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Session 3520

Introducing CPU Scheduling Algorithms: The Photocopier Scenario

John K. Estell Bluffton College

Retention is obtained from a combination of repetition and association. One methodology for promoting retention is to introduce a topic by first alluding to an association to which students can relate. This serves as a foundation upon which the technical material can then be based, making the learning of the involved concepts easier. Unfortunately, in the area of operating systems there is little time for repetition in the lecture and few materials are available that show associations. This paper is a first step in providing materials suitable for an operating systems course that illustrate concepts through associations.

The subject of CPU (central processing unit) scheduling algorithms is not one that students can easily comprehend in the abstract. Normally students are presented in lecture with one algorithm after another, and as the concept of a process is often new and fuzzy the students have no intuitive grasp of the material. However, there are associations between a CPU and a photocopying machine that can be used to assist in the teaching of CPU scheduling algorithms. Both devices can be used by at most one entity at a time and there is often a queue awaiting access. Students can readily relate to waiting in line to use the photocopier, and because of this familiarity the associations can be presented in a short amount of time. By introducing the concepts behind the various types of scheduling algorithms using concrete examples in this context, students are given the framework that allows them to understand how the algorithms work. When the actual scheduling algorithms are then introduced using the traditional methodology, the concepts have already been explained, and the associations allow them to remember the technical material.

The lectures on this topic begin with some background explanatory material. One of the fundamental actions of a multitasking operating system is the scheduling of resources. The CPU is one of the primary resources available on a computer system; the goal of CPU scheduling is to assign processes to be executed by the CPU over time in such a way that system objectives are met. There are a variety of schedulers that affect the long-term, medium-term, and short-term performance; the focus of this paper is on the short-term CPU scheduling algorithms performed by the dispatcher. The success of CPU scheduling depends on the property that process execution consists of a cycle of CPU execution and I/O wait; when a process is in an I/O wait state, another process should be allowed to use the currently idle CPU to increase the utilization of the resource. Ideally, the allocation of the CPU will be performed so as to optimize one or more aspects of system behavior, such as minimal response time, minimal overhead, maximal resource utilization, or maximal throughput.

Once the basic definitions are presented, the concepts behind some well-known CPU scheduling algorithms can now be introduced by stating the following problem: In a library we have several people and one photocopier. Each person has a variety of items to photocopy - some have one page, others a few pages out of several books, and there are also those who want

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Estell, J. K. (1997, June), Introducing Cpu Scheduling Algorithms: The Photocopier Scenario Paper presented at 1997 Annual Conference, Milwaukee, Wisconsin. 10.18260/1-2--6651