CPU scheduling algorithms with examples

1. CPU scheduling
Mohd Amir khan

2. CPU scheduling
 A process execution consist office cycle of CPU execution and input output
execution
 Every process begins with CPU burst that may be followed by input output
bus, then another CPU burst and then input output burst and so on
eventually in the last will end up on CPU burst
 CPU burst there are those processes which require most of time on CPU
 Input output bus which require most of the time on input output devices
for peripherals
 A good CPU scheduling idea should choose the mixture of both so that
both input and output devices and CPU can be utilized efficiently

3. Basic Concepts
 Maximum CPU utilization obtained with
multiprogramming
 CPU–I/O Burst Cycle – Process execution
consists of a cycle of CPU execution and
I/O wait
 CPU burst followed by I/O burst
 CPU burst distribution is of main concern
For example
o cricket team
o printing document
o addition of two numbers

4. CPU scheduling terminology
 Burst time /execution time /running time:- is the time causes required for
running on CPU
 Waiting time:- time spent by a process in ready state waiting for CPU
 Arrival time:- When a process enters ready state
 Turnaround time:- total time spent by a process in the system and around time
equal to {exit time - arrival time} or {burst time + waiting time}
 Response time:- Time between a process enters ready queue and get
scheduled on the CPU for the first time
 Exit time:- when process completes execution and exit from system

5. Histogram of CPU-burst Times

6. CPU scheduling Algorithm
 Two approaches
1. Non-Preemptive Scheduling:
2. Preemptive Scheduling:
 Non-preemptive
 Once resources(CPU Cycle) are allocated to a process, the process holds it till it completes its burst time or switches
to waiting state
 Process can not be interrupted until it terminates itself or its time is up.
 Rigid, not cost association
 Preemptive
 In this resources(CPU Cycle) are allocated to a process for a limited time
 Process can be interrupted in between.
 Flexible and cost associated

7. Scheduling Algorithm Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time

8. First Come First serve (FCFS)
 It assign CPU to the process which arrives first
 Easy to understand and can easily be implemented using queue data structure
 Allows non-pre-emptive in nature

9. First- Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
 Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
 Waiting time for P1 = 0; P2 = 24; P3 = 27
 Average waiting time: (0 + 24 + 27)/3 = 17
P P P1 2 3
0 24 3027
A Gantt chart is a horizontal bar chart
developed as a production control tool in
1917 by Henry L. Gantt,
an American engineer and social scientist

10. Solve this
Pid AT BT TAT WT
A 3 4
B 5 3
C 0 2
D 5 1
E 4 3
Create Gantt Chart
Calculate Waiting Time of each and
average waiting time

11. Convoy Effect
Pid AT BT WT AWT
P1 0 100 0
P2 1 1 99
Create Gantt Chart
Pid AT BT WT AWT
P1 1 100 1
P2 0 1 0
Create Gantt Chart
Smaller process have to wait for long time for bigger process to release CPU

12. Advantages
 Easy to learn
 Easy to understand
 Easy to implement
 We used for background process where execution is not urgent

13. Disadvantages
 Suffer from Convoy effect
 Normally long average waiting time
 No consideration to burst time or arrival time/Priority
 Should not be used for interactive system

14. Shortest job first (non-preemptive) shortest
remaining time first(SRTF) (preemptive)
 Out of all available process CPU is assigned to the process having smallest burst time
requirement no priority, no seniority
 If there is tie first Come First serve is used to break tie
 Can we used both with non primitive and primitive approach
 Primitive version STRF is also called as optimal as it currently minimum average waiting
time

15. Example of SJF
Process Arrivall Time Burst Time
P1 1 6
P2 2 8
P3 4 7
P4 0 3
 SJF scheduling chart
 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P3
0 3 24
P4
P1
169
P2

16. SJF (non-preemptive)
Pid AT BT TAT WT
A 3 1
B 1 4
C 4 2
D 0 6
E 2 3
Create Gantt Chart
Calculate Waiting Time of Processes

17. Example of Shortest-remaining-time-first
 Now we add the concepts of varying arrival times and preemption to
the analysis
Process Arrival Time Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
 Preemptive SJF Gantt Chart
 Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5
P4
0 1 26
P1
P2
10
P3
P1
5 17

18. SRTF(Pre-emptive)
Pid AT BT TAT WT
A 3 4
B 5 3
C 0 2
D 5 1
E 4 3
Create Gantt Chart
Calculate Waiting Time of Processes

