Operating System - CPU scheduling - Lecture 4

Ch 4 : CPU scheduling

It s the basis of multiprogramming operating systems. By switching the CPU among processes, the operating system can make the computer more productive

In a single-processor system, only one process can run at a time. Others must wait until the CPU is free and can be rescheduled. The objective of multiprogramming is to have some process running at all times, to maximize CPU utilization. The idea is relatively simple. A process is executed until it must wait, typically for the completion of some I/O request. In a simple computer system, the CPU then just sits idle. All this waiting time is wasted; no useful work is accomplished. With multiprogramming, we try to use this time productively. Several processes are kept in memory at one time.

Note: what are burst times?
A CPU can run instructions from cache until it needs to fetch more instructions or data from memory. That ends the CPU burst and starts the I/O burst. The I/O burst read or writes data until the requested data is read/written or the space to store it cache runs out. That ends an I/O burst.

When one process has to wait, the operating system takes the CPU away from that process and gives the CPU to another process. This pattern continues. Every time one process has to wait, another process can take over use of the CPU. Scheduling of this kind is a fundamental operating-system function. Almost all computer resources are scheduled before use. The CPU is, of course, one of the primary computer resources. Thus, its scheduling is central to operating-system design.