Round Robin CPU Scheduling Algorithm: A Comprehensive Explanation with Example

Understand the Round Robin CPU scheduling algorithm and its effectiveness in improving fairness and responsiveness. This tutorial provides a detailed explanation of the algorithm's operation, including a step-by-step example with calculations of turnaround and waiting times, and a visual Gantt chart.

Round Robin CPU Scheduling Algorithm: A Detailed Explanation

Introduction to Round Robin Scheduling

Round Robin is a CPU scheduling algorithm designed to improve fairness and responsiveness in time-sharing systems. Unlike simpler algorithms that might lead to long wait times for some processes, Round Robin aims for a more equitable distribution of CPU time. It achieves this by assigning each process a fixed time slice, called a time quantum, to use the CPU. After its time quantum, a process is returned to the ready queue, even if it hasn't completed its task, allowing other processes to run.

Key Characteristics of Round Robin

  • Preemptive: A running process can be interrupted.
  • Time Quantum: A fixed time slice allocated to each process.
  • Cyclic Scheduling: Processes are scheduled in a circular fashion.
  • Fairness: Aims for an even distribution of CPU time.
  • Avoids Starvation: Every process gets a chance to run.

Advantages of Round Robin

  • Fair allocation of CPU time among processes.
  • Easy to implement.
  • Avoids convoy effects (where a short process is blocked by a long process) and starvation.

Disadvantages of Round Robin

  • Performance depends heavily on the choice of time quantum.
  • High context-switching overhead (saving and restoring process states).
  • Doesn't inherently prioritize processes; all processes are treated equally.

Example Round Robin Calculation

Let's consider six processes (P1-P6) with their arrival and burst times:

Process ID Arrival Time (AT) Burst Time (BT)
P1 0 7
P2 1 4
P3 2 15
P4 3 11
P5 4 20
P6 4 9

Assume a time quantum (TQ) of 5 time units. The ready queue will contain processes based on their arrival order. A Gantt chart would visually represent the process scheduling. Based on this, the Completion Time, Turnaround Time, and Waiting Time can be calculated for each process.

Process ID Arrival Time Burst Time Completion Time Turnaround Time Waiting Time
P1 0 7 31 31 24
P2 1 4 9 8 4
P3 2 15 55 53 38
P4 3 11 56 53 42
P5 4 20 66 62 42
P6 4 9 50 46 37

Average Waiting Time: (24 + 4 + 38 + 42 + 42 + 37) / 6 = 31.17

Average Turnaround Time: (31 + 8 + 53 + 53 + 62 + 46) / 6 = 42.17

Average Completion Time: (31 + 9 + 55 + 56 + 66 + 50) / 6 = 44.5