Os notes

GANDHI INSTITUTE FOR EDUCATION & TECHNOLOGY Baniatangi, Bhubaneswar

FOR 6TH SEM (CSE, ECE, EEE)

Prepared By:

Santosh Kumar Rath

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Introduction to Operating System: An operating system is a program that acts as an intermediary between a user of a computer

and the computer hardware. The purpose of an operating system is to provide an environment

in which a user can execute program.

(i) Hardware

System Components

An operating system is an important part of almost every computer system. A computer system

can be divided roughly into four components:

(ii) Operating system

(iii) Applications programs

(iv) The users

1. Hardware

The hardware- the central processing unit (CPU), the memory, and the input /output

(I/O) devices - provides the basic computing resources for the system.

2. Operating System

An operating system is a control program.

A control program controls the execution of user programs to prevent errors and

improper use of the computer.

It provides a basis for the application program and acts as an intermediary between

the user and computer.

3. Application Program

It help the user to perform the singular or multiple related tasks.

It help the program to solve in real work.

The applications programs- such as compilers, database systems, games, and business

programs- define the ways in which these resources are used to solve the computing

problems of the users.

Simple Batch Systems

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To speed up processing, jobs with similar needs were batched together and were run through

the computer as a group. Thus, the programmers would leave their programs with the

operator. The operator would sort programs into batches with similar requirements and, as the

computer became available, would run each batch. The output from each job would be sent

back to the appropriate programmer.

Demerits

The lack of interaction between the user and the job while that job is executing.

In this execution environment, the CPU is often idle.

SPOOLING

The expansion of spooling is the simultaneous peripheral operation on-line .For

example if two or more users issue the print command, and the printer can accept the requests

.Even the printer printing some other jobs. The printer printing one job at the same time the

“spool disk” can load some other jobs.

‘Spool disk’ is a temporary buffer, it can read data from the secondary storage

devices directly. Simultaneously the CPU executing some other job in the spool disk , at the

same time the printer printing the third job. So three jobs are running simultaneously.

Multi-programming Systems: It is a technique to execute the number of the program simultaneously by a single

processor. In this case the number of the processes are reside in main memory at a time. The

operating system picks and begins to execute one of the job in the main memory.

In non-multiprogramming system , the CPU can execute only one program at a time , if

the running program waiting for any I/O device , the CPU become idle, so it will effect on the

performance of the CPU. But in the multiprogramming the CPU switches from that job to

another job in the job pool. So the CPU is not idle at any time.

This idea is common in other life situations. A lawyer does not have only one client at a time.

Rather, several clients may be in the process of being served at the same time..)

Multiprogramming is the first instance where the operating system must make decisions for the

users.

Multiprogramming increases CPU utilization .

Advantages:-

CPU is never idle , so the performance of CPU will increase.

Throughput of the CPU may also increase.

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Time-Sharing Systems

Time sharing, or multitasking, is a logical extension of multiprogramming. Multiple jobs are

executed by the CPU switching between them, but the switches occur so frequently that the

users may interact with each program while it is running.

Time-sharing systems were developed to provide interactive use of a computer system at a

reasonable cost. A time-shared operating system uses CPU scheduling and multiprogramming

to provide each user with a small portion of a time-shared computer.

Time-sharing operating systems are CTTS , Multics , Cal ,Unix .

Distributed Systems

Increases computation speed

A distributed system is a collection of physically separate, possibly heterogeneous computer

systems that are networked to provide the users with access to the various resources that the

system maintains. The processor can’t share the memory or clock , each processor has its.

Advantages:

Increases functionality

Increases the data availability, and reliability.

Network Operating System :

A network operating system is an operating system that provides features such as file sharing

across the network and that includes a communication scheme that allows different processes

on different computers to exchange messages. A computer running a network operating

system acts autonomously from all other computers on the network, although it is aware of the

network and is able to communicate with other networked computers.

Parallel System:

A system consisting of more than one processor and it is tightly coupled then it is known as

parallel system. In parallel systems number of the processor executing their job parallel.

Tightly Coupled:-The system having more than one processor in close communication ,sharing

the computer bus , the clock , and some times memory and peripheral devices. These are

referred to as the “tightly coupled” system.

