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Chapter 1 - Why Threads

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

Why Use Threads Over Processes?
If both the process model and threads model can provide concurrent program execution, why use threads over processes?
Creating a new process can be expensive. It takes time. (A call into the operating system is needed, and if the process creation triggers process rescheduling activity, the operating system's context-switching mechanism will become involved.) It takes memory. (The entire process must be replicated.) Add to this the cost of interprocess communication and synchronization of shared data, which also may involve calls into the operating system kernel, and threads provide an attractive alternative.
Threads can be created without replicating an entire process. Furthermore, some, if not all, of the work of creating a thread is done in user space rather than kernel space. When processes synchronize, they usually have to issue system calls, a relatively expensive operation that involves trapping into the kernel. But threads can synchronize by simply monitoring a variable—in other words, staying within the user address space of the program.
We'll spell out the advantages of threads over the multiprocess model of multitasking in our performance measurements in Chapter 6, Practical Considerations. In the meantime, we'll show you how to build a multithreaded program.

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