Lecture 9 Summary: Round Robin (RR)

1. Core Concept

• Round Robin: Designed specifically for Time-sharing systems.
• It is essentially Preemptive FCFS.
• Time Quantum (q): A small unit of time (typically 10-100 milliseconds) that each process is allowed to run.

2. How it works

• The CPU scheduler goes around the ready queue, giving each process the CPU for up to 1 time quantum.
• If the process burst < 1 quantum: The process finishes and releases the CPU.
• If the process burst > 1 quantum: The process is interrupted after the quantum, and it is moved to the end of the ready queue.

3. Performance Factors

• Time Quantum size (q):
  • If q is very large: RR becomes identical to FCFS.
  • If q is very small: RR leads to "Processor Sharing," but causes too many Context Switches (overhead), which slows down the system.
• Rule: The time quantum should be large compared to the context switch time.

4. Calculations in RR

• Because processes can be interrupted and resume later, Response Time is usually very good.
• Waiting Time increases as the process is pushed back into the queue multiple times.