The document discusses various CPU scheduling algorithms. It begins by classifying CPU scheduling as either preemptive or non-preemptive. It then lists performance criteria for good schedulers such as CPU utilization and response time. The document proceeds to describe scheduling algorithms including First-Come First-Served (FCFS), Shortest Process First (SPF), Shortest Remaining Time First (SRTF), Round Robin (RR), Priority Scheduling, and Multilevel Feedback Queues. Examples are provided to demonstrate how each algorithm works.

1. CPU Scheduling
 Classification of CPU Scheduling
 Performance Criteria of Good Scheduler
 CPU Scheduling Algorithms

2. *Property of STI J0024
CPU scheduling may be classified as:
 Non-preemptive scheduling
CPU scheduling is non-preemptive if once the CPU has been assigned to a
process and the process starts executing, the CPU cannot be taken away from
that process.
 Preemptive scheduling
In preemptive CPU scheduling, even though the CPU has been assigned to a
process and the process is already executing, the CPU scheduler may decide to
assign the CPU to another process in the ready queue.

3. *Property of STI J0024
A good scheduler should optimize the following performance criteria:
 CPU utilization
 Throughput
 Turnaround time
 Response time
 Waiting time

4. *Property of STI J0024
The different CPU scheduling algorithms are:
 First-Come, First-Served Algorithm
 Shortest Process First Algorithm
 Shortest Remaining Time First Algorithm
 Round Robin Algorithm
 Priority Scheduling
 Multilevel Feedback Queues

5. *Property of STI J0024
First-Come, First-Served Algorithm (FCFS)
- the one that enters the Ready queue first gets to execute at the CPU first
a.
b.
c.
A
0 8
A
0
B
8 12
A
0
B C
8 12 17

6. *Property of STI J0024
First-Come, First-Served Algorithm (FCFS)
d.
e.
WTA = 0 – 0 = 0 ms
WTB = 8 – 3 = 5 ms
WTC = 12 – 4 = 8 ms
WTD = 17 – 6 = 11 ms
WTE = 20 – 10 = 10 ms
The average waiting time is
(0 + 5 + 8 + 11 + 10)/5 = 34/5 = 6.8 ms
A
0
B C D
8 12 17 20
A
0
B C D
8 12 17 20
E
22

7. *Property of STI J0024
First-Come, First-Served Algorithm (FCFS)
TAA = 8 - 0= 8 ms
TAB = 12 - 3 = 9 ms
TAC = 17 - 4 = 13 ms
TAD = 20 - 6 = 14 ms
TAE = 22 - 10 = 12 ms
The average turnaround time is
(8 + 9 + 13 + 14 + 12)/5 = 56/5 = 11.2 ms

8. *Property of STI J0024
Shortest Process First Algorithm (SPF)
- the process with the shortest CPU burst
a.
b.
c.
A
0 8
A
0
D
8 11
A
0
D
8 11 13
E

9. *Property of STI J0024
Shortest Process First Algorithm (SPF)
d.
e.
WTA = 0 – 0 = 0 ms
WTB = 13 – 3 = 10 ms
WTC = 17 – 4 = 13 ms
WTD = 8 – 6 = 2 ms
WTE = 11 – 10 = 1 ms
The average waiting time is
(0 + 10 + 13 + 2 + 1)/5 = 26/5 = 5.2 ms
A
0
BD
8 11 13 17
E
A
0
B CD
8 11 13 17
E
22

10. *Property of STI J0024
Shortest Process First Algorithm (SPF)
TAA = 8- 0 = 8 ms
TAB = 17 - 3 = 14 ms
TAC = 22 - 4 = 18 ms
TAD = 11 - 6 = 5 ms
TAE = 13 - 10 = 3 ms
The average turnaround time is
(8 + 14 + 18 + 5 + 3)/5 = 48/5 = 9.6 ms

11. *Property of STI J0024
Shortest Remaining Time First Algorithm (SRTF)
- the preemptive version of SPF
a.
b.
c.
d.
Process ID Arrival Time CPU Burst
A 0 8
B 3 4
C 4 5
D 6 3
E 10 2
0
A
0
A
3
B
0
B
7
A
3
0
B
7
DA
3

12. *Property of STI J0024
Shortest Remaining Time First Algorithm (SRTF)
e.
f.
g.
h.
WTA = (0 – 0) + (12 – 3) = 9 ms
WTB = 3 – 3 = 0 ms
WTC = 17 – 4 = 13 ms
WTD = 7 – 6 = 1 ms
WTE = 10 – 10 = 0 ms
The average waiting time is
(9 + 0 + 13 + 1 + 0)/5 = 23/5 = 4.6 ms
0
B D
7 10
A
3
0
B D
7 10 12
EA
3
A
0
B D
7 10 12 17
EA
3
A
0
B CD
7 10 12 17
E
22
A
3

13. *Property of STI J0024
Shortest Remaining Time First Algorithm (SRTF)
TAA = 17 - 0= 17 ms
TAB = 7 - 3 = 4 ms
TAC = 22 - 4 = 18 ms
TAD = 10 - 6 = 4 ms
TAE = 12 - 10 = 2 ms
The average turnaround time is
(17 + 4 + 18 + 4 + 2)/5 = 45/5 = 9.0 ms

