CPU Scheduling in Operating System

• CPU Burst Time – In simple terms, the duration for which a process gets control of the CPU is the CPU burst time, and the concept of gaining control of the CPU is the CPU burst. Similarly, an I/O burst is the concept of performing I/O operations.
• CPU Utilization – CPU utilization can be defined as the the percentage of time CPU was handling process execution to total time

Formulae –

CPU Utilization = (Total time – Total idle time)/(Total Time)

A computer with 75% CPU utilization is better than 50%, since the CPU time was better utilized to handle process execution in 75% and lesser time was wasted in idle time.

• Waiting Time is the total amount of time spent in the ready queue to gain the access of the CPU for execution.
• Turn Around Time – From the time the process is submitted to the time the process is completed, is defined as Turn Around Time.
• Throughput – In a unit time the number of processes that can be completed is called the throughput.
• Load Time – It is defined as the average number of process that are pending in the Ready Queue and waiting for execution time.
• Response Time – can be defined as the time interval between when the process was submitted and when the first response is given to the process.

Different types of Scheduling Algorithms –

• First come first serve (FCFS)
• Shortest Job First (SJF) – non-preemptive
• Shortest Job First (SJF) – preemptive
• Priority Scheduling
• Round Robin

Listed below are some other algorithms:

1. Highest Response Ratio Next (HRRN)
2. Multilevel Queue Scheduling:
3. Multi level Feedback Queue Scheduling

Preemptive vs Non Preemptive Scheduling

Non Preemptive Scheduling

In this type of scheduling, if a process enters the CPU and gets processing time. The process will keep on executing until it has terminated or has to forcibly go to waiting state as it needs a resource that it locked by another parallel process.

Example – SJF i.e. Shortest Job First (Non Preemptive)

Preemptive Scheduling

In this type of scheduling, if a process enters the CPU and gets processing time. It can get switched by another process that have higher priority.

Priority may be forced i.e. may be assigned to each process or time based dynamic priority as in the case of SRTF algorithm. 

Example – SRTF i.e. Shortest Remaining time First (Also known as SJF – Preemptive) 

Note – This information is given wrong on Geeks4Geeks they have mentioned SRTF non preemptive.

Note – It’s okay to not to understand the above completely as of now. We suggest going through all the different scheduling algorithms and solve first and then come back here to understand it again, then it will be easy.