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Chapter 2 - Designing Threaded Programs

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

Suitable Tasks for Threading
To find the places in your program where you can use threads, you essentially must look for the properties we identified in Chapter 1, Why Threads?: potential parallelism, overlapping I/O, asynchronous events, and real-time scheduling. Whenever a task has one or more of these properties, consider running it in a thread. You can identify a task that is suitable for threading by applying to it the following criteria:
 It is independent of other tasks.
Does the task use separate resources from other tasks? Does its execution depend on the results of other tasks? Do other tasks depend on its results? We want to maximize concurrency and minimize the need for synchronization. The more tasks depend on each other and share resources, the more the threads executing them will end up blocked waiting on each other.
 It can become blocked in potentially long waits.
Can the task spend a long time in a suspended state? A program can typically perform millions of integer operations in the time it would take to perform a single I/O operation. If you dedicate a thread to the I/O task, the rest of the program could accomplish a lot more work in less time.
 It can use a lot of CPU cycles.
Does the task perform long computations, such as matrix crunching, hashing, or encryption? Time-consuming calculations that are independent of activities elsewhere in the program are good candidates for threading. In a multiprocessing environment, you might let a thread executing on one CPU process a long computation while other threads on other CPUs handle input.
 It must respond to asynchronous events.
Must the task handle events that occur at random intervals, such as network communications or interrupts from hardware and the operating system? Use threads to encapsulate and synchronize the servicing of these events, apart from the rest of your application.
 Its work has greater or lesser importance than other work in the application.
Must the task perform its work in a given amount of time? Must it run at specific times or specific time intervals? Is its work more time critical than that of other tasks? Scheduling considerations are often a good reason for threading a program. For instance, a window manager application would assign a high priority thread to user input and a much lower priority thread to memory garbage collection.
Server programs—such as those written for database managers, file servers, or print servers—are ideal applications for threading. They must be continuously responsive to asynchronous events—requests for services coming over communications channels from a number of client programs. Processing these requests typically requires I/O to secondary storage.
Computational and signal-processing applications that will run on multiprocessing systems are another good candidate for threading. They contain many CPU-intensive tasks that can be spread out over a number of available CPUs.
Finally, real-time developers are attracted to threads as a model for servers and multiprocessing applications. Multithreaded applications are more efficient than multiprocess applications. The threads model also allows the developers to set specific scheduling policies for threads. What's more, threads eliminate some of the complexity that comes with asynchronous programming. Threads wait for events whereas a serial program would be interrupted and would jump from context to context.

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