MSDN Magazine - October 2007 - (Page 64)

make the thread pool invoke FreeLibrary on the passed module handle when the specified callback instance completes execution. It’s up to the application to ensure that all outstanding callbacks have completed execution and that all pending callbacks on the thread pool’s queue have been canceled before the callback function returns. Earlier, the SetThreadpoolCallbackLibrary function was described as a way to prevent a DLL from being unloaded prematurely while a callback function is still executing code inUsing multiple timers with side the DLL. The cost of different window sizes this insurance is fairly high because of the overhead allows you to see the effect involved in acquiring and of the system coalescing releasing the loader lock timer expirations together. before and after the invocation of the callback function. Note also that there is a single loader lock in the process and it may have heavy contention on it, which means that relying on the thread pool’s mechanism to ensure a DLL doesn’t unload prematurely may have a negative impact on the performance of the application. Depending upon the application scenario, it may be more efficient to build an application-specific mechanism that provides for safe unloading of a DLL using a combination of the functions described in this section and Window’s synchronization primitives. Try It Out with the Sample Applications As you’ve seen, the new thread pool component includes many improvements to help you easily write applications that are both highly reliable and scalable. Key among these improvements is the ability to host more than one thread pool per process, where each thread pool has its own separate set of characteristics so you can partition processes by type of work performed. I hope this overview of the new thread pool has whetted your appetite to explore how using it can benefit your applications. To get you started, I’ve included two sample applications with this article. The first, ThreadPoolDemo, lets you experiment with work, wait, and timer objects so you can explore how they work. ThreadPoolDemo will run on either the process’s default thread pool or on a custom thread pool, allowing the minimum and maximum thread counts to be specified through command line parameters. The work object demonstration, which is executed by specifying the -Work command-line option, creates a work object. By using the supplied count with the command-line parameter -I, it will submit the work item to the thread pool the number of times specified. You can use the -B option to define how long the callback function should block, and the -E option to specify how long the callback function should execute. By default, the program will run until all the submitted work items have been executed. Specifying either –CC or –CW on the command line causes the application to immediately call CloseThreadpoolCleanupGroupMembers after submitting the work item the specified number of times. The difference between the two options is that –CC cancels all callbacks that have not yet started while –CW waits for them to execute before returning from the CloseThreadpoolCleanupGroupMembers call. You can also experiment with timer objects. You can configure multiple timer objects where each can have its own due time and an optional period and window. Using multiple timers with different window sizes allows you to see the effect of the system coalescing timer expirations together. Finally, the wait-object demonstration let’s you define one or more event specifications for the thread pool to wait on. Each event specification can include an optional due time to indicate when the event is to be signaled, as well as an optional timeout interval for defining when the wait should expire. These options let you experiment with combinations of events that become signaled, timeout from waits, or never become signaled and never timeout. For details on command-line parameters for each of the callback object types, type one of the following in a command prompt: “ThreadPoolDemo –Work ?”, “ThreadPoolDemo –Timer ?”, or “ThreadPoolDemo –Wait ?”. The second sample application included with this article is called CopyFile. Essentially it is an update of the Windows SDK file-copying example that demonstrates how a completion port works. CopyFile will copy a source file to a destination file using the thread pool and demonstrates how I/O Completion Objects work. Typing “CopyFile –Usage” at the command line displays a complete description of the program parameters in the console window. I’d like to thank Rob Earhart, Eric Li, and Sandeep Ranade for answering my questions and providing insightful feedback; and I’d like to thank Rob Shewan for reviewing the content and providing me with valuable feedback. ■ 64 msdnmagazine Thread Pools

Table of Contents Feed for the Digital Edition of MSDN Magazine - October 2007

Cover Contents Toolbox CLR Inside Out Basic Instincts Data Points Cutting Edge Pooled Threads WPF Threads Parallel Linq Parallel Performance Mobile Apps Test Run Foundations Windows with C++ Netting C++ .NET Matters { End Bracket } Net Nuptials

MSDN Magazine - October 2007

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