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Advantages:-

• Increases throughput.

• Increases reliability.

A real-time operating system (RTOS) is an

Special-Purpose Systems:

The discussion thus far has focused on general-purpose computer systems that we are all

familiar with. There are, however, different classes of computer systems whose functions are

more limited and whose objective is to deal with limited computation domains.

Real-Time Embedded Systems

operating system (OS) intended to serve real-

time application requests. It must be able to process data as it comes in, typically without buffering

delays. A real-time system is used when rigid time requirements have been placed on the

operation of a processor or the flow of data; thus, it is often used as a control device in a

dedicated application. Sensors bring data to the computer. The computer must analyze the

data and possibly adjust controls to modify the sensor inputs. Systems that control scientific

experiments, medical imaging systems, industrial control systems, and certain display systems

are realtime systems. Some automobile-engine fuel-injection systems, home-appliance

controllers, and weapon systems are also real-time systems.

A real-time system has well-defined, fixed time constraints. Processing must be done within the

defined constraints, or the system will fail. For instance, it would not do for a robot arm to be

instructed to halt after it had smashed into the car it was building. A real-time system functions

correctly only if it returns the correct result within its time constraints. Contrast this system with

a time-sharing system, where it is desirable (but not mandatory) to respond

quickly, or a batch system, which may have no time constraints at all.

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SYSTEM STRUCTURES

User interface: All OS have a user interface(UI).Interfaces are of three types-

Command Line Interface: uses text commands and a method for entering them Batch

interface: commands and directives to control those commands are entered into

files and those files are executed. Graphical user interface: This is a window

system with a pointing device to direct I/O, choose from menus and make

selections and a keyboard to enter text.

Operating-System Services

OS provides an environment for execution of programs. It provides certain services to

programs and to the users of those programs. OS services are provided for the

convenience of the programmer, to make the programming task easier.

One set of SOS services provides functions that are helpful to the user –

Program execution: System must be able to load a program into memory

and run that program. The program must be able to end its execution either

normally or abnormally.

I/O operations: A running program may require I/O which may involve a file

or an I/O device. For efficiency and protection, users cannot control I/O devices

directly.

File system manipulation: Programs need to read and write files and

directories. They also need to create and delete them by name, search for a

given file, and list file information.

Communications: One process might need to exchange information with another

process.Such communication may occur between processes that are executing on

the same computer or between processes that are executing on different

computer systems tied together by a computer network. Communications may be

implemented via shared memory or through message passing.

Error detection: OS needs to be constantly aware of possible errors. Errors may

occur in the CPU and memory hardware, in I/O devices and in the user program. For

each type of error, OS takes appropriate action to ensure correct and consistent

computing.

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Another set of OS functions exist for ensuring efficient operation of the system. They are-

a. Resource allocation: When there are multiple users or multiple jobs running at the same

time, resources must be allocated to each of them. Different types of resources such

as CPU cycles, main memory and file storage are managed by the operating system.

b. Accounting: Keeping track of which users use how much and what kinds of

computer resources.

c. Protection and security: Controlling the use of information stored in a multiuser or

networked computer system. Protection involves ensuring that all access to system

resources is controlled. Security starts with requiring each user to authenticate himself or

herself to the system by means of password and to gain access to system resources.

An example to illustrate how system calls are used:

System Calls

System calls provide an interface to the services made available by an operating system.

Writing a simple program to read data from one file and copy them to another file-

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a) First input required is names of two files – input file and output file. Names can

be specified in many ways- One approach is for the program to ask the user for the names of

two files. In an interactive system, this approach will require a sequence of system

calls, to write a prompting message on screen and then read from the keyboard the

characters that define the two files. On mouse based and icon based systems, a menu of

file names is displayed in a window where the user can use the mouse to select the

source names and a window can be opened for the destination name to be specified.

b) Once the two file names are obtained, program must open the input file and

create the output file. Each of these operations requires another system call. Possible error

conditions –When the program tries to open input file, no file of that name may exist

or file is protected against access. Program prints a message on console and terminates

abnormally. If input file exists, we must create a new output file. If the output file with the

same name exists, the situation caused the program to abort or delete the existing file and

create a new one. Another option is to ask the user(via a sequence of system calls) whether to

replace the existing file or to abort the program.

