- Different CPU scheduling algorithms has different properties. Selection decision depends on the properties of various algorithms.
- There are many criteria need to be consider for comparing CPU scheduling algorithms.
- This characteristic is used to determine the best algorithm. The criteria are as following:
- We want to keep the CPU as busy as possible. It may range from 0 to 100%. In real time system, it suits range from 40% (for a lightly loaded system ) to 90% ( for heavily loaded system ).
- If the CPU is busy executing processes, then work is being done. One measure of work is the number of processes completed per time unit for throughput. For time processes, these may be a one process for one minute. For shorter transactions, throughput might be 10 processes per minute.
- From the submission time of a process to its completion time is turnaround time. It is the sum of total time period spends waiting to get into memory, waiting in the ready queue, executing in the CPU and doing input/output operations.
- The CPU scheduling algorithm does not affect the amount of the time during which process execute or does input/output. It affects only the amount of time that a process spends waiting in the ready queue. Waiting time is the sum of period spends waiting in the ready queue.
- In an interactive system, a process can produce some output early and can continue, computing new results. Previous results are being displayed the user. Thus another measure is the time from the submission to the request until the first response is produced. It is called Response time. The turnaround time is normally limited by speed of output device.