Post on 06-Jan-2017
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Baljit Singh Saini
Priority SchedulingPre-emptive
Baljit Singh Saini
Key PointsIn priority scheduling a number is assigned
to each process which indicates its priority level.
Lower the number, higher is the priority.If a new process arrives which is having
higher priority than the currently running process, then the currently running process is preempted.
It suffers from starvation problem.“Aging” can be used to increase the priority
of a process and thus avoid starvation.
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
0Timer
P1
P2 Both P1 and P2 arrive at time
ZERO
0
Which is having higher priority P1 or
P2 ?
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4/3 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
1Timer
P2
0
P1
At time 1, no new process arrives so we
continue with P1
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4/2 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
2Timer
P2
0
P1
At time 2, no new process arrives so we
continue with P1
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4/1 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
3Timer
P2
0
P1
At time 3, no new process arrives so we
continue with P1
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
4Timer
P2
0
P1
At time 4, P1 has finished its
processing, so processor is assigned
to P2 now.
4
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/2 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
5Timer
0
P1
4
P2
At time 5, no new process arrives so we
continue with P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
6Timer
0
P1
4
P2
At time 6, P3 arrives P3
Which is higher priority process P2 or
P3?
6
P2 is preempted and P3 starts executing
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
6Timer
0
P1
4
P2
P2
6
P3
System state at time 6
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/6 6
P4 3 4 11
p5 2 2 12
7Timer
0
P1
4
P2
6
P3
At time 7, no new process arrives, so we
continue with P3P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/5 6
P4 3 4 11
p5 2 2 12
8Timer
0
P1
4
P2
6
P3
At time 8, no new process arrives, so we
continue with P3P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/4 6
P4 3 4 11
p5 2 2 12
9Timer
0
P1
4
P2
6
P3
At time 9, no new process arrives, so we
continue with P3P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/3 6
P4 3 4 11
p5 2 2 12
10Timer
0
P1
4
P2
6
P3
At time 10, no new process arrives, so we
continue with P3P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
11Timer
0
P1
4
P2
6
P3
At time 11, P4 arrives
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
Which Process is having higher priority
P3 or P4?
Since p3 had higher priority so it
continues executionP4
P2
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
12Timer
0
P1
4
P2
6
P3
At time 12, P5 arrives
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/1 6
P4 3 4 11
p5 2 2 12
Which Process is having higher priority
P3 or P5?
Since p3 had higher priority so it
continues executionP4
P2
P5
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
13Timer
0
P1
4
P2
6
P3
At time 13, P3 is finished with its
execution
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/0 6
P4 3 4 11
p5 2 2 12
Which Process is having higher priority
P2, P4 or P5?
P2 and P5 are having same priorityP
4P2
P5
13
Since P2 arrived first so P2 is selected for
execution
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
13Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
P4
P2
P5
13
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
14Timer
0
P1
4
P2
6
P3
At time 14, P2 is finished with its
execution
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/0 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
P4
P2
P5
13
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
14Timer
0
P1
4
P2
6
P3
At time 14, P2 is finished with its
execution
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
P4
P2
P5
13
14
Out of P4 and P5, P5 is having higher
priority
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
15Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2/1 12
P4
P2
P5
13
14
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
16Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2/0 12
P4
P2
13
14
At time 16, P5 is finished with its
execution
P5
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
16Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
P4
P2
13
14
At time 16, P5 is finished with its
execution
P5
16
P4 is the only process left
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
17Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4/3 11
p5 2 2 12
P4
P2
13
14
P5
16
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
18Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4/2 11
p5 2 2 12
P4
P2
13
14
P5
16
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
19Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4/1 11
p5 2 2 12
P4
P2
13
14
P5
16
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
20Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4/0 11
p5 2 2 12
P4
P2
13
14
P5
16
20
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
p5 2 2 12
20Timer
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3 0
P3 1 7 6
P4 3 4 11
p5 2 2 12
P4
P2
13
14
P5
16
20
Baljit Singh Saini
Process Priority Burst Time Arrival Time
P1 1 4 0
P2 2 3/1 0
P3 1 7/2 6
P4 3 4 11
0
P1
4
P2
6
P3
Process Priority Burst Time Arrival Time
P1 1 4
P2 2 3
P3 1 7
P4 3 4 11
p5 2 2 12
P4
P2
13
14
P5
16
20
Waiting Time = Start time – arrival time + wait time for next burst
P1 =
0 – 0 = 0
0
P2 =
0
4 – 0 + 7 = 11 P3 =
6
6 - 6 = 0 P4 = 16 – 11 = 5P5 = 14 – 12 = 2
Av. Waiting time = (0+11+0+5+2)/5 = 18/5 = 3.6