The Thread Pool in Visual C#

Creation QR Code in Visual C# The Thread Pool

The .NET Framework provides a thread pool, which is a collection of worker threads available to perform short pieces of work. The thread pool continuously adjusts the number of threads that are allowed to process work items simultaneously in an attempt to optimize throughput. The exact algorithm used to adjust the thread count is not documented, but as a general rule, if the system is not busy, work will typically be serviced very quickly after you queue it up. But as the computer becomes busier, items will sit in the queue for longer the thread pool tries to avoid the overheads of preemption, thread switching, and resource contention by not running too much concurrent work. When a system is already busy, trying to process more work items would probably slow it down further, and so

keeping items queued up and processing fewer at a time is likely to result in better overall performance. The simplest way to use the thread pool is through its QueueUserWorkItem method. Example 16-6 shows a modification to the previous examples rather than creating threads, it uses the thread pool. QueueUserWorkItem takes a delegate to any method that accepts a single object as its argument, so it s happy with the same Go method as Example 16-2. (Unlike the Thread constructor, there s no overload that accepts a method without an argument the thread pool insists on there being an argument whether you have a use for one or not.)

static void Main(string[] args) { ThreadPool.QueueUserWorkItem(Go, "One"); ThreadPool.QueueUserWorkItem(Go, "Two"); Go("Main"); // Problem: not waiting for work items to complete!

}

This example has a problem: if the main thread finishes first, the program may exit before the thread pool work items complete. So this only illustrates how to start work on the thread pool. This might not be a problem in practice it depends on your application s typical life cycle, but you may need to add additional code to coordinate completion. Running on a quad-core machine, this particular example behaves in much the same way as the previous ones, because the thread pool ends up creating a thread for both work items. On a single-core machine, you might see a difference it could decide to let the first item run to completion and then run the second afterward. The thread pool is designed for fairly short pieces of work. One of the most important jobs it was originally introduced for was to handle web requests in ASP.NET web applications, so if you re wondering how much work constitutes fairly short, a reasonable answer is about as much work as it takes to generate a web page. .NET 4 introduces a new way to use the thread pool, the Task Parallel Library, which offers a couple of advantages. First, it handles certain common scenarios more efficiently than QueueUserWorkItem. Second, it offers more functionality. For example, tasks have much more comprehensive support for handling errors and completion, issues Example 16-6 utterly fails to address. If the main thread finishes before either of the work items is complete, that example will simply exit without waiting for them! And if you want the main thread to discover exceptions that occurred on the thread pool threads, there s no easy way to do that. If any of these things is important to you, the Task Parallel Library is a better way to use the thread pool. There s a whole section on that later in this chapter. For now, we ll continue looking at some aspects of threading that you need to know, no matter what multithreading mechanisms you may be using.