Round Robin(RR) Process Scheduling Algorithm: This is preemptive version of FCFS algorithm based on time quantum. Very large time quantum makes Round Robin as FCFS algorithm. It minimizes average response time. It has circular queue to hold processes to be executed next.
Example: Consider the following table:
Process no.

Arrival Time

Burst Time

P1

0

6

P2

2

1

P3

4

4

P4

5

3

Find the average waiting time and average turn arround time using RR(Round Robin) algorithm with Time Quantum size 2?
Solution: Using round robin algorithm with time quantum size 2, ready queue and gantt chart are:
Ready Queue: P1, P2, P1, P3, P1, P4, P3, P4
Gantt chart is:
Therefore,
Waiting Time

Turn around Time

Completion Time

Process No.

Arrival Time

Burst Time

9  6 = 3

9  0 = 9

9

P1

0

6

1  1 = 0

3  2 = 1

3

P2

2

1

8  4 = 4

13  5 = 8

13

P3

5

4

8  3 = 5

14  6 = 8

14

P4

6

3

So,
Average Turn arround time = (9 + 1 + 8 + 8) / (4) = 6.5
Average Waiting time = (3 + 0 + 4 + 5) / (4) = 3