Operating System presentation on priority Scheduling, Queue Scheduling and multi level feedback scheduling with examples.

1. Operating system
Presentation
Group 3rd

2. Jawad Haider (20011519-055)
Sana Khalid (20011519-019)
Faiza Ijaz (20011519-047)

3. Topics
Priority
Scheduling
Multilevel Queue
Scheduling
Multilevel Feedback
Queue Scheduling

4. Jawad Haider
20011519-055
01
Priority Scheduling

5. Priority Scheduling
01
Introduction
What is priority
Scheduling
02
Preemptive priority
Scheduling
03
Non- preemptive
Priority
Scheduling
04
Example
Gantt chart, Process
waiting time and average
waiting time
05
Problem
Solution of this problem
06
Question
Using preemptive
scheduling

6. Priority Scheduling(Basic Concept)
● A priority is associated with each process, and the CPU is
allocated to the process with the highest priority.
● Equal-priority processes are scheduled in FCFS order.
Types of Priority Scheduling
1. Preemptive Priority Scheduling
2. Non-preemptive Priority Scheduling

7. Preemptive priority
Scheduling
A Preemptive Priority scheduling algorithm
will pre-empt the CPU if the priority of the
newly arrived process is higher than the
priority of the currently running process.

8. Non-Preemptive priority
Scheduling
A Non-preemptive Priority scheduling
algorithm will put the new process at the
head of the ready queue. Here, the
currently running process will not be pre-
empted by the new process.

9. The priority of the process is determined in terms of high
priority and low priority. Some fixed range of numbers generally
indicates priorities. However, there is no general agreement on whether
0 is the highest or lowest priority. Some systems use low numbers to
represent low priority; others use low numbers for high priority. This
difference can lead to confusion. Here, we will assume that low numbers
represent high priority.
Deciding the priority of a
process

10. Example
Consider the following set of processes, assumed to have arrived at
time 0, in the order P1, P2, P3, P4, P5, with the length of the CPU
burst given in milliseconds:
Process ID Brust Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2

11. Gantt chart:
P2 P5 P1 P3 P44
0 1 6 16 18 19
Waiting Time for P1 = 6 milliseconds.
Waiting Time for P2 = 0 milliseconds.
Waiting Time for P3 = 16 milliseconds.
Waiting Time for P4 = 18 milliseconds.
Waiting Time for P5 = 1 milliseconds.
So, the Average Waiting Time = (6+0+16+18+1)/5 = 8.2
milliseconds.

12. Problem with Priority Scheduling
A major problem with priority scheduling algorithms
is indefinite blocking or starvation.
A process that is ready to run but waiting for the CPU can
be considered blocked.
If a low priority process is waiting in the queue for CPU and
new high priority processes keep entering the queue, the
higher priority processes will keep executing. In such a
case, the process with low priority will be delayed, or it may
never get the CPU in the worst case. A priority scheduling
algorithm can leave some low priority processes waiting
indefinitely.
In a heavily loaded computer system, a steady stream of
higher-priority processes can prevent a low-priority
process from ever getting the CPU.

13. Solution
•A solution to the problem of indefinite blockage of low-priority
processes is aging.
•Aging is a technique of gradually increasing the priority of processes
that wait in the system for a long time.
For Example: If priorities range from 127 (low) to 0 (high), we could increase
the priority of a waiting process by 1 every 15 minutes. Eventually, even a
process with an initial priority of 127 would have the highest priority in the
system and would be executed.
Thus, we have discussed how priority scheduling works, the problem of
starvation the low priority processes face, and its solution in the form of
aging.

14. Solve Example
Consider the set of processes with arrival time (in milliseconds), CPU
burst time (in milliseconds), and priority (0 is the highest priority)
shown below. None of the processes has I/O burst time.
Process ID Arrival Time Brust Time Priority
P1 0 11 2
P2 5 28 0
P3 12 2 3
P4 2 10 1
P5 9 16 4
The average waiting time (in milliseconds) of all the
processing using preemptive priority scheduling algorithm is
_________________.
(A) 29 (B) 30 (C)31 (D)32

15. Gantt Chart
0 2 5 33 40 49 51 67
P1 P4 P2 P4 P1 P3 P5
Waiting Time = Total Waiting Time – No. of milliseconds
Process executed – Arrival Time
Waiting Time for P1 = (40 -2-0) = 38 milliseconds.
Waiting Time for P2 = (5-0-5) = 0 milliseconds.
Waiting Time for P3 = (49-0-12) = 37 milliseconds.
Waiting Time for P4 = (33-3-2) = 28 milliseconds.
Waiting Time for P5 = (51-0-9) = 42 milliseconds.
Average Waiting time = (38+0+37+28+42)/ 5 = 145/5 = 29
milliseconds

16. Sana Khalid
20011519-019
02
Multilevel Queue
Scheduling

17. Basic
concept
Multilevel queue scheduling is used when processes in
the ready queue can be divided into different classes
where each class has its own scheduling needs. For
instance, foreground or interactive processes and
background or batch processes are commonly divided.