When both files are set up, a loop reads from the input file and writes to the output file

(system calls respectively). Each read and write must return status information regarding

various possible error conditions. After entire file is copied, program closes both files,

write a message to the console or window and finally terminate normally.

Application developers design programs according to application programming interface

(API). API specifies set of functions that are available to an application programmer.

The functions that make up the API typically invoke the actual system calls on behalf

of the application programmer. Benefits of programming rather than invoking actual system

calls:

Program portability – An application programmer designing a program using an API

can expect program to compile and run on any system that supports the same API.

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Actual system calls can be more detailed and difficult to work with than the

API available to an application programmer.

The run time support system ( a set of functions built into libraries included with a compiler) for

most programming languages provides a system call interface that serves as a link to

system calls made available by OS. The system call interface intercepts function calls

in the API and invokes the necessary system call within the operating system. A number is

associated with each system call and the system call interface maintains a table indexed

according to these numbers. System call interface then invokes the intended system call

in the OS kernel and returns the status of the system call and return any values.

System calls occur in different ways, depending on the computer in use – more

information is required than simply the identity of the desired system call. The exact

type and amount of information vary according to the particular OS and call.

Three general methods are used to pass parameters to OS-

I. Pass the parameters in registers

II. Storing parameters in blocks or tables in memory and the address of the block id passed as

a parameter in a register

III. Placing or pushing parameters onto the stack by the program and popping off the stack by

the OS.

• end, abort

Types of system calls

Five major categories:

1) Process control

• load, execute

• create process, terminate process

• get process attributes, set process attributes

• wait for time

• wait event, signal event

• allocate and free memory

2) File Management

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• create file, delete file

• open, close

• read, write, reposition

• get file attributes, set file attributes

3) Device management

• request device, release device

• read, write, reposition

• get device attributes, set device attributes

• logically attach or detach devices

4) Information maintenance

• get time or date, set time or date

• get system data, set system data

• get process, file or device attributes

• set process, file or device attributes

5) Communications

• create, delete communication connection

• send, receive messages

• transfer status information

• attach or detach remote devices

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System programs provide a convenient environment for program development and

execution. They can be divided into these categories-

File management: These programs create, delete, copy, rename, print, dump, list and

manipulate files and directories.

Status information: Some programs ask the system for the date, time, and amount

of available memory or disk space, number of users.

File modification: Text editors may be available to create and modify the content of files

stored on disk or other storage devices.

Programming language support: Compilers, assemblers, debuggers and interpreters for

common programming languages are often provided to the user with the OS.

Program loading and execution: Once a program is assembled or compiled, it must

be loaded into memory to be executed. System provides absolute loaders, relocatable

loaders, linkage editors and overlay loaders.

Communications: These programs provide the mechanism for creating virtual connections

among processes, users and computer systems.

System Programs

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In addition to systems programs, OS are supplied with programs that are useful in

solving common problems or performing operations. Such programs include web

browsers, word processors and text formatters, spread sheets, database systems etc.

These programs are known as system utilities or application programs.

PROCESS MANAGEMENT

Current day computer systems allow multiple programs to be loaded into memory and

executed concurrently. Process is nothing but a program in execution. A process is the unit

of work in a modern time sharing system. A system is a collection of processes:

operating system processes executing operating system code and user processes

executing user code. By switching CPU between processes, the operating system can make

the computer more productive.

Overview

A batch system executes jobs whereas a time shared system has user programs or

tasks. On a single user system, user may be able to run several programs at one

time: a word processor, a web browser and an e-mail package. All of these are called

processes.

The Process

A process is a program in execution. A process is more than a program code sometimes known

as text section. It also includes the current activity as represented by the value of the program

counter and the contents of the processor’s registers. A process generally also includes the

process stack which contains temporary data and a data section which contains global

variables.

A process may also include a heap which is memory that is dynamically allocated

during process run time. Program itself is not a process, a program is a passive entity

such as a file containing a list of instructions stored on disk (called an executable file)

whereas process is an active entity with a program counter specifying the next

instruction to execute and a set of associated resources. A program becomes a

process when an executable file is loaded into memory. Although two processes may be

associated with the same program, they are considered two separate execution sequences.

