Question

(d) Pre-emptive priority scheduling [4 marks]:

Consider the following set of processes, with the length of the CPU-burst time given in milliseconds:

Process	Burst	Priority	Arrival Time
P1	8	3	0
P2	3	4	1
P3	6	2	3
P4	3	1	5
P5	1	5	7
P6	3	8	14
P7	8	5	18

(d-1) Draw Gantt chart illustrating the execution of these processes using pre-emptive priority (a smaller priority number implies a higher priority) [2 marks]

00   01   02   03   04   05   06   07   08   09   10   11   12   13   14   15   16   17   18   19   20   21   22   23   24   25   26   27   28   29   30   31   32   33   34

(d-2) Compute the turnaround time of all processes in the previous question [2 mark]

Process	Turnaround Time
P1
P2
P3
P4
P5
P6
P7
Average Turnaround Time

Answer 1

Q(d)

(d)-1

Sol:

We have the required Gantt Chart as follows:

Gantt Chart P1 P3 P4 P3 P1 P2 P5 P7 P6 3 5 8 12 17 20 21 29 32

Which can be explained as follows:

• P1 enters at time=0 so P1 is executed first.
• P2 enters at t=1 but its priority is less than P1 so P1 continues execution
• P3 enters at t=3 with higher priority than P1 so P3 starts execution and P1 is pre-empted
• Now, P3 does execution till t=5.
• P4 enters execution at t=5 since it now has highest priority and it will complete its execution till t=8 because of highest priority.
• Now, after P4, P3 has the highest priority so it will complete its execution till t=12
• Now, after P3, P1 has the highest priority so it will complete its execution till t=17
• Now, after P1, P2 has the highest priority so it will complete its execution till t=20
• Now, after P2, P5 has the highest priority so it will complete its execution till t=21
• Now, after P5, P7 has the highest priority so it will complete its execution till t=29
• Now, after P5, P6 will complete its execution till t=32

(Note that all times are in millisecond(ms))

(d)-2

Sol:

We know that Turnaround Time = Completion Time - Arrival Time

So, we have the required table as follows (here completion time is found using the gantt chart where last executing time of the process is written):

Process	Arrival Time(in ms)	Completion Time(in ms)	Turnaround Time(in ms)
P1	0	17	17 - 0 = 17
P2	1	20	20 - 1 = 19
P3	3	12	12 - 3 = 9
P4	5	8	8 - 5 = 3
P5	7	21	21 - 7 = 14
P6	14	32	32 - 14 = 18
P7	18	29	29 - 18 = 11

Average Turnaround Time = Sum of Turnaround times of all the processes/Number of processes

= (17 + 19 + 9 + 3 + 14 + 18 + 11)/7

= 91/7

= 13 ms (Ans.)

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