Consider a pre-emptive priority processor scheduler. There are three processes P

Question

Consider a pre-emptive priority processor scheduler. There are three processes P1, P2, and P3 in the job mix that have the following characteristics:

Process

Arrival

Time

Priority

Activity

P1

0 sec

1

8 sec CPU burst followed by

4 sec I/O burst followed by

6 sec CPU burst and quit

P2

2 sec

3

64 sec CPU burst and quit

P3

4 sec

2

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst and quit

        Diagram showing process execution:

1) What is the turnaround time for each of P1, P2, and P3?

Solution:

2) What is the average waiting time for this job mix?

Solution:  

Process

Arrival

Time

Priority

Activity

P1

0 sec

1

8 sec CPU burst followed by

4 sec I/O burst followed by

6 sec CPU burst and quit

P2

2 sec

3

64 sec CPU burst and quit

P3

4 sec

2

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst followed by

2 sec I/O burst followed by

2 sec CPU burst and quit

CPU P1 I/O Time 64 P3 Pl P3 P1 P3 P1 P3 Pl 2 2 2 2 2 2 2 6 P3 P3 P3 P1 2 2 2 2 2

Explanation / Answer

1)

turnaround time = completion time - arrival time

P1:
turnaround time = 88 - 0 = 88 sec

P2:
turnaround time = 66 - 2 = 64 sec

P3:
turnaround time = 80 - 4 = 76 sec

2)

Average waiting time :-

waiting time = turnaround time - burst time

P1:
waiting time = 88 - 18 = 70 sec

p2:
waiting time = 64 - 64 = 0 sec

p3:
waiting time = 76 - 14 = 62 sec

averagae waiting time = (70+0+62)/3 = 132 /3 = 44 sec

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