Introduction
In a modern computer there is usually one CPU (or a few cores) but many processes wanting to run at the same time — your browser, a music player, background updates, the OS itself. CPU scheduling is the mechanism the operating system uses to decide which ready process gets the CPU next, and for how long. Good scheduling is what makes a single CPU feel like it is running everything "simultaneously".
For BCS, Bank IT, and IT-officer exams this is one of the highest-yield Operating Systems topics. Questions are almost always calculative — you are given a set of processes with arrival and burst times and asked to draw a Gantt chart and compute average waiting time or average turnaround time. If you master the five core algorithms and the three formulas below, you can answer nearly every question on this topic with full marks.
Learning Objectives
After studying this lesson you should be able to:
- Explain why CPU scheduling exists and the CPU–I/O burst cycle.
- Distinguish preemptive vs non-preemptive scheduling and the role of the dispatcher.
- Define and compute the five scheduling criteria: CPU utilization, throughput, turnaround time, waiting time, response time.
- Apply and hand-trace FCFS, SJF, SRTF, Priority, and Round Robin on any process set.
- Draw a correct Gantt chart and calculate average waiting and turnaround times.
- Recognise the convoy effect, starvation, and aging, and pick the right algorithm for a goal.
- Avoid the classic exam mistakes that cost easy marks.