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Chapter 1 - Why Threads

Pthreads Programming
Bradford Nichols, Dick Buttlar and Jacqueline Proulx Farrell
 Copyright © 1996 O'Reilly & Associates, Inc.

Choosing Which Applications to Thread
The major benefit of multithreaded programs over nonthreaded ones is in their ability to concurrently execute tasks. However, in providing concurrency, multithreaded programs introduce a certain amount of overhead. If you introduce threads in an application that can't use concurrency, you'll add overhead without any performance benefit.
So what makes concurrency possible? First, of course, your application must consist of some independent tasks—tasks that do not depend on the completion of other tasks to proceed. Secondly, you must be confident that concurrent execution of these tasks would be faster than their serial execution.
On a uniprocessing system, the concurrent execution of independent tasks will be faster than their serial execution if at least one of these tasks issues a lot of I/O requests and must wait for the device to complete each request. On a multiprocessor, even CPU-bound tasks can benefit from concurrency because they can truly proceed in parallel.
If you are writing an application for a uniprocessor, look at overlapping I/O and asynchronous events as the motivation for threading an application. If your program is hung up in doing a lot of disk, file, or network accesses when it could be doing other useful things, threads offer a means of doing them while the thread that handles the I/O is waiting.* If your program must deal with many asynchronous events, such as the receipt of an out-of-band message, threads give you an efficient way to structure its event handling where the only alternatives for a single-threaded process would be to either abruptly change context or put off handling the event to a more convenient time. The server portion of a client/server program often meets both of these criteria for concurrency: it must handle asynchronous requests and wait while retrieving and storing data in secondary storage.
 *A side benefit is that your code is ready to take advantage of multiprocessing systems in the future. Multiprocessing UNIX hosts are not restricted to exotic scientific number crunching anymore as two- to four-CPU server and desktop platforms have become commonplace.
If your application has been designed to use multiple processes, it's likely that it would benefit from threading. A common design model for a UNIX server daemon is to accept requests and fork a child image of itself to process the request. If the benefits of concurrency outweighed the overhead of using separate processes in the application, threading is bound to improve its performance because threads involve less overhead.
The remaining class of applications that can benefit from threads are those that execute on multiprocessing systems. Purely CPU-bound applications can achieve a performance boost with threads. A matrix-multiply program (or similar analytical program) with independent computational tasks but no excessive I/O requirements would not benefit from threads on a uniprocessing system. However, on a multiprocessor, this same application could speed up dramatically as the threads performed their computations in parallel.
As we'll see in Chapter 6, there are commonly three different types of Pthreads implementations. To take full advantage of a multiprocessing system, you'll need an implementation that's sophisticated enough to allow multiple threads in a single process to access multiple CPUs.

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