This document summarizes a presentation on CPU scheduling algorithms. It discusses why CPU scheduling is necessary due to underutilization of resources when processes are waiting. It then covers the differences between preemptive and non-preemptive scheduling, listing advantages and disadvantages of each. Specific algorithms like FCFS are explained, showing examples of how it schedules processes without and with arrival times and calculating waiting times, turnaround times and completion times. The document aims to explain the basics of CPU scheduling to optimize resource usage.

1. University of Science & Technology
Chittagong
Course title: Operating Systems
Course code: CSE 327
Department of Computer Science & Engineering
(CSE)
1
Submitted by :
Piku Das (17010110)
Mizanur Rahman (17010108)
Israt Liza (17010111)
Eftykher Mahmud( 17010112)
Sushmita Dey (17010113)
Batch No : 29
Submitted to :
Fatema Tuj Zohra
(Lecturer of CSE
Department)
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2. Presentation on
CPU Scheduling
2

3. Why CPU scheduling ?
One of the key goal of an operating system is to
utilize the the CPU as much as possible.
There is a situation for underutilization of CPU
because most of the component of a computer is
not as fast as the microprocessor.
To overcome the problem of underutilization of
CPU and main memory, the multiprogramming
was introduced. The objective of
multiprogramming is to have some process
running all the time. 3

4. Why CPU scheduling ? (cont… .)
So to implement the concept of multiprogramming we
need to design scheduling algorithms.
Whenever the CPU becomes idle , the OS must choose
one of the process in the ready queue to be executed .
The selection process is carried out by a short term
scheduler or CPU scheduler .
4

5. 5
Preemptive Vs Non-Preemptive
Preemptive Non-Preemptive
1. Allocation For Resource
to process for Limited Time.
1. Process holds the resource till
Terminated or Switch to Waiting
State.
2. Process Can be
Interrupted in between
2. Process Can’t be Interrupted
until it terminated or switching
waiting state
3. High CPU Utilization 3. Low CPU Utilization
4. In Preemptive
Scheduling, a running
process may be replaced by
a higher priority process at
any time
4. In Non- Preemptive Scheduling,
Once the CPU has been allocated to
a process ,the process keeps the
CPU it releases the CPU either by
terminated by switching the

6. 6
Preemptive Non-Preemptive
5. Preemptive Scheduling is
Flexible
5. Preemptive Scheduling is
Rigid
6. Preemptive Scheduling is
more Complex
6. Non-Preemptive
Scheduling is Simple but it is
less efficient
7. Preemptive Scheduling is
not attractive
7. Non-Preemptive
Scheduling is more attractive
because of its simplicity
8. Preemptive Scheduling is
cost associative
8. Non-Preemptive
Scheduling is not cost
associative
Preemptive Vs Non-Preemptive

7. 7
9.Used in time sharing
System
9.Not suitible for time sharing
System.
10.Switching Overhead 10.Not Switching Overhead
11.SJF (Shortest Job First )
Scheduling,
Round Robin Scheduling
11. FCFS(First come First
Serve)
Preemptive Vs Non-Preemptive

8. 8
CPU Scheduling Criteria
Scheduling Criteria:
 There are several different criteria to consider when
trying to select the "best" scheduling algorithm for a
particular situation and environment, including:
 CPU utilization - Ideally the CPU would be busy 100% of
the time, so as to waste 0 CPU cycles. On a real system CPU
usage should range from 40% ( lightly loaded ) to 90% (
heavily loaded. )
 Number of processes completed per unit time. May range
from 10 / second to 1 / hour depending on the specific
processes.

9. 9
 Turnaround time - Time required for a particular process
to complete, from submission time to completion.
( Wall clock time. )
 Waiting time - How much time processes spend in the
ready queue waiting their turn to get on the CPU. ( Load
average - The average number of processes sitting in the
ready queue waiting their turn to get into the CPU.
 Reported in 1-minute, 5-minute, and 15-minute averages
by "uptime" and "who". ) Response time - The time taken in
an interactive program from the issuance of a command to
the commence of a response to that command.
CPU Scheduling Criteria

10. 10
In general one wants to optimize the average value of a
criteria ( Maximize CPU utilization and throughput, and
minimize all the others. ) However some times one wants to do
something different, such as to minimize the maximum
response time.
 Sometimes it is most desirable to minimize the variance of a
criteria than the actual value. I.e. users are more accepting of a
consistent predictable system than an inconsistent one, even if
it is a little bit slower.
CPU Scheduling Criteria

11. What is FCFS?
 FCFS or First come first serve is an
operating system process Scheduling
algorithm that automatically executes queued
requests and processes by order of their
arrival.
5

12. i. Jobs are executed on first come, first serve
basis.
ii. It is an non-preemptive scheduling
algorithm.
iii.It’s implementation is based on FIFO queue.
iv.OS runs the process at head of the queue.
FCFS Basic Concepts
12
Two kinds of non-preemptive FCFS
 With priority scheme.
 No priority scheme.

13. Advantages
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i. Simple Algorithm.
ii. It is simple to understand and code.
iii. Suitable for batch system.
i. Because of non-preemptive algorithm the waiting time
can be large if short request behind the long process.
ii. Not suitable for sharing system.
iii. Because of long average waiting time the performance
is poor.
Disadvantages

14. FCFS CPU Scheduling Algorithm
Without Arrival Time
14

15. Process:
The processes arrive in the order P1, P2, P3 and are served
as per the FCFS algorithm. The Gantt chart is as shown:
P1 P2 P3
0 25 29 32
Fig: Gantt Chart

16. 16
Process Burst Time Waiting
Time
Completion
Time
Turn Around
Time
P1 25 0 25 25
P2 4 25 29 29
P3 3 29 32 32
 Formula:
 Completion Time: Time at which process completes its
execution.
 Turn Around Time: Turn Around Time = Completion Time –
Arrival Time.
 Waiting Time(W.T): Waiting Time = Turn Around Time –
Burst Time.
Average waiting time = (0+25+29)/3
= 18millisecond.
Average T.A.T = (25+29+32)/3

17. FCFS CPU Scheduling With Arrival Time
17
Process Arrival Time Burst Time
P0 0 2
P1 1 6
P2 2 4
P3 3 9
P4 4 12

18. Process:
Process Arrival Time Burst Time
P0 0 2
P1 1 6
P2 2 4
P3 3 9
P4 4 12
The processes arrive in the order P1, P2, P3 and P4 are served as per
the FCFS algorithm. The Gantt chart is as shown:
P0 P1 P2 P3 P4
8 12 21 330 2
Fig: Gantt chart

19. 19
Process Arrival Time Burst Time Completion
Time
Waiting Time Turn Around
Time
P0 0 2 2 0 2
P1 1 6 8 1 7
P2 2 4 12 6 10
P3 3 9 21 9 18
P4 4 12 33 17 29
 Formula:
 Completion Time: Time at which process completes its
execution.
 Turn Around Time: Turn Around Time = Completion Time –
Arrival Time.
 Waiting Time(W.T): Waiting Time = Turn Around Time – Burst
Time.
Average waiting time = (0+1+6+9+17) / 5
= 6.6ms.
Average T.A.T = (2+7+10+18+29) / 5

20. Thanks Everyone
THE END
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