Priority Scheduling
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 |
9 - 5 = 4 |
9 |
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
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