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Performing a Simple Compute-Bound Operation
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To queue an asynchronous compute-bound operation to the thread pool, you typically call one of the following methods defined by the ThreadPool class:
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static Boolean QueueUserWorkItem(WaitCallback callBack); static Boolean QueueUserWorkItem(WaitCallback callBack, Object state);
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These methods queue a work item and optional state data to the thread pool s queue, and then all of these methods return immediately . A work item is simply a method identified by the callback parameter that will be called by a thread pool thread . The method can be passed a single parameter specified via the state (the state data) argument . The version of QueueUserWorkItem without the state parameter passes null to the callback method . Eventually, some thread in the pool will process the work item, causing your method to be called . The callback method you write must match the System.Threading.WaitCallback delegate type, which is defined as follows:
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delegate void WaitCallback(Object state);
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Note The signatures of the WaitCallback delegate, the TimerCallback delegate (discussed
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in this chapter s Performing a Periodic Compute-Bound Operation section), and the ParameterizedThreadStart delegate (discussed in 25, Thread Basics ) are all identical . If you define a method matching this signature, the method can be invoked by using ThreadPool.QueueUserWorkItem, by using a System.Threading.Timer object, or by using a System.Threading.Thread object .
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Part V
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Threading
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The following code demonstrates how to have a thread pool thread call a method asynchronously:
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using System; using System.Threading; public static class Program { public static void Main() { Console.WriteLine("Main thread: queuing an asynchronous operation"); ThreadPool.QueueUserWorkItem(ComputeBoundOp, 5); Console.WriteLine("Main thread: Doing other work here..."); Thread.Sleep(10000); // Simulating other work (10 seconds) Console.WriteLine("Hit <Enter> to end this program..."); Console.ReadLine(); } // This method's signature must match the WaitCallback delegate private static void ComputeBoundOp(Object state) { // This method is executed by a thread pool thread Console.WriteLine("In ComputeBoundOp: state={0}", state); Thread.Sleep(1000); // Simulates other work (1 second) // When this method returns, the thread goes back // to the pool and waits for another task } }
When I compile and run this code, I get the following output:
Main thread: queuing an asynchronous operation Main thread: Doing other work here... In ComputeBoundOp: state=5
And, sometimes when I run this code, I get this output:
Main thread: queuing an asynchronous operation In ComputeBoundOp: state=5 Main thread: Doing other work here...
The difference in the order of the lines in the output is attributed to the fact that the two methods are running asynchronously with respect to one another . The Windows scheduler determines which thread to schedule first, or it may schedule them both simultaneously if the application is running on a multi-CPU machine . Note If the callback method throws an exception that is unhandled, the CLR terminates the process (unless the host imposes its own policy) . Unhandled exceptions are discussed in 20, Exceptions and State Management .
26
Compute-Bound Asynchronous Operations
Execution Contexts
Every thread has an execution context data structure associated with it . The execution context includes things such as security settings (compressed stack, Thread s Principal property, and Windows identity), host settings (see System.Threading. HostExecutionContextManager), and logical call context data (see System.Runtime. Remoting.Messaging.CallContext s LogicalSetData and LogicalGetData methods) . When a thread executes code, some operations are affected by the values of the thread s execution context settings . This is certainly true for the security settings . Ideally, whenever a thread uses another (helper) thread to perform tasks, the issuing thread s execution context should flow (be copied) to the helper thread . This ensures that any operations performed by helper thread(s) are executing with the same security settings and host settings . It also ensures that any data stored in the initiating thread s logical call context is available to the helper thread . By default, the CLR automatically causes the initiating thread s execution context to flow to any helper threads . This transfers context information to the helper thread, but it comes at a performance cost because there is a lot of information in an execution context, and accumulating all of this information and then copying it for the helper thread takes a fair amount of time . If the helper thread then employs additional helper threads, then more execution context data structures have to be created and initialized as well . In the System.Threading namespace, there is an ExecutionContext class that allows you to control how a thread s execution context flows from one thread to another . Here is what the class looks like:
public sealed class ExecutionContext : IDisposable, ISerializable { [SecurityCritical] public static AsyncFlowControl SuppressFlow(); public static void RestoreFlow(); public static Boolean IsFlowSuppressed(); // Less commonly used methods are not shown }
You can use this class to suppress the flowing of an execution context, thereby improving your application s performance . The performance gains can be quite substantial for a server application . There is not much performance benefit for a client application, and the SuppressFlow method is marked with the [SecurityCritical] attribute, making it impossible to call in some client applications (like Silverlight) . Of course, you should suppress the flowing of execution context only if the helper thread does not need or access the context information . If the initiating thread s execution context does not flow to a helper thread, the helper thread will use whatever execution context it last associated with it . Therefore, the helper thread really shouldn t execute any code that relies on the execution context state (such as a user s Windows identity) .
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