ABSTRACT
High-level language programmers and computer users deal with what is really a virtual computer. That virtual computer they see is facilitated by a software bridge that plays the role of interlocutor between the actual computer hardware and the computer user’s environment. This software, described in general in Chapter 80, is the operating system. The computer’s operating system (OS) is made up of a group of systems programs that serve two basic ends:
To control the allocation and use of the computing system’s resources among the various users and tasks
To provide an interface between the computer hardware and the programmer or user that simplifies and makes feasible the creation, coding, debugging, maintenance, and use of applications programs
Thus, the OS creates and maintains an environment in which users can have programs executed. That is, it provides a structure in which the user can request and monitor execution of his or her programs and can receive the resulting output. To this end, the OS must make available to the user’s program the system resources needed for its execution. These system resources are the processor, primary memory, secondary memory (including the file system), and the various devices. Because most modern computing systems are powerful enough to allow multiple user programs or at least multiple tasks to execute in the same time
frame, the OS must allocate these resources among the potentially competing needs of the multiple tasks in such a way as to ensure that all tasks can execute to completion. Furthermore, these resources must be allocated so that no one task is unnecessarily or unfairly delayed. This requires that the OS schedule its resources among the various and competing tasks. The detailed characterization of the problem of scheduling computer system resources in a number of settings; the techniques, algorithms, and policies that have been set forth for its solution; and the criteria and method of assessment of the efficacy of these solutions form the subject of this chapter.
