Consider the following process and find the average waiting time when the longest remaining time first (preemptive) scheduling algorithm is applied:

Process ID	Arrival Time	Bust Time
P0	2	1
P1	4	4
P2	5	4
P3	8	10
P4	10	11

All time is in milli seconds.
(A) 17.5 ms
(B) 20 ms
(C) 25 ms
(D) 14 ms


Answer: (D)

Explanation: According to the longest remaining time first (preemptive) algorithm processes are scheduled as following below:

Waiting time = Turn around time – burst time
Turn around time = Completion time – arrival time

Process ID	Arrival Time	Bust Time	Turn around Time	Waiting Time
P0	2	1	1	0
P1	4	4	26	22
P2	5	4	26	22
P3	8	10	24	14
P4	10	11	23	12

Average waiting time = summation of all processes waiting time / number of processes
And average waiting time = 70 / 5 = 14 ms.

So, option (D) is correct.

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