Page 85 - DCAP403_Operating System

Operating System

Notes Figure 5.4: Priority Scheduling

Flow 1

Flow 2 Classifier Highest Priority

Scheduler
Flow 3
Port
Middle Priority
Flow 4
Flow 5
Lowest Priority
Flow 6

Flow 7

Flow 8

Advantages and Disadvantages

The main advantage is the important jobs can be finished earlier as much as possible. The main
disadvantage is the lower priority jobs will starve.

Example:

Process Burst time Arrival Priority
P 10 0 3
1
P 2 1 0 1
P 3 2 0 4
P 1 0 5
4
P 5 0 2
5
Gantt Chart

P 2 P 5 P 1 P 3 P 4
0 1 6 16 18 19

Average waiting time: (0+1+6+16+18)/5 = 8.2

5.3.5 Round-Robin (RR)

Round-robin scheduling is really the easiest way of scheduling. All processes form a circular
array and the scheduler gives control to each process at a time. It is off course very easy to
implement and causes almost no overhead, when compared to all other algorithms. But response
time is very low for the processes that need it. Of course it is not the algorithm I want, but it can
be used eventually.
This algorithm is not the best at all for general-purpose Operating Systems, but it is useful for bath-
processing Operating Systems, in which all jobs have the same priority, and in which response
time is of minor or no importance. And this priority leads us to the next way of scheduling.

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