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Wednesday, August 26, 2009

Overview of Threads

A thread is an encapsulation of the flow of control in a program. Most people are used to writing single-threaded programs - that is, programs that only execute one path through their code "at a time". Multi-threaded programs may have several threads running through
different code paths "simultaneously".
A thread is similar to the sequential programs: a single thread also has a beginning, an end, a sequence, and at any given time during the run time of the thread there is a single point of execution. However, a thread itself is not a program--it cannot run on its own--but runs within a program.

Why use threads?
Threads should not alter the semantics of a program. They simply change the timing of operations. As a result, they are almost always used as an elegant solution to performance related problems. Here are some examples of situations where you might use threads :
* Doing lengthy processing: When a windows application is calculating it cannot process any more messages. As a result, the display cannot be updated.
* Doing background processing: Some tasks may not be time critical, but need to execute continuously.
* Doing I/O work: I/O to disk or to network can have unpredictable delays. Threads allow you to ensure that I/O latency does not delay unrelated parts of your application.

In the program, some operations incur a potentially large delay or CPU hogging, but this delay or CPU usage is unacceptable for other operations; they need to be serviced now.
* Making use of multiprocessor systems: You can't expect one application with only one thread to make use of two or more processors! Chapter 3 explains this in more detail.
* Efficient time sharing: Using thread and process priorities, you can ensure that everyone gets a fair allocation of CPU time.

Similarities between process and threads :
* Like processes threads share CPU and only one thread active (running) at a time.
* Like processes, threads within a processes, threads within a processes execute sequentially.
* Like processes, thread can create children.
* And like process, if one thread is blocked, another thread can run.

Differences between process and threads :
* Unlike processes, threads are not independent of one another.
* Unlike processes, all threads can access every address in the task.
* Unlike processes, thread are design to assist one other. Note that processes might or might not assist one another because processes may originate from different users.

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