Priority scheduling

Priority Scheduling

Priority is assigned for each process.
Process with highest priority is executed first and so on.
Processes with same priority are executed in FCFS manner.
Priority can be decided based on memory requirements, time requirements or any other resource requirement.
for example:

Priority Scheduling

0Comment

thumbs up down

Priority Based Process Scheduling (Preemptive/Non-preemptive)

Priority based (Non-preemptive) Process Scheduling: Priority can be internal or external and static or dynamic. This algorithm selects process with highest priority to execute.

Example: Consider the following table:

Process no.	Arrival Time	Burst Time	Priority
P1	0	6	2
P2	2	1	4 (Lowest)
P3	4	4	1 (Highest)
P4	5	3	3

Find the average waiting time and average turn arround time using Priority(Non-preemptive) algorithm?

Solution: Using Priority(non-preemptive) algorithm, gantt chart is:

Therefore,

Waiting Time	Turn around Time	Completion Time	Process No.	Arrival Time	Burst Time	Priority
6 - 6 = 0	6 - 0 = 6	6	P1	0	6	2
12 - 1 = 11	14 - 2 = 12	14	P2	2	1	4 (Lowest)
6 - 4 = 2	10 - 4 = 6	10	P3	4	4	1 (highest)
8 - 3 = 5	13 - 5 = 8	13	P4	5	3	3

So,
Average Turn arround time = (6 + 12 + 5 + 8) / (4) = 8

Average Waiting time = (0 + 11 + 2 + 5) / (4) = 4.5

Priority based (preemptive) Process Scheduling: This algorithm selects process with highest priority to execute and it’s preempted lower priority process.

Example: Consider the following table:

Process no.	Arrival Time	Burst Time	Priority
P1	0	6	2
P2	2	1	4 (Lowest)
P3	4	4	1 (Highest)
P4	5	3	3

Find the average waiting time and average turn arround time using Priority(Preemptive) algorithm?

Solution: Using Priority(Preemptive) algorithm, gantt chart is:

Therefore,

Waiting Time	Turn around Time	Completion Time	Process No.	Arrival Time	Burst Time	Priority
10 - 6 = 4	10 - 0 = 10	10	P1	0	6	2
12 - 1 = 11	14 - 2 = 12	14	P2	2	1	4 (Lowest)
4 - 4 = 0	8 - 4 = 4	8	P3	4	4	1 (highest)
7 - 3 = 4	13 - 6 = 7	13	P4	6	3	3

So,
Average Turn arround time = (10 + 12 + 4 + 7) / (4) = 8.25

Average Waiting time = (4 + 11 + 0 + 4) / (4) = 4.75

4Comments

thumbs up down

Vikas @vikasrajput

6 Dec 2019 11:07 pm

completion time for P3 is incorrect in the solution table, it should be 8

Rohit Panwar @panwarrohit

7 Dec 2019 12:47 am

@vikas u are right it was typo ...now it has been corrected.Thanx for make us correct.

SHIVAM KUMAR @shivamkumar12

13 Dec 2019 09:04 pm

it should be 6 in place of 5
priority (non-preemption)
Average Turn arround time = (6 + 12 + 5 + 8) / (4) = 8

SHIVAM KUMAR @shivamkumar12

13 Dec 2019 09:09 pm

in priority (preemption)
P4 arrival time is 5 . please make correction accordingly
Average Turn arround time = (10 + 12 + 4 + 7) / (4) = 8.25 // its correct answer is 8.5

Average Waiting time = (4 + 11 + 0 + 4) / (4) = 4.75 // its correct answer is 5