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DeltaEngine

Playing around with VS2010 and the Parallel Extensions

by Benjamin Nitschke 26. May 2009 23:28
Last week when I installed VS2010 I played around with it for a few minutes, but I've been very busy at work because our game is at its final stage and we got a lot of heat going on (this is not a hint, no no, no hint from me, I'm not allowed to talk about it. Damn it, this could be a hint).

Currently the most interesting new feature besides the cool VS2010 IDE is the Parallel Extensions for me. Sadly the IDE still unusable for real work IMO because all the addins just don't work, there isn't even a fix for TestDriven.net yet, Jamie Cansdale is probably busy too ^^

VS2010 support for parallel programming goes beyond just adding a few extra classes in .NET 4.0: You also got a great IDE implementation which lots of useful features and new tool debugging and profiling windows for checking out parallel tasks, threads and the scheduling. Next there are native C++ libraries that work with Parallel Extensions too (using lambda functions) and work good together with STL. You can check out all the new features at the official VS2010 page!

Let's take a quick look on how to use these Parallel Extensions. I wrote this in a few minutes last week, I was just too lazy (I mean busy of course) to post it yet, it is obviously very simple stuff, but was still useful to test out some of the new parallel IDE features and check out some new .NET 4.0 classes. First of all let's do a boring foreach loop, which displays numbers from 0 to 9, which get added to expectedTotalNum. This should obviously be 45 because sum(0..9)=45. Later we will do some parallel foreach adding and check if we got the same result. Since it does not matter in which order we add these numbers, it is also a good test to just start a bunch of parallel tasks and let them do their work. Obviously you would never write parallel code just to add some numbers, but this should illustrate the point and it does not hurt your performance much anyway (as long as you have a few lines of code executing that take more than a few instructions).
 // Initialize a list for some parallel testing :)
List<int> someInts = new List<int>();
for (int num = 0; num < 10; num++)
someInts.Add(num);

// Print out numbers sequentially
int expectedTotalNum = 0;
foreach (int num in someInts)
{
Console.WriteLine("sequential num=" + num);
expectedTotalNum += num;
}

Console.WriteLine("expectedTotalNum=" + expectedTotalNum);
This outputs the obvious sequential adding of 0 to 9 to expectedTotalNum, which is 45 at the end of the loop:
sequential num=0
sequential num=1
sequential num=2
sequential num=3
sequential num=4
sequential num=5
sequential num=6
sequential num=7
sequential num=8
sequential num=9
expectedTotalNum=45
Now let's do the same in parallel, just by replacing foreach with Parallel.ForEach:
 // And do it with the new Parallel Programming classes in .NET 4
 int totalNum = 0;
 System.Threading.Parallel.ForEach(someInts, num =>
   {
     Console.WriteLine("parallel num=" + num);
     totalNum += num;
   });
 Console.WriteLine("totalNum=" + totalNum);
While the result is still the same because totalNum is 45 at the end of this loop, we get there in a different way. As you can see this is a little bit confusing at first because the adding does not happen sequentially anymore, but in parallel instead. Also note that this output can change when you execute it again and can even be much different on different platforms with different number of CPUs:
parallel num=0
parallel num=2
parallel num=5
parallel num=7
parallel num=8
parallel num=9
parallel num=6
parallel num=3
parallel num=4
parallel num=1
totalNum=45
Okay, good stuff so far, but you might not always have a foreach loop and you might not want to wait for it to complete anyway. Maybe you just have some work tasks that need to be executed, no matter in what order and you might not even care if they complete their work right away or in a little bit. A method could just add some work that has to be done and then return while the work tasks are executed in the background. This is when you would have used the Thread or ThreadPool classes in the past, which are great on their own, but you always have some setup code and it was hard to test and you could not use them all over the place because setting up threads is a costly operation and if your work is just few lines of code, it was always a better idea to execute it right away. But fear no more, now you can just create new tasks with the new Task class in the System.Threading namespace. This is a much easier job and has many advantages because .NET 4.0 handles all the thread creation for you, will reuse threads once they are completed with their task and all this works in a very performant way. Setting up tasks is a little bit more work than just executing Parallel.ForEach, but it allows much greater flexibility and you can add more tasks from whereever you are. Tasks can even have children and you can have a lot of complex code using all these tasks. Testing multi-threaded code is obviously harder than just writing sequential code, but with all the great additions to the IDE, it is now easier than ever with the new Parallel Tasks window and by checking out the Parallel Stacks, which shows you all the running threads and were all the task code is.

