local machine, and uses the permissions of the locally logged on user.

Nothing out of the ordinary. Well, maybe the Reverse Debugging with IntelliTrace in VS2010, which is very cool.

 

Hardware Specific and CrapWare

I don't know about you, but I keep my development PC as stable as possible. I rarely install new software, so that I keep the overall performance stable over time. I will most of the time install new software versions only after having tested them on other PCs to determine their behavior.

Call me maniac, that's what it is :)

But then, what to do when the need for testing an installation program comes up ? Or when you need to debug plugins for NI TestStand or Labview ? Or when the software needs a very specific kind of hardware that cannot be installed on your development PC ? (Rainbow Keys, anyone ?)


The answer is simple : The Remote Debugger ! When possible, I will test and debug my software on a virtual machine, or on a physical machine that has the appropriate environment to execute the software.

That way, the development environment stays stable, and I don't need to make installation of software that could add some crapware and eat up the few bytes of RAM left :)

The Remote Debugger ?

The idea is to continue using the development machine, where the source code is and to connect via the network on a machine that will execute the program. After that, the remote debugging session is very similar to a local session, with the exception of the "Edit and Continue" that is not supported. But most of the time, we can live without it.

 

Running the debugger from Visual Studio

It is possible to run the execution on the remote machine by using the "Use Remote Machine" option in the "Debug" tab of a C# project. It is important to note that checking this option implies that all paths specified in "Working Directory" or "External Program" are those of the remote machine.

Aditionnally, Visual Studio will not copy the binaries and PDB files on the remote machine. You have to make the copy of the files at the appropriate location, by using a "Post Build Action", a UNC path in the form of "\\mymachine\c$\temp".

 

Attach to a Running Process

It is also possible to attach to a running process, by using the "Debug / Attach To Process" option. You just need to fill in the "Qualifier" and set the name of the remote debugger, and to choose the process to debug.

Quick hint: The option "Show processes from all users" is not enabled by default. This means that is you want to debug a Windows Service, you will not see it in the list until you enable it.

Finally, the "Attach To Process" window is also very useful with local processes. It is a very handy feature to create a memory dump of a process that takes too much memory, and analyze it.

 

Installing the Remote Debugger

The Remote Debugger is an additional Visual Studio component that is located on the installation media, in the "Remote Debugger" folder. Three versions exist : x86, x64 and ia64 (RIP, Itanium...). If you have to debug a 32 process on 64 bits machine, I advise that you install both the x86 and x64 versions. You will have to choose which remote debugger to run depending on the .NET runtime that is used. You can see which version to use in the "Type" column of the "Attach to Process" window.

Here's what to do :

  • If you are using VS2008 SP1, you can download it here, and for VS2010 you can use the install located on the DVD
  • Once installed on the remote machine, install the RDBG service with the wizard, using the LocalSystem account.
  • You may have a message about a security issue. If you do, follow these steps :
    • Open the "Local Security Policy" section of the "Administrative Tools" control panel
    • Go to the "Local Policies" / "Security Options"
    • Double click on "Network access: Sharing and security model for local accounts" and set the value to "Classic : Local users authenticate as themselves"
    • Close the window
  • If your machine is not on the same domain as your development machine, or even if it's not on a domain at all, add a local use account on the remote machine that has the same name as your current username, and make it a member of the administrators group. The password also has to be the same.
  • Start the remote debugger on the remote machine. Note that to debug a 32 bits process, you have to run the 32 Bits version of the debugger.
  • On the development machine, open the "Attach to process" window, and type the identifier of the remote debuger (shown on the remote debugger window). It should look like this: administrator@my-machine.

Note that the firewall on both the development and the remote machine can prevent the remote debugger from working properly. You can temporarily disable it, but make sure to enable it back after. If you only want to enable specific ports, the port 135/TCP is used. The Remote Debugger uses DCOM as its communication protocol.

 

And if my breakpoints stay empty red circles ?

This is a very common situation that means that the pdb files do not match the loaded binaries. Make sure that you've copied the pdb files at the same time you did the dlls.

The "Debug / Windows / Modules" shows if the debug symbols have been loaded properly, and if it's not the case, the "View / Output / Debug" window will most of the time show why.


Happy debugging !

About me

My name is Jerome Laban, I am a Software Architect, C# MVP and .NET enthustiast from Montréal, QC. You will find my blog on this site, where I'm adding my thoughts on current events, or the things I'm working on, such as the Remote Control for Windows Phone.