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Thread pools abstract away the management o f threads, amortizing the cost of creating and deleting them over the life of your process and optimizing the total number of threads to achieve superior all-around performance and scaling Using a thread pool instead of explicit thread ing gets you away from thread management minutia and back to solving your business or domain problems Most programmers can be very suc cessful at concurrent programming without ever having to create a sin gle thread by hand, thanks to carefully engineered Windows and CLR thread pool implementations Identifying patterns that emerge, abstracting them away, and hiding the use of threads and thread pools are also other useful techniques It's com mon to layer systems so that most of the threading work is hidden inside of concrete components A server program, for example, usually doesn't have any thread based code in callbacks; instead, there is a top-level pro cessing loop that is responsible for moving work to run on threads No mat ter what mechanisms you use, however, synchronization requirements are always pervasive unless alternative state management techniques (such as isolation) are employed Nevertheless, threads are a basic ingredient of life Examining them in depth before looking at the abstractions that sit atop them will give you a better understanding of the core mechanisms in the OS, and from there, we can build up those (important and necessary) layers of abstraction without sacrificing knowledge of what underlies them And perhaps you' ll find yourself one day building such a layer of abstraction Last, a word of caution Deciding precisely when it's a good idea to intro duce additional threads is not as straightforward as you might imagine Introducing too many can negatively impact your program's performance due to various fixed overheads and because the OS will spend increasingly more time trying to schedule them fairly as the ratio of threads to processors grows (we'll see details on this later) At the same time, introducing too few will lead to underutilized hardware and wasted opportunity In some cases, the platform will help you create additional concurrency by using separate threads for some core system services (the CLR's ability to perform multi threaded garbage collections is one example), but more often than not, it's left to you to decide and manage
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As with most things, threads have a beginning and an end Let's take a look at what causes the creation of a new thread, what causes the termination of an existing thread, and what precisely goes on during these two events We'll also look at the D l lMa i n method, which is a way for native code to receive notifications of thread creation and termination events
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Thread Creation During the creation of a new process, Windows will automatically create a new thread to run the program's entry point code That's typically your main function in your programming language of choice (ie, ( w ) ma i n i n C++ , Ma i n i n C#, and s o forth) Without a t least one thread, the process wouldn't be able to do anything because processes themselves don't exe cute code-threads do Once the process has been bootstrapped, additional threads may be created by code run within the process itself by the mech anisms we're about to review
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When creating a new thread, you must specify a few pieces of information, including the function at which the thread should begin running-the thread start routine-and the Windows kernel takes care of everything thereafter When the creation request returns successfully, the new thread will have been initialized, and, so long as it wasn't created as suspended (specified by an optional flag), registered into a queue of threads to be run and later scheduled onto a processor When the thread actually gets to run on a processor is subject to the thread scheduler and, therefore, system load and available resources In fact, the new thread may have already begun (or finished) running by the time the request for creation returns Once the new thread runs, its thread start routine can call any other code in the process, and so forth, accessing any shared memory in the process' s address space, using other process-wide resources, and perhaps even creating additional threads of its own The thread start routine can return normally or throw an unhand led exception, both of which termi nate the thread, or alternatively the thread can be terminated via some
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