), and then press the start button. Very soon the task of starting, interrupting, and
restarting processes became the bailiwick of the OS.
The question of which task to schedule for execution at any moment has been the subject of much research
and considerable design effort. The answer depends in part on the type of operating system. If the system is a
batch system, the scheduling plan should optimize throughput (the number of jobs completed per hour), CPU
utilization, and/or ???turnaround time??? (the average elapsed time between the submission of a job and its completion).
If the system is an interactive system, the scheduler should provide responsiveness and the impression
of fairness to all users. Interactive systems often have less well-defined boundaries of jobs, so the criteria
mentioned for batch systems are not as relevant. A more important criterion is response time to the user, the
time delay between input from the user and some response back from the process.
If the system is a real-time system, with important constraints on execution time, the scheduler will be
judged by the consistency with which applications meet their deadlines, and the predictability of performance.
Some real-time systems are hard real-time systems, meaning that important absolute limits bound performance
requirements (imagine a system that coordinates process equipment in a chemical plant).
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