Programming with POSIX® Threads

"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less."

— Lewis Carroll, Through the Looking-Glass

This book will use several critical terms that may be unfamiliar to you unless you've already had some experience with parallel or asynchronous programming. Even if you are familiar with them, some of the terms have seen assorted and even contradictory uses within research and industry, and that is clearly not going to help communication. We need to begin by coming to a mutual agreement regarding the meaning of these terms, and, since I am writing the book, we will agree to use my definitions. (Thank you.)

1.2.1 Asynchronous

Asynchronous means that things happen independently (concurrently) unless there's some enforced dependency. Life is asynchronous. The dependencies are supplied by nature, and events that are not dependent on one another can occur simultaneously. A programmer cannot row without the oars, or bail effectively without the bucket—but a programmer with oars can row while another programmer with a bucket bails. Traditional computer programming, on the other hand, causes all events to occur in series unless the programmer takes "extraordinary measures" to allow them to happen concurrently.

The greatest complication of "asynchrony" has been that there's little advantage to being asynchronous unless you can have more than one activity going at a time. If you can start an asynchronous operation, but then you can do nothing but wait for it, you're not getting much benefit from the asynchrony.

1.2.2 Concurrency

Concurrency, which an English dictionary will tell you refers to things happening at the same time, is used to refer to things that appear to happen at the same time, but which may occur serially. Concurrency describes the behavior of threads or processes on a uniprocessor system. The definition of concurrent execution in POSIX requires that "functions that suspend the execution of the calling thread shall not cause the execution of other threads to be indefinitely suspended."

Concurrent operations may be arbitrarily interleaved so that they make progress independently (one need not be completed before another begins), but concurrency does not imply that the operations proceed simultaneously. Nevertheless, concurrency allows applications to take advantage of asynchronous capabilities, and "do work" while independent operations are proceeding.

Most programs have asynchronous aspects that may not be immediately obvious. Users, for example, prefer asynchronous interfaces. They expect to be able to issue a command while they're thinking about it, even before the program has finished with the last one. And when a windowing interface provides separate windows, don't you intuitively expect those windows to act asynchronously? Nobody likes a "busy" cursor. Pthreads provides you with both concurrency and asynchrony, and the combination is exactly what you need to easily write responsive and efficient programs. Your program can "wait in parallel" for slow I/O devices, and automatically take advantage of multiprocessor systems to compute in parallel.

1.2.3 Uniprocessor and multiprocessor

The terms uniprocessor and multiprocessor are fairly straightforward, but let's define them just to make sure there's no confusion. By uniprocessor, I mean a computer with a single programmer-visible execution unit (processor). A single general-purpose processor with superscalar processing, or vector processors, or other math or I/O coprocessors is still usually considered a uniprocessor.

By multiprocessor, I mean a computer with more than one processor sharing a common instruction set and access to the same physical memory. While the processors need not have equal access to all physical memory, it should be possible for any processor to gain access to most memory. A "massively parallel processor" (MPP) may or may not qualify as a multiprocessor for the purposes of this book. Many MPP systems do qualify, because they provide access to all physical memory from every processor, even though the access times may vary widely.

1.2.4 Parallelism

Parallelism describes concurrent sequences that proceed simultaneously. In other words, software "parallelism" is the same as English "concurrency" and different from software "concurrency." Parallelism has a vaguely redeeming analogy to the English definition: It refers to things proceeding in the same direction independently (without intersection).