What do you mean by Priority Based Scheduling ?
A priority is associated with each process and the scheduler always picks
up the highest priority process for execution from the ready queue . A major
problem with the priority based scheduling is that there is indefinite blocking
of a lost priority process by a high priority process. A solution to this
problem is aging priority.. Aging Priority is a technique of gradually increasing
the priority of a process that wait in the system for a long time.
Eventually the older process attain a higher priority and are endured of
completion in a finite term.
Let us understand with the help of an example:
Process Processing Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Using Priority scheduling we would be scheduling the processes according
to the following Gantt Chart .
P2 P5 P1 P3 P4
0 1 6 16 18 19
Time Process Completed Turn Around Time Waiting Time
0 ----- ---- ----
1 P2 1-0=1 1-1=0
6 P5 6-0 =6 6-2=4
16 P1 16-0=16 16-10=6
18 P3 18-2=16 18-2=16
19 P4 19-1=18 19-1=18
Average Turn around Time= 1+6+16+16+18/5 = 60/5 = 12
Average waiting Time = 6+0+16+18+1/5= 8.2
Throughput = 5/19 = 0.26
Processor utilization = 30/30 * 100 = 100%