Process State

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As a process executes, it changes state. The state of a process is defined in part by the

current activity of that process. Each process may be in one of the following states-

New: The process is being created.

Running: Instructions are being executed.

Waiting: The process is waiting for some event to occur.

Ready: The process is waiting to be assigned to the processor.

Terminated: The process has finished execution.

Process Control Block

Each process is represented in the operating system by a process control block (PCB)

also called task control block. It contains many pieces of information associated with a

specific process including:

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Process State: The state may be new, ready, running, waiting, halted etc.

Process Number: Unique number which is assigned to a program at time of its execution.

Program Counter: The counter indicates the address of the next instruction to be executed for

this process.

CPU registers: The registers vary in number and type depending on the computer architecture.

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CPU scheduling information: This information includes a process priority, pointers to scheduling

queues, and other scheduling parameters.

Memory management information: This information may include such information as the value

of base and limit registers etc.

Accounting information: This information includes the amount of CPU and real time used, time

limits etc.

I/O status information: This information includes the list of I/O devices allocated to the process,

etc.

Threads

Process is a program that performs a single thread of execution. A single thread of

control allows the process to perform only one task at one time.

Process Scheduling

The objective of multi programming is to have some process running at all times to

maximize CPU utilization. The objective of time sharing is to switch the CPU among

processes so frequently that users can interact with each program while it is running.

To meet these objectives, the process scheduler selects an available process for

program execution on the CPU. For a single processer system, there will never be more than

one running process.

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Scheduling Queues

Job queue: As processes enter the system, they are put inside the job queue, which consists of

all processes in the system.

Ready queue: The processes that are residing in main memory and are ready and

waiting to execute are kept on a list called the ready queue. This queue is stored as a

linked list. A ready queue header contains pointers to the first and final PCB’s in the list. Each

PCB includes a pointer field that points to the next PCB in the ready queue.

Device queue: When a process is allocated the CPU, it executes for a while and eventually

quits, is interrupted or waits for the occurrence of a particular event such as the completion of

an I/O request. The list of processes waiting for a particular I/O device is called a device

queue. Each device has its own device queue.

Queuing diagram

A common representation for process scheduling is queuing diagram. Each rectangular box represents a queue. Two types of queues are present: ready queue and a set of device queues. Circles represent the resources that serve the queues and the arrows indicate the flow of processes in the system.

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A new process is initially put in the ready queue. It waits there until it is selected for execution

or is dispatched. Once the process is allocated the CPU and is executing, one of the

following events might occur –

a) The process could issue and I/O request and then be placed in the I/O queue.

b) The process could create a new sub process and wait for the sub process’s termination.

c) The process could be removed forcibly from the CPU as a result of an interrupt

and be put back in the ready queue.

In a batch system, more processes are submitted than can be executed immediately.

These processes are spooled to a mass storage device (disk) where they are kept for later

execution. The long term scheduler or job scheduler selects processes from this pool

and loads them into memory for execution. The short term scheduler or CPU scheduler

selects from among the processes that are ready to execute and allocates the CPU to one of

them. The distinction between these two lies in the frequency of execution. The long

term scheduler controls the degree of multi programming (the number of processes in memory).

Most processes can be described as either I/O bound or CPU bound. An I/O bound process is

one that spends more of its time doing I/O than it spends doing computations. A CPU bound

process generates I/O requests infrequently using more of its time doing computations.

Schedulers

A process migrated among the various scheduling queues throughout its life time. The OS must

select for scheduling purposes, processes from these queues and this selection is

carried out by scheduler.

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Long term scheduler must select a good mix of I/O bound and CPU bound processes.

A system with best performance will have a combination of CPU bound and I/O bound

processes.

Some operating systems such as time sharing systems may introduce

additional/intermediate level of scheduling. Idea behind medium term scheduler is that

sometimes it can be advantageous to remove processes from memory and thus reduce

the degree of multiprogramming. Later the process can be reintroduced into memory

and its execution can be continued where it left off. This scheme is called swapping. The

process is swapped out and is later swapped in by the medium term scheduler.

Context switch

Interrupts cause the OS to change a CPU from its current task and to run a kernel routine.

