This document discusses operating systems and CPU scheduling. It covers the functions of operating systems including process management, file management, and I/O management. It then discusses different CPU scheduling algorithms like first-come first-served, shortest job first, priority scheduling, and round robin scheduling. The goals of scheduling algorithms are to maximize throughput and minimize waiting time and turnaround time for processes.

1. Operating Systems

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Agenda
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4 CPU Scheduling
1 Introduction
2 Functions of Operating Systems
Process Management
5 File management & Trace utilities
6 Signals & Resource Limits
7 Introduction to IPC

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Introduction
Module 1

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 Resource allocator
– manages and allocates resources
 Control program
– controls the execution of user programs and operations of I/O devices
 Kernel
– the one program running at all times (all else being application programs)
Functions of Operating Systems

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 General Purpose Operating System (GPOS)
– Desktop System
 Real-Time Operating System (RTOS)
– Time sharing Operating Systems
Types of OS

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 Hard real-time
– Secondary storage maybe absent or limited, data stored in short term
memory, or read-only memory (ROM)
– Conflicts with time-sharing systems, and not supported by general-
purpose operating systems
 Soft real-time
– Limited utility in industrial control of robotics
– Useful in applications (multimedia, virtual reality) requiring advanced
operating-system features
Real-Time Systems (Contd.).

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Functions of Operating System
Module 2

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 An operating system performs large number of functions. Each
function is carried out by a component of the operating system.
 The components of an operating system are:
 Process management sub system
 CPU Scheduling
 File management sub system
 Signals & Resource Limits
 Introduction to IPC
Operating System Functionalities

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 The main functionalities of File management system are:
– The creation and deletion of files
– The creation and deletion of directories
– The support of primitives for manipulating files and directories
– The mapping of files onto secondary storage
– The back up of files on permanent storage media
File Management

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 Program to be executed, it need to be in primary memory.
 Major task of OS
– keep track of which part of memory are currently being used and by
which program
– decide which process are loaded into memory when memory space
becomes available
– Allocate and reallocated memory space as needed
Main-Memory Management

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 A process is a program in execution. A process needs certain
resources, including CPU time, memory, files, and I/O devices, to
accomplish its task
 The operating system is responsible for the following activities in
connection with process management
– Process creation and deletion
– Process continuation and suspension
– Provision of mechanisms for:
– process synchronization
– process communication
Process Management

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 Each program needs an input and generates output.
 User desires that the I/O has been performed without any details.
 User looks for convenience to run programs.
 Hides the user the details of underlying H/W for the I/O.
I/ O Management - Operations

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CPU Scheduling
Module 4

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 First-Come, First-Served (FCFS) Scheduling
 Shortest-Job-First (SJF) Scheduling
 Priority Scheduling
 Round Robin (RR) Scheduling
 Real-Time Scheduling
Scheduling Algorithm

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Process Burst Time
PI 24.0
PII 3.0
PIII 3.0
Suppose that processes appear in the order: P1 , PII , PIII
The Gantt Chart for the schedule is:
Waiting time for PI = 0.0; PII = 24.0; PIII = 27.0
Average waiting time: (0.0 + 24 .0+ 27.0)/3 = 17.0
First-Come, First-Served (FCFS)
Scheduling
PI PII PIII
24 27 30
0

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Suppose that processes appear in the order PII , PIII , P1
The Gantt chart for the schedule is:
Waiting time for PI = 6.0; PII = 0.0; PIII = 3.0
Average waiting time: (6.0 + 0.0 + 3.0)/3 = 3.0
Much better than previous case
FCFS Scheduling (Contd.).
PI
PIII
PII
6
3 30
0

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 Associate with each process the length of its next CPU burst. Use
these lengths to schedule the process with the shortest time
 Two schemes:
– Non-preemptive
– preemptive
 SJF is optimal
Shortest-Job-First (SJF) Scheduling

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 Each process associates with priority.
 The maximum priority process will be allocated by the CPU.
– Pre-emptive
– Non-pre-emptive
 SJF is a unique of priority scheduling with priorities associated
according to the no. of CPU burst necessary by the processes.
 Problem =Starvation – low priority processes will not get a chance to
execute
 Solution =Aging – as time progresses increase the priority of the
process
Priority Scheduling

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 On the principle that CPU must be shared between each ready
process.
 CPU time is divided into time-quantum or time-slice.
 The ready processes are queued up in a circular queue.
 Context switching takes place whenever either the running process
exhausts its allocated time-quantum or completes its execution.
Round Robin (RR) Scheduling

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In this module, we discussed:
 CPU Scheduling
 Scheduling Criteria
 Range of Scheduling Algorithms
Summary

21. Thank You