CPU Scheduling Criteria
Different CPU scheduling algorithms have different properties and the choice of a particular algorithm depends on the various factors. Many criteria have been suggested for comparing CPU scheduling algorithms.
The criteria include the following:
- CPU utilisation: The main objective of any CPU scheduling algorithm is to keep the CPU as busy as possible. Theoretically, CPU utilisation can range from 0 to 100 but in a real-time system, it varies from 40 to 90 percent depending on the load upon the system.
- Throughput:A measure of the work done by the CPU is the number of processes being executed and completed per unit of time. This is called throughput. The throughput may vary depending upon the length or duration of the processes.
- Turnaround time: For a particular process, an important criterion is how long it takes to execute that process. The time elapsed from the time of submission of a process to the time of completion is known as the turnaround time. Turn-around time is the sum of times spent waiting to get into memory, waiting in the ready queue, executing in CPU, and waiting for I/O. The formula to calculate Turn Around Time = Compilation Time – Arrival Time.
- Waiting time: A scheduling algorithm does not affect the time required to complete the process once it starts execution. It only affects the waiting time of a process i.e. time spent by a process waiting in the ready queue. The formula for calculating Waiting Time = Turnaround Time – Burst Time.
- Response time: In an interactive system, turn-around time is not the best criteria. A process may produce some output fairly early and continue computing new results while previous results are being output to the user. Thus another criteria is the time taken from submission of the process of request until the first response is produced. This measure is called response time. The formula to calculate Response Time = CPU Allocation Time(when the CPU was allocated for the first) – Arrival Time
- Completion time: The completion time is the time when the process stops executing, which means that the process has completed its burst time and is completely executed.
There are various CPU Scheduling algorithms such as-
- First Come First Served (FCFS)
- Shortest Job First (SJF)
- Longest Job First (LJF)
- Priority Scheduling
- Round Robin (RR)
- Shortest Remaining Time First (SRTF)
- Longest Remaining Time First (LRTF)