When an interrupt occurs, the system needs to save the current context of the process currently

running on the CPU so that it can restore that context when its processing is done, essentially

suspending the process and then resuming it. The context is represented in the PCB of

the process, it includes the value of CPU registers, process state and memory

management information. We perform a state save of the current state of the CPU and then

state restore to resume operations.

Switching the CPU to another process requires performing a state save of the current process

and a state restore of a different process. This task is known as context switch. Kernel saves

the context of the old process in its PCB and loads the saved context of the new process

scheduled to run. Context switch time is pure overhead. Its speed varies from machine to

machine depending on the memory speed, the number of registers that must be copied

and the existence of special instructions. Context switch times are highly dependent on

hardware support.

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Multithreading

Thread is a basic unit of CPU utilization. Support for threads may be provided either at the

user level for user threads or by the kernel for kernel threads. User threads are

supported above the kernel and are managed without kernel support whereas kernel

threads are supported and managed directly by the operating system.

In a single processor system, only one process can run at a time; others must wait until CPU is

free and can be rescheduled. The objective of multi programming is to have some

process running at all times to maximize CPU utilization. Under multi programming,

several processes are kept in memory at one time. When one process has to wait, the OS

Benefits of multi threaded programming are:

Responsiveness: Multi threading an interactive application may allow a program to

continue running even if part of it is blocked or is performing a lengthy operation,

thereby increasing responsiveness to the user.

Resource interaction: Threads share the memory and the resources of the process to which they

belong. The benefit of sharing code and data is that it allows an application to

have several different threads of activity within the same address space.

Economy: Allocating memory and resources for process creation is costly. Because threads

share resources of the process to which they belong, it is more economical to create and

context switch threads.

Utilization of multi processor architecture: The benefits of multi threading can be

greatly increased in a multi processor architecture where threads may be running in parallel

on different processors. Multi threading on a multi CPU machine increases concurrency.

User threads are supported above the kernel and are managed without kernel

support whereas kernel threads are supported and managed directly by the operating

system.

CPU scheduling is the basis of multi programmed operating systems. By switching CPU among

processes, the OS can make the computer more productive. On operating systems that

support threads, it is kernel level threads and not processes that are scheduled by operating

system. But process scheduling and thread scheduling are often used interchangeably.

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takes the CPU away from that process and gives the CPU to another process. As CPU is one

of the primary computer resources, its scheduling is central to operating system design.

Three common ways of establishing a relationship between user level and kernel level

threads are:

Many to one model - Maps many user level threads to one kernel thread. Thread

management is done by the thread library in user space hence it is efficient but

entire process will block if a thread makes a blocking system call. As only one thread can

access the kernel at a time, multiple threads are unable to run in parallel on multi processors.

One to one model - Maps each user thread to kernel thread. It allows more

concurrency by allowing another thread to run when a thread makes a blocking

system call; it also allows multiple threads to run in parallel on multi processors.

Disadvantage is that creating a user thread requires creating the corresponding kernel thread.

Many to many model - Multiplexes many user level threads to a smaller or equal

number of kernel level threads. The number of kernel threads may be specific to

either a particular application or a particular machine. Developers can create as many user

threads as necessary and the corresponding kernel threads can run in parallel on a

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multi processor. Also, when a thread performs a blocking system call, the kernel can

schedule another thread for execution.

On operating systems that support threads, it is kernel level threads that are being scheduled

by operating system. User level threads are managed by thread library. To run on a CPU,

user level threads must be mapped to an associated kernel level thread although this

mapping may be indirect and may use a light weight process.

PROCESS SCHEDULING:

Process is an executing program with a single thread of control. Most operating systems

provide features enabling a process to contain multiple threads of control. A thread is a basic

unit of CPU utilization; it comprises of a thread ID, a program counter, a register set

and a stack. It shares with other threads belonging to the same process its code

section, data section and other operating system resources such as open files and signals. A

traditional or heavy weight process has a single thread of control.

CPU – I/O burst cycle:

Process execution consists of a cycle of CPU execution and I/O wait. Processes

alternate between these two states. Process execution begins with CPU burst followed by an

I/O burst and so on. The final CPU burst ends with a system request to terminate execution.

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An I/O bound program has many short CPU bursts. A CPU bound program might

have a few long CPU bursts.