The following code will create 10 tasks and do the same thing as above, but with several additions. To be able to wait for all the tasks to finish we will add all 10 tasks to the allTasks list. We also have to make sure that our local foreach variable does not change while we are executing tasks because the foreach loop will quickly create all tasks, but might not execute them right away theirfore using num can cause problems. Instead we just create a local copy of num and use that instead, which can't change because we never increase it like we do with num. Finally we add some boring thread sleeping to make it easier to debug this code and check out whats going on by adding breakpoints. Without the sleep the code still works, but checking out the Parallel Tasks and Parallel Stacks windows will most likely give us no results or only the last few tasks that are still being executed at the end of the foreach loop because the tasks are so simple and executed very quickly. We even wait a little after the foreach to make sure all the tasks have been added and are being executed right now or are scheduled (waiting for execution).
 // And finally some tasks, yeah!
 totalNum = 0;
 var allTasks = new List<Task>();
 foreach (int num in someInts)
 {
   // We need a local variable for num because num itself can change
   // at the end of this loop before the task might even be executed!
   int numToBeAdded = num;
   allTasks.Add(Task.Factory.StartNew(delegate
   {
     totalNum += numToBeAdded;
     Console.WriteLine("Adding " + numToBeAdded + " in task with id=" + Task.Current.Id);
     // Wait a little for checking out the tasks in the new Tasks window in VS2010!
     Thread.Sleep(1000);
   }));
 } // foreach
 Console.WriteLine("Done with foreach loop. Tasks might still be pending");

 // Wait a little for all tasks to start
 Thread.Sleep(100);
 Task.WaitAll(allTasks.ToArray());

 // And finally return the result (45 again if everything worked)
 Console.WriteLine("totalNum=" + totalNum);
First of all the results again, which is 45 again. This only work with numToBeAdded, if you use num instead it will sometimes give you different results because at the time you execute a task num already might have changed, especially if you have more tasks than CPUs used for execution and time consuming code like Console.Writeline or even a Thread.Sleep is in there!
Adding 1 in task with id=1
Adding 7 in task with id=6
Adding 8 in task with id=7
Adding 6 in task with id=5
Adding 4 in task with id=3
Adding 3 in task with id=4
Done with foreach loop. Tasks might still be pending
Adding 9 in task with id=8
Adding 0 in task with id=10
Adding 5 in task with id=2
Adding 2 in task with id=9
totalNum=45
As you can see this even looks more confusing than the way Parallel.ForEach added those numbers because while we might create those tasks sequentially (number 1-10) it does not mean there are also executed in exactly that way. To make it easier to check these things out there is the Parallel Tasks window, which shows the following after starting all 10 tasks by adding a breakpoint after Thread.Sleep(100) just before we wait for all tasks to complete:



Not only do we see all over our 10 tasks here, we also see right away that 8 of them are currently being executed (because I have 8 CPUs with my hyper-threaded i7) and all of them are waiting because of our stupid Thread.Sleep(1000) we added for each of those tasks. A second later those tasks are done and the last 2 are also executed, most likely with 2 thread ids already created earlier. You can click on each task and see where it is currently executing in the source code and you can also check out all the thread information in the normal Threads window. But even more useful than the Parallel Tasks window is the Parallel Stacks, which shows how all this code is related, which task or thread created which new task and so on:



All good stuff. While I already have some ideas how to use this on some of my current projects, I have not ported anything to .NET 4.0 / VS2010 yet because of the many issues I have with the IDE (no addins, color theme not really working, I always have to reset it when starting VS2010, also I don't like that the project and solution formats have changed so I cannot easily switch back to VS2008, etc.). But hopefully more and more addins will work for VS2010 and some of the issues are fixed, then it will be great to use all this new .NET 4.0 stuff (dynamics, parallel extensions, MEF, etc.).

Comments


4/18/2010 7:39:23 AM #

Useful info. Hope to see more good posts in the future.

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4/21/2010 7:51:27 AM #

A good read, definitely worth a cut and paste. Thanks!

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5/11/2010 7:54:18 AM #

The greatest scientific achievements in mankind were only as a result of guesswork.

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Comments are closed
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