.NET Parallel Programming

All about Async/Await, System.Threading.Tasks, System.Collections.Concurrent, System.Linq, and more…

FAQ - .NET Parallel Programming

FAQ :: StartNew() with TaskScheduler.FromCurrentSynchronizationContext() doesn’t work?

October 15, 2010 Oct 15, 2010 10/15/10
Danny Shih
We’ve seen a number of folks write the following code to execute on the UI thread and get unexpected behavior. TaskScheduler uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();Task uiTask = Task.Factory.StartNew(delegate{    // … Update UI component; BUG!}, uiScheduler);The issue is that the ...

FAQ :: TaskScheduler.UnobservedTaskException event doesn’t work?

August 5, 2010 Aug 5, 2010 08/5/10
Danny Shih
Recall that if exceptions thrown from Task bodies are left unobserved, they will be escalated.  In .NET 4, this means that TPL will throw them on the finalizer after the Task objects are available for garbage collection.  The UnobservedTaskException event on the TaskScheduler class was added as a last-resort method to observe such ...

FAQ :: Parallel.ForEach and non-generic collections?

March 2, 2010 Mar 2, 2010 03/2/10
Danny Shih
.NET 2.0 introduced Generics to allow enhanced code reusability and type safety.  Since then, generic collections (IEnumerable<T>, List<T>, Dictionary<T>, etc.) have become standard and are recommended over their non-generic counterparts (IEnumerable, ArrayList, HashTable, etc.).  As a result, Parallel.ForEach only ...

FAQ :: Which .NET language is best for parallelism?

February 11, 2010 Feb 11, 2010 02/11/10
Danny Shih
The new parallelization support in the .NET Framework 4 is implemented purely in libraries and the runtime and does not require special compiler support.  Therefore, it is available to all compliant .NET languages.  This includes all of the managed languages that ship as part of Visual Studio 2010 (Visual C#, Visual Basic, Visual F...

FAQ :: Are all of the new concurrent collections lock-free?

January 26, 2010 Jan 26, 2010 01/26/10
Danny Shih
(This answer is based on the .NET Framework 4.  As the details below are undocumented implementation details, they may change in future releases.)No.  All of the collections in the new System.Collections.Concurrent namespace employ lock-free techniques to some extent in order to achieve general performance benefits, but traditional ...

FAQ :: You talk about performance, speedup, and efficiency…what do you mean exactly?

January 19, 2010 Jan 19, 2010 01/19/10
Danny Shih
All of these terms are overloaded, even in the context of parallel computing.  However, we’ve used them extensively to describe how well our parallel algorithms and demo applications work.  And sometimes, we throw them around carelessly on the blog, forums, etc., so here are our general definitions.Performance is an attribute ...

FAQ :: The Debugger does not correctly handle Task exceptions?

January 11, 2010 Jan 11, 2010 01/11/10
Danny Shih
The following code correctly observes and handles a Task exception and should print “gotcha!” to the console.  By default though, the Debugger will report a crash.Task t = Task.Factory.StartNew(() => { throw new Exception("poo"); }); try { t.Wait(); } catch (AggregateException) { Console.WriteLine("gotcha!"); } The issue ...

FAQ :: Why is the speedup not X on my X-way machine?

January 5, 2010 Jan 5, 2010 01/5/10
Danny Shih
We’ll be regularly posting answers to frequently asked questions that we’ve gotten on the forum, internal email lists, etc.  Here’s the first – enjoy! Why is the speedup not X on my X-way machine?  Or, why does my parallel code run slower?  Less than ideal speedup can typically be attributed to two things: 1.  ...