This document discusses CPU scheduling algorithms. It begins with basic concepts of CPU scheduling and scheduling criteria like CPU utilization, throughput, turnaround time, and waiting time that scheduling aims to optimize. It then explains key types of scheduling algorithms like first come first served, shortest job first, priority, and round robin. Code implementation and demonstrations of these algorithms in C++ are also covered. The document concludes with the involvement of the operating system in scheduling.

1. CPU
Scheduling
Algorithms
1
Group No. 2

2. Overview
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Implementation in C++
 Demonstration
 Involvement of Operating System
2
CPU Scheduling Algorithms

3. CPU Scheduling Algorithms 3
Basic Concepts
 Main objective of multiprogramming is to keep on
running processes all the time for maximum CPU
utilization.
 Scheduling is fundamental function of OS.
 The task of selecting the processes in memory that
are ready to execute, and allocating them to the
CPU is performed by the CPU Scheduler.

4. 4
CPU Scheduler
 CPU scheduling decisions may take place when a
process:
o 1. Switches from running to waiting state
o 2. Switches from running to ready state
o 3. Switches from waiting to ready
o 4. Terminates
 Scheduling under 1 and 4 is non preemptive.
 All other scheduling is preemptive.
CPU Scheduling Algorithms

5. 5
 Nonpreemptive
Once a process is allocated the CPU, it does not
leave unless:
o it has to wait, e.g., for I/O request
o it terminates
 Preemptive
o OS can force (preempt) a process from CPU at
anytime
o E.g., to allocate CPU to another higher-priority
process
CONT…
CPU Scheduling Algorithms
CPU Scheduler

6. 6
Scheduling Criteria
 CPU utilization: keep the CPU as busy as possible
◦ Maximize
 Throughput: No of processes that complete their
execution per time unit
◦ Maximize
 Turnaround time: amount of time to execute a
particular process (time from submission to
termination)
◦ Minimize
CPU Scheduling Algorithms

7. 7
CONT…
CPU Scheduling Algorithms
 Waiting time: amount of time a process has been
waiting in the ready queue (sum of time waiting in
ready queue)
o Minimize
 Response time – amount of time it takes from when
a request was submitted until the first response is
produced, not output (for time-sharing
environment)
o Minimize
Scheduling Criteria

8. 8
Scheduling Algorithms
 First Come, First Served
 Shortest Job First
 Priority
 Round Robin
CPU Scheduling Algorithms

9. 9
CPU Scheduling Algorithms
Implementation in C++
 Class: cpuschedule
 Attributes:
o n – number of processes
o Bu[ ] – Array to store Burst Time
o A[ ] – Array to store Arrival Time
o Wt[ ] – Array to store Waiting Time
o Twt – Total Waiting Time
o Awt – Average Waiting Time

10. 10
CPU Scheduling Algorithms
Implementation in C++
o Getdata() – To get number of processes and
Burst Times from the user
o Fcfs() – First Come, First Served Algorithm
o Sjf() – Shortest Job First (normal) Algorithm
o SjfP() – Shortest Job First (Preemption)
Algorithm
o SjfNp() – Shortest Job First (non
preemption) Algorithm
o Priority() – Priority Algorithm
o RoundRobin() – Round Robin Algorithm
CONT…
 Operations:

11. 11
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
CPU Scheduling Algorithms
First Come, First Served
P1 P2 P3
24 27 30
0

12. 12
 Suppose that the processes arrive in the order :
P2 , P3 , P1 (P1:24,P2:3,P3:3)
 The Gantt chart for the schedule is:
 Waiting time for P1 = 6;P2 = 0; P3 = 3
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case
 Convoy effect - short process behind long process
CONT…
CPU Scheduling Algorithms
First Come First
Served
P1
P3
P2
6
3 30
0

13. 13
CPU Scheduling Algorithms
Shortest Job First
Normal SJF
Process Burst Time
P1 7
P2 3
P3 4
 The Gantt Chart for SJF (Normal) is:
 Average waiting time = (0 + 3 + 7)/3 = 3.33
P2 P3 P1
7
3
0 14

14. 14
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 The Gantt Chart for SJF (non-preemptive) is:
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Non-Preemptive SJF
CPU Scheduling Algorithms
Shortest Job First
CONT…
P1 P3 P2
7
2 16
0
P4
8 12
4 5

15. 15
CONT…
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 The Gantt Chart for SJF (preemptive) is:
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3
P2
4
2 11
0
P4
5 7
P2 P1
16
CPU Scheduling Algorithms
Preemptive SJF
Shortest Job First

16.  Associate with each process the length of its next CPU
burst.
 Use these lengths to schedule the process with the
shortest time.
 Two schemes:
o Non-Preemptive: once CPU given to the process it
cannot be preempted until completes its CPU burst.
o Preemptive: if a new process arrives with CPU burst
length less than remaining time of current executing
process, preempt. This scheme is know as the
Shortest-Remaining-Time-First (SRTF).
 SJF is optimal: gives minimum average waiting time for a
given set of processes.
Shortest Job First
CONT…
CPU Scheduling Algorithms 16

17. 17
CPU Scheduling Algorithms
Priority
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
 Gantt Chart
 Average waiting time = (6 + 0 + 16 + 18 + 1)/5 = 8.2
P2 P1
P5
6
1 16
0
P3
18
P4
19

18.  A priority number (integer) is associated with each
process.
 Lager the CPU burst lower the priority.
 The CPU is allocated to the process with the highest
priority (smallest integer ≡ highest priority)
 Starvation (Infinity blocking): low priority processes
may never execute.
 Aging: as time progresses increase the priority of the
process.
Priority
CONT…
CPU Scheduling Algorithms 18

19. 19
CPU Scheduling Algorithms
Round Robin
Process Burst Time
P1 24
P2 3
P3 3
 Quantum time = 4 milliseconds
 The Gantt chart is:
 Average waiting time = {[0+(10-4)]+4+7}/3 = 5.6
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30

20.  Typically, higher average turnaround than SJF, but
better response
 Each process gets a small unit of CPU time (time
quantum), usually 10-100 milliseconds. After this
time has elapsed, the process is preempted and
added to the end of the ready queue.
 Performance
o q large ⇒ FCFS
o q small ⇒ q must be large with respect to
context switch, otherwise overhead is too high
CPU Scheduling Algorithms 20
Round Robin
CONT…

21. CPU Scheduling Algorithms 21
Involvement of OS
Source Code (.c)
Conversion
Executable (.exe)
Compiler
Microsoft
Windows
Micro-kernel
(.i , .o)
Memory
CPU
(load executable
directly to memory)
Execut
e
Scheduling
Algorithms

22. CPU
Scheduling
Algorithms
Group No. 2 22
Group No. 2