19. Advantages and disadvantages of shortest job
first scheduling algorithm
Advantages
 SRTF guarantees minimal average waiting
time
 Standard for other algorithm in terms of
average waiting time
 Better Average response time compare to
First Come First serve
Disadvantages
 Algorithm cannot be implemented as there
is no way to know the burst time of a
process
 Process with the longest CPU burst time
requirement will go into starvation
 No idea of priority, process with large burst
time have work response time

20. Priority Algorithm
 Here a priority is associated with each process
 At any instance of time out of all available process, CPU is allocated to the process
which posses the highest Priority
 Tie is broken with FCFS order
 No importance to arrival time or burst time
 Supports both non pre-emptive and pre-emptive version

21. Continue..
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest priority
 Preemptive
 Non-preemptive
 SJF is priority scheduling where priority is the inverse of predicted
next CPU burst time
 Problem  Starvation – low priority processes may never execute
 Solution  Aging – as time progresses increase the priority of the
process

22. Example of Priority
Scheduling(Non-preemptive)
Process Arrival Time Burst Time Priority
P1 1 10 5
P2 0 1 3
P3 2 2 5
P4 3 1 4
P5 1 5 7
Priority scheduling Gantt Chart
 Average waiting time

23. Non Pre-emptive
Pid AT BT Priority TAT WT
P1 0 4 2
P2 1 3 3
P3 2 1 4
P4 3 5 5
P5 4 2 5
Create Gantt Chart
Calculate Waiting Time of Processes

24. Pre-emptive
Pid AT BT Priority TAT WT
P1 0 4 2
P2 1 3 3
P3 2 1 4
P4 3 5 5
P5 4 2 5
Create Gantt Chart
Calculate Waiting Time of Processes

25. Advantages and disadvantages of priority
scheduling algorithm
Advantages
 Provide a facility of priority specially for
system process
 Allows to run important process first even
if it is a user process
Disadvantages
 Here process with smaller priority may
starve for CPU
 No idea of response time or waiting time
 Note aging is a technique of gradually
increasing the priority of process that wait
in the system for long time

26. Round Robin (RR)
 Each process gets a small unit of CPU time (time quantum q),
usually 10-100 milliseconds. After this time has elapsed, the
process is preempted and added to the end of the ready queue.
 If there are n processes in the ready queue and the time quantum
is q, then each process gets 1/n of the CPU time in chunks of at
most q time units at once. No process waits more than (n-1)q
time units.
 Timer interrupts every quantum to schedule next process
 Performance
 q large  FIFO
 q small  q must be large with respect to context switch, otherwise
overhead is too high

27. Continue.
 It is design for time sharing system where it is not necessary to complete one process
and then start another, but to be responsive and divide time of CPU among the
process in the ready state
 Here ready q to be treated as circular queue
 We will fix the time quantum up to which process can hold the CPU in one go within
which either a process terminates or process must release the CPU and re-enter in the
circular queue and wait for the next chance
 Round Robin is called preemptive in nature

28. Example of RR with Time Quantum =
4
Process Burst Time
P1 24
P2 3
P3 3
 The Gantt chart is:
 Typically, higher average turnaround than SJF, but better response
 q should be large compared to context switch time
 q usually 10ms to 100ms, context switch < 10 usec
P P P1 1 1
0 18 3026144 7 10 22
P2
P3
P1
P1
P1

29. Pre-emptive
Pid AT BT TAT WT
A 0 5
B 1 3
C 2 1
D 3 2
E 4 3
TQ=2
Create Queue
Create Gantt Chart
Calculate Waiting Time of Processes

30. Advantages and disadvantages of Round Robin
scheduling algorithm
 Advantages
 Perform best in terms of average response
time
 Works well in call to time sharing system,
client server architecture and interactive
system
 Kind of shortest job first implementation
 Disadvantages
 Longer process may starve
 Performance depends heavily on time
Quantum
 No idea of reality

31. 1. The following is the set of processes whose arrival time, burst time and the priorities are given
below:
S.No. Process Arrival time (in ms) Bunt Time(in ms) Priority
1 P1 0 10 5
2 P2 0 5 2
3 P3 2 3 1
4 P4 5 20 4
5 P5 10 2 3
If Shortest job first ( vith pre-emption) scheduling policy and Priority scheduling policy(with pre-
emption) is used then, what will be the average waiting time in both the cases?
Ops:
A. 5.6 ms, 8 ms
B. None of the mentioned options
C. 6.8 ms, 7.6 ms
D. 0 5.6 ms, 7.6 ms
Question