Programming Embedded Systems, 2nd Edition

The Scheduler

We have already talked about multitasking and the idea that an operating system makes it possible to execute multiple “programs” at the same time. But what does that mean? How is it possible to execute several tasks concurrently? In actuality, the tasks are not executed at the same time. Rather, they are executed in pseudoparallel. They merely take turns using the processor. This is similar to the way several people might read the same copy of a book. Only one person can actually use the book at a given moment, but each person can read it if everyone takes turns.

An operating system is responsible for deciding which task gets to use the processor at a particular moment. The piece of the operating system that decides which of the tasks has the right to use the processor at a given time is called the scheduler. The scheduler is the heart and soul of any operating system. Some of the more common scheduling algorithms are:

First-in-first-out (FIFO)

This scheduling (also called cooperative multitasking) allows each task to run until it is finished, and only after that is the next task started.

Shortest job first

This algorithm allows each task to run until it completes or suspends itself; the next task selected is the one that will require the least amount of processor time to complete.

Priority

This algorithm is typically used in real-time operating systems. Each task is assigned a priority that is used to determine when the task executes once it is ready. Priority scheduling ...