Round Robin CPU Scheduling
Subject: Operating Systems
Preemptive CPU scheduling based on fixed time quantum allocation.
Concept Summary
Round Robin scheduling is designed for time-sharing systems. The CPU scheduler cycles through the ready queue, giving each process a fixed slice of time (quantum). If a process does not complete within its quantum, it is preempted and pushed to the back of the ready queue. As the quantum approaches infinity, no process is ever preempted, resulting in standard FCFS execution.
Key Revision Rules & Formulas
- Round Robin (RR) allocates a fixed time quantum q to each ready process in a FIFO queue.
- If time quantum q is extremely large (q -> ∞), RR behaves exactly like First-Come First-Served (FCFS).
- If time quantum q is too small, context-switching overhead dominates CPU utilization.
- RR is preemptive and guarantees responsive turn-around time for interactive processes.
Common Exam Pitfalls
- Thinking small time quantum is always better: Extremely small quantums cause high context-switch overhead.
- Confusing RR with SJF: RR ignores process burst times, whereas SJF selects the shortest burst time first.
Sample Practice Questions
Question 1: In CPU scheduling, what happens to Round Robin (RR) algorithm as the time quantum approaches infinity?
- It degenerates into First-Come First-Served (FCFS)
- It degenerates into Shortest Job First (SJF)
- It causes starvation for long processes
- Priority Inversion occurs
Explanation: When time quantum is infinitely large, processes run to completion without preemption, behaving like FCFS.