Definition
Round Robin
Round Robin (RR) is a preemptive scheduling strategy based on time slicing. The processor allocates fixed-size quanta (time slices) to processes in cyclic order.
Mechanism
Selection
Same as FCFS — oldest process in Ready Queue.
Preemption
When time slice expires (clock interrupt), process moves to back of Ready Queue, next process dispatched.
Performance
Quantum Length
- Too large: Behaves like FCFS
- Too small: Frequent process switching degrades performance
- Ideal: Slightly longer than interactive burst, much longer than interrupt/scheduler overhead
I/O Bias
RR disadvantages I/O-bound processes — they block before time slice expires, then find themselves “overtaken” by CPU-bound processes.
Virtual Round Robin
VRR
Virtual Round Robin addresses I/O bias with an Auxiliary Queue:
- I/O-returning processes go to Auxiliary Queue (higher priority than Ready Queue)
- They run for the remainder of their unused time slice
- Ensures fair CPU share relative to non-blocking tasks