In order to achieve an efficient processor management, OS tries to select the most appropriate process from the ready queue. For selection, the relative importance of the
followings may be considered as performance criteria.
Processor Utilization: The ratio of busy time of the processor to the total time passes for processes to finish. We would like to keep the processor as busy as possible.
Processor Utilization = (Processor buy time) / (Processor busy time + Processor idle time)
Throughput: The measure of work done in a unit time interval.
Throughput = (Number of processes completed) / (Time Unit)
Turnaround Time (tat): The sum of time spent waiting to get into the ready queue, execution time and I/O time.
tat = t(process completed) – t(process submitted)
Waiting Time (wt): Time spent in ready queue. Processor scheduling algorithms only affect the time spent waiting in the ready queue. So, considering only waiting time instead of turnaround time is generally sufficient.
Response Time (rt): The amount of time it takes to start responding to a request. This criterion is important for interactive systems.
rt = t(first response) – t(submission of request)
We, normally, want to maximize the processor utilization and throughput, and minimize tat, wt, and rt. However, sometimes other combinations may be required depending on to processes.