Calculate the average waiting time for the following processes:

Consider the preemptive shortest job first scheduling algorithm and all the time in nanoseconds.

(A) 7.10 ns
(B) 10 ns
(C) 11.6 ns
(D) None of these.


Answer: (D)

Explanation: Using preemptive shortest job first scheduling algorithm, Gantt chart will be

Since, Turn Round Time = Completion time – arrival time, so

P1 = 15 - 5 = 10
P2 = 32 - 2 = 30
P3 = 8 - 0 = 8
P4 = 3 - 0 = 3
P5 = 23 - 1 = 22
P6 = 57 - 15 = 42 

And, Waiting time = Turn around time – burst time, so

P1 =  10 - 7 = 3
P2 = 30 - 9 = 21
P3 = 8 - 5  = 3
P4 = 3 -3 = 0
P5 = 22 - 8 = 14
P6 = 42 - 25 = 17 

Hence,

Average waiting time  
= (3 + 21 + 3 + 14 + 17) / 6 = 9.66 ns 

So, option (D) is correct.

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