18. Cont.…
Each queue is assigned a priority and can use its own
scheduling algorithm which makes it convenient to
use many scheduling algorithms at the same time.
Generally, topmost level of queue has highest priority
which decreases as we move to lower levels.

19. Advantage
The major advantage of this
algorithm is that we can use various
algorithms such as FCFS, SJF, LJF,
etc. At the same time in different
queues.
Disadvantage
The lowest level processes suffer
from starvation problem.

20. ● Priority queue scheduling. It
executes processes based upon
their priorities .
● It is both preemptive and non
preemptive in nature.
● There is no idea of average
waiting time and response
time.
● MLQ. Processes are executed
depending on priority of that
particular level of queue to
which process belongs.
● It can be both non-preemptive
and preemptive in nature
depending upon conditions.
● Average waiting time and
average response time depends
upon algorithms used in various
levels of multi level queue for
scheduling.
Difference between MLQ and priority
queue scheduling

21. For Example
a common division is
a foreground
(interactive) process and
a background (batch) process.
These two classes have
different scheduling needs. For
this kind of situation Multilevel
Queue Scheduling is used

22. Ready Queue
READY QUEUE is divided into separate
queues for each class of processes. For
example, let us take three different types
of processes System processes,
Interactive processes, and Batch
Processes. All three processes have their
own queue.

23. The Description of the processes are
as follows:
System Processes: The CPU itself
has its own process to run which is
generally termed as System Process.
Interactive Processes: An Interactive
Process is a type of process in which
there should be same type of
interaction.

24. Batch Processes: Batch processing is generally a
technique in the Operating system that collects the
programs and data together in the form of
the batch before the processing starts.
All three different type of processes has their own
queue. Each queue has its own Scheduling
algorithm.

25. Solve Example
Consider the below table of four processes under Multilevel Queue
scheduling. Queue number denoted the queue of the process. Priority
of queue 1 is greater then queue 2. Queue one has Round Robin and
queue 2 uses FCFS.
Process Queue AT BT CT TAT WT
P1 1 0 4
P2 1 0 3
P3 2 0 8
P4 1 10 5

26. Faiza Ijaz
20011519-047
03
Multilevel Feedback
Queue Scheduling

27. Difference between (MLQ)and(MLFQ)
CPU scheduling algorithms:
MLQ: The priority is fixed in
this algorithm. When all
processes in one queue get
executed completely then
only processes in other queue
are executed.
MLFQ: The priority for process is dynamic
as process is allowed to move between
queue. A process taking longer time in
lower priority queue can be shifted to
higher priority queue and vice versa.
Since, processes do not move
between queues, it has low
scheduling overhead and is
inflexible.
Since, processes are allowed to move
between queues, it has high scheduling
overhead and is flexible.

28. ● Multilevel Feedback Queue
Scheduling In this processes
can move between the queues.
And thus, much more efficient
than multilevel queue
scheduling.
Multilevel Feedback Queue Scheduling:

29. Multilevel feedback queue scheduling, however,
allows a process to move between queues. The idea is
to separate processes with different CPU-burst
characteristics. If a process uses too much CPU time,
it will be moved to a lower-priority queue. Similarly, a
process that waits too long in a lower-priority queue
may be moved to a higher-priority queue.
Working of MLFQS:

30. Multilevel feedback queue
scheduling, however, allows a
process to move between queues.
Multilevel Feedback
Queue Scheduling (MLFQ) keeps
analyzing the behavior (time of
execution) of processes and
according to which it changes its
priority.
Cont.…

31. In general, a multilevel feedback queue scheduler is
defined by the following parameters:
• The number of queues.
• The scheduling algorithm for each queue.
• The method used to determine when to upgrade a process
to a higher-priority queue.
• The method used to determine when to demote a process
to a lower-priority queue.
• The method used to determine which queue a process will
enter when that process needs service.

32. Exercise Question: Consider a system that has a CPU-bound process, which
requires a burst time of 40 seconds. The multilevel Feed Back Queue
scheduling algorithm is used and the queue time quantum ‘2’ seconds and in
each level it is incremented by ‘5’ seconds. Then how many times the
process will be interrupted and in which queue the process will terminate
the execution?

33. Solution:
Process P needs 40 Seconds for total execution.
At Queue 1 it is executed for 2 seconds and then interrupted and shifted to
queue 2.
At Queue 2 it is executed for 7 seconds and then interrupted and shifted to
queue 3.
At Queue 3 it is executed for 12 seconds and then interrupted and shifted
to queue 4.
At Queue 4 it is executed for 17 seconds and then interrupted and shifted
to queue 5.
At Queue 5 it executes for 2 seconds and then it completes.
Hence the process is interrupted 4 times and completed on queue 5.

34. Advantage
 It is more flexible.
 It allows different processes to
move between different queues.
 It prevents starvation by moving
a process that waits
too long for the lower priority
queue to the higher priority
queue.
Disadvantage
 It produces more CPU
overheads.
 It is the most complex algorithm.