1. Understand Process Management
• Process Scheduling Algorithms
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3. Recap
In the last class, you have learnt
Shortest Job First scheduling algorithm and its
-Advantages
-Disadvantages
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4. Objectives
On completion of this class, you would be able to know
• Priority Scheduling
• Round Robin Scheduling
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5. Priority Scheduling
• Priority scheduling algorithm is special case of SJF
• A priority is associated with each process
• Priorities are indicated by fixed range of numbers
• In this scheduling CPU is allocated to the process with
the highest priority
• Equal priority process scheduled in FCFS order
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6. Priority Scheduling
• Consider the following set of processes assumed to have
arrived at time 0,
– in order P1,P2,P3-----P5
– with the length of the CPU burst time given in milliseconds
– and their associated priorities
Process Burst time Priorities
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
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7. Priority Scheduling
• Using Priority Scheduling we should schedule these
processes according to the following Gantt chart
P2 P5 P1 P3 P4
0 1 6 16 18 19
• The average waiting time is 8.2 milliseconds
i.e. (0+1+6+16+18)/5 = 8.2 milliseconds
Priority can be defined internally or externally
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8. Priority Scheduling
• Internally defined priorities use some measurable
quantity or quantities to compute the priority of a
process i.e.
– Time limits
– Memory requirements
– Open files
– The ratio of average I/O burst to average CPU burst
• Externally priorities are set by criteria process type,
work, department sponsoring 8

10. Priority Scheduling
• Priority scheduling can be either be Preemptive or Non
Preemptive
• When a process arrives at the ready queue, its priority is
compared with the priority of currently running process
• Preempt the CPU if the priority of the newly arrived
process is higher than the priority of the currently
running processes
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11. Priority Scheduling
• If the priority scheduling algorithm is Non –
Preemptive put the new process at the head of the
ready queue
• A major problem with this is indefinite blocking
• A solution to this problem is Aging
• Aging is a technique which gradually increases the
priority of processes that wait in the system for a
long time 11

12. Round-Robin Scheduling Algorithm
• The round robin scheduling algorithm is designed
especially for Time Sharing Systems
• It is similar to FCFS scheduling, but Preemption is added
to switch between processes
• A small unit of time called Time quantum or time slice is
defined
• A time quantum is generally from 10 to 100 mille seconds
• The CPU scheduler allocates the CPU to each process for
a time interval of up to 1 time quantum
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13. Round-Robin Scheduling Algorithm
• The average waiting time under Round Robin policy
is often long
• Consider the following set of processes that arrive at
time 0, with the
– length of the CPU burst time given in milliseconds:
Process Burst Time
P1 24
P2 3
P3 3
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14. Round-Robin Scheduling Algorithm
• Use a time quantum of 4milliseconds
• Process P1 gets the first 4msec it is Preempted after the
first time quantum
• P2 & P3 does not require 4msec they quit before their time
quantum
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15. Round-Robin Scheduling Algorithm
• The CPU is returned to process P1
• The average waiting time 17 / 3= 5.66milliseconds
• The average turn around time for 3 processes of time
quantum 10 is 29
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17. Round-Robin Scheduling Algorithm
• Round Robin scheduling is Preemptive
• If CPU burst time of process, is larger than the
process time quantum then CPU is Preempted
from the process
• No process is allocated CPU for more than 1 time
quantum

18. Round-Robin Scheduling Algorithm
• Performance of RR scheduling depends on the
size of time quantum
• If time quantum is extremely very large RR
approach is same as FCFs
• If it is extremely small, RR approach is called
processor sharing
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19. Summary
In this class, you have learnt
• Priority Scheduling Algorithm
• Round Robin Scheduling
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20. Frequently Asked Questions
1. Explain Round Robin Scheduling
2. Explain Priority Scheduling
3. What are the Internal & External priorities
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21. Quiz
1. In __________ scheduling CPU allocated to the
process with the highest priority
a) FCFS
b) SJF
c) RR
d) Priority
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22. Quiz
2. Priority scheduling can be __________
a) Preemptive
b) Non Preemptive
c) Both
d) None
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23. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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24. Quiz
3. RR scheduling is designed for _______ systems
a) Time sharing
b) Real time
c) Distributed
d) None
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25. Quiz
4. If the time quantum is extremely large than RR is
same as
a) SJF
b) FCFS
c) Priority
d) None
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