ISRO | ISRO CS 2011 | Question 4

The following table shows the processes in the ready queue and time required for each process for completing its job.

Process    Time
P1          10
P2          5
P3          20
P4          8
P5          15

If round-robin scheduling with 5 ms is used what is the average waiting time of the processes in the queue?
(A) 27 ms
(B) 26.2 ms
(C) 27.5 ms
(D) 27.2 ms

Answer: (B)
Explanation: The Gantt chart for the processes is shown:

Waiting time of a process = Sum of the periods spent waiting in the ready queue.

 Waiting time = completion time - burst time
Waiting time of P1 = 30 -10 = 20
Waiting time of P2 = 10 - 5 = 5
Waiting time of P3 = 58 - 20 = 38
Waiting time of P4 = 38 - 8 = 30
Waiting time of P5 = 53 - 15 = 38

Average waiting time = 20 + 5 + 38 + 30 + 38
                     =131/5 = 26.2

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