Multitasking and Multithreading

You can concentrate on more than Whats six times nine? one thing at once. You won get very far reading this book if someone is interrupting you every five seconds asking you to do arithmetic problems. But any computer with a modern operating system can do many things at once. More precisely, it can simulate that ability by switching very quickly back and forth between tasks.

In a multitasking operating system, each program, or process, gets its own space in memory and a share of the CPUs time. Every time you start the Ruby interpreter, it runs in a new process. On Unix-based systems, your script can spawn subprocesses: this feature is very useful for running external command-line programs and using the results in your own scripts (see Recipes 20.8 and 20.9, for instance).

The main problem with processes is that they e expensive. Its hard to read while people are asking you to do arithmetic, not because either activity is particularly difficult, but because it takes time to switch from one to the other. An operating system spends a lot of its time as overhead, switching between processes, trying to make sure each one gets a fair share of the CPUs time.

The other problem with processes is that its difficult to get them to communicate with each other. For simple cases, you can use techniques like those described in Recipe 20.8. You can implement more complex cases with Inter-Process Communication and named pipes, but we say, don bother. If you want your Ruby program to do two things at once, you e better off writing your code with threads.

A thread is a sort of lightweight process that runs inside a real process. One Ruby process can host any number of threads, all running more or less simultaneously. Its faster to switch between threads than to switch between processes, and since all of a processs threads run in the same memory space, they can communicate simply by sharing variables.

Recipe 20.3 covers the basics of multithreaded programming. We use threads throughout this book, except when only a subprocess will work (see, for instance, Recipe 20.1). Some recipes in other chapters, like Recipes 3.12 and 14.4, show threads used in context.

Ruby implements its own threads, rather than using the operating systems implementation. This means that multithreaded code will work exactly the same way across platforms. Code that spawns subprocesses generally work only on Unix.

If threads are faster and more portable, why would anyone write code that uses subprocesses? The main reason is that its easy for one thread to stall all the others by tying up an entire process with an uninterruptible action. One such action is a system call. If you want to run a system call or an external program in the background, you should probably fork off a subprocess to do it. See Recipe 16.18 for a vivid example of thisa program that we need to spawn a subprocess instead of a subthread, because the subprocess is going to play a music file.