14. *Property of STI J0024
Round Robin Algorithm (RR)
- the process are selected on a first-come, first-served basis. However, each
process is given a time limit to execute at the CPU.
Assume a time quantum of 3 ms.
a.
b.
Process ID Arrival Time CPU Burst
A 0 8
B 3 4
C 4 5
D 6 3
E 10 2
0
A
3
0
B
6
A
3

15. *Property of STI J0024
Round Robin Algorithm (RR)
c.
d.
e
f.
g.
h
i.
0
B A
6 9
A
3
0
B A
6 9 12
CA
3
D
0
B A
6 9 12 15
CA
3
D
0
B A
6 9 12 15
CA
3
B
16
D
0
B A
6 9 12 15
CA
3
B
16
A
18
D
0
B A
6 9 12 15
CA
3
B
16
A
18
E
20
D
0
B CA
6 9 12 15
C
22
A
3
B
16
A
18
E
20

16. *Property of STI J0024
Round Robin Algorithm (RR)
WTA = (0 – 0) + (6 – 3) + (16 – 9) = 10 ms
WTB = (3 – 3) + (15 – 6) = 9 ms
WTC = (9 – 4) + (20 – 12) = 13 ms
WTD = 12 – 6 = 6 ms
WTE = 18 – 10 = 8 ms
The average waiting time is
(10 + 9 + 13 + 6 + 8)/5 = 46/5 = 9.2 ms

17. *Property of STI J0024
Round Robin Algorithm (RR)
TAA = 18 - 6= 18 ms
TAB = 16 - 3 = 13 ms
TAC = 22 - 4 = 18 ms
TAD = 15 - 6 = 9 ms
TAE = 20 - 10 = 10 ms
The average turnaround time is
(18 + 13 + 18 + 9 + 10)/5 = 68/5 = 13.6 ms

18. *Property of STI J0024
Priority Scheduling
- Each process is assigned a priority and the CPU scheduler selects the process in
the ready queue with the highest priority to execute next.
Using the non-preemptive priority algorithm:
a.
b.
Process ID Arrival
Time
CPU Burst Priority
A 0 8 4
B 3 4 3
C 4 5 1
D 6 3 2
E 10 2 2
A
0 8
A
0
C
8 13

19. *Property of STI J0024
Priority Scheduling
c.
d.
e.
WTA = 0 – 0 = 0 ms
WTB = 18 – 3 = 15 ms
WTC = 8 – 4 = 4 ms
WTD = 13 – 6 = 7 ms
WTE = 16 – 10 = 6 ms
The average waiting time is
(0 + 15 + 4 + 7 + 6)/5 = 32/5 = 6.4 ms
A
0
C D
8 13 16
A
0
C D
8 13 16 18
E
A
0
BC D
8 13 16 18
E
22

20. *Property of STI J0024
Priority Scheduling
TAA = 8 - 0= 8 ms
TAB = 22 – 3 = 19 ms
TAC = 13 - 4 = 9 ms
TAD = 16 - 6 = 10 ms
TAE = 18 - 10 = 8 ms
The average turnaround time is
(8 + 19 + 9 + 10 + 8)/5 = 54/5 = 10.8 ms

21. *Property of STI J0024
Priority Scheduling
Using the preemptive priority algorithm:
a.
b.
c.
d.
Process ID Arrival
Time
CPU Burst Priority
A 0 8 4
B 3 4 3
C 4 5 1
D 6 3 2
E 10 2 2
0
A
0
BA
3
0
B C
4 9
A
3
0
B C
4 9
A
3
D

22. *Property of STI J0024
Priority Scheduling
e.
f.
WTA = (0 – 0) + (17 – 3) = 14 ms
WTB = (3 – 3) + (14 - 4) = 10 ms
WTC = (4 – 4) = 0 ms
WTD = (9 – 6) = 3 ms
WTE = (12 – 10) = 2 ms
The average waiting time is
(14 + 10 + 0 + 3 + 2)/5 = 29/5 = 5.8 ms
0
B C D
4 9 12
A
3
A
0
B C D
4 9 12 14
E
22
A
3
B
17

23. *Property of STI J0024
Priority Scheduling
TAA = 22 – 0 = 22 ms
TAB = 17 - 3 = 14 ms
TAC = 9 - 4 = 5 ms
TAD = 12 - 6 = 6 ms
TAE = 14 - 10 = 4 ms
The average turnaround time is
(22 + 14 + 5 + 6 + 4)/5 = 51/5 = 10.2 ms

24. *Property of STI J0024
Multilevel Feedback Queues
Q0All processes enter
the queuing network
at the rear of the
highest priority
queue
Processes that
finish executing or
request for an I/O
operation within the
given time slice
leave the queuing
network
FCFS (time slice = 10 ms)
Q1
FCFS (time slice = 20 ms)
Processes that do not finish executing
within the given time slice move to the next
lower-priority queue
Processes that
finish executing or
request for an I/O
operation within the
given time slice
leave the queuing
network
.
.
.
Qn
RR
Processes that do not finish executing
within the given time slice move to the next
lower-priority queue
Q2
FCFS (time slice = 30 ms)
Processes that
finish executing or
request for an I/O
operation within the
given time slice
leave the queuing
network
Processes that
finish executing or
request for an I/O
operation within the
given time slice
leave the queuing
network