Gate2019 cs Q51

51Consider the following four processes with arrival times (in milliseconds) and their length of CPU bursts (in milliseconds) as shown below:

Process	P1	P2	P3	P4
Arrival time	0	1	3	4
CPU burst time	3	1	3	Z

These processes are run on a single processor using preemptive Shortest Remaining Time First scheduling algorithm. If the average waiting time of the processes is 1 millisecond, then the value of Z is________.

Note – Numerical Type question

Option : A
Explanation :
Let’s assume Z = 2, then gantt chart will be,
P1 P2 P1 P1 P4 P3
0 1 2 3 4 6 9

Average waiting time,
= {(4-0-3) + (2-1-1) + (9-3-3) + (6-4-2)} / 4
= (1 + 0 + 3 + 0) / 4
= 4 / 4
= 1
So, answer is 2.

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