CPU scheduler:

Whenever the CPU becomes idle, the operating system must select one of the

processes in the ready queue to be executed. The selection process is carried out by

the short term scheduler or CPU scheduler. The scheduler selects a process from the

processes in memory that are ready to execute and allocates the CPU to that process.

The ready queue is not necessarily a first in first out queue. A ready queue can be

implemented as a FIFO queue, a priority queue, a tree or an unordered linked list.

All the processes in the ready queue are lined up waiting for a chance to run on the

CPU. The records in the queue are process control blocks of the processes.

Pre-emptive scheduling:

CPU scheduling decisions may take place under the following four conditions-

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a) When a process switches from the running state to the waiting state

b) When a process switches from the running state to the ready state

c) When a process switches from the waiting state to the ready state

d) When a process terminates

When scheduling takes place under conditions 1 and 4, scheduling scheme is non - pre emptive

or co-operative. Else it is called pre emptive.

Under non pre emptive scheduling, once the CPU has been allocated to a process,

the process keeps the CPU until it releases the CPU either by terminating or by

switching to the waiting state. Pre emptive scheduling incurs cost associated with access to

shared data. Pre emption also affects the design of the operating system kernel.

Dispatcher:

The dispatcher is the module that gives control of the CPU to the process selected by

the short term scheduler. This function involves-

a) Switching context

b) Switching to user mode

c) Jumping to the proper location in the user program to restart that program

Dispatcher should be as fast as possible since it is invoked during every process switch. The

time it takes for the dispatcher to stop one process and start another running is

called dispatch latency.

Scheduling criteria:

Different CPU scheduling algorithms have different properties. Criteria for comparing CPU

scheduling algorithms-

a) CPU utilization: Keep the CPU as busy as possible

b) Through put: One measure of work of CPU is the number of processes that are completed

per time unit called through put.

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c) Turnaround time: The interval from the time of submission of a process to the time

of completion is the turnaround time. Turnaround time is the sum of the periods spent

waiting to get into memory, waiting in the ready queue, executing on the CPU and doing I/O.

d) Waiting time: It is the sum of the periods spent waiting in the ready queue.

e) Response time: The time it takes for the process to start responding but is not the time it

takes to output the response.

It is desirable to maximize CPU utilization and through put and to minimize

turnaround time, waiting time and response time.

Scheduling algorithms:

First Come First Serve:

This is the simplest CPU scheduling algorithm. The process that requests the CPU first

is allocated the CPU first. The implementation of FCFS is managed with a FIFO queue.

When a process enters the ready queue, its PCB is linked onto the tail of the queue.

When the CPU is free, it is allocated to the process at the head of the queue. The running

process is then removed from the queue.

The average waiting time under FCFS is often quite long. The FCFS scheduling algorithm is non

pre-emptive.

Once the CPU has been allocated to a process, that process keeps the CPU until it

releases the CPU either by terminating or by requesting I/O. It is simple, fair, but

poor performance. Average queuing time may be long.

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Shortest Job First:

This algorithm associates with each process the length of the process’s next CPU burst. When

the CPU is available, it is assigned to the process that has the smallest next CPU burst. If the

next CPU bursts of two processes are the same, FCFS scheduling is used to break the tie.

Priority

The SJF is a special case of the general priority scheduling algorithm. A priority is

associated with each process and CPU is allocated to the process with highest priority.

Equal priority processes are scheduled in FCFS order. An SJF algorithm is simply a

priority algorithm where the priority is the inverse of the predicted next CPU burst.

:

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The larger the CPU burst, the lower the priority. Priorities are generally indicated by some

fixed range of numbers usually 0 to 7.

Priorities can be defined either internally or externally. Internally defined priorities use

some measurable quantity to computer the priority of a process. External priorities are

set by criteria outside the operating system.

Priority scheduling can be either pre emptive or non pre emptive. When a process arrives at

the ready queue, its priority is compared with the priority of the currently running

process. A preemptive priority scheduling algorithm will preempt the CPU is the

priority of the newly arrived process is higher than the priority of the currently

running process. A non preemptive priority scheduling algorithm will put the new process at

the head of the ready queue.

The major problem with priority scheduling algorithm is indefinite blocking or

starvation. A process that is ready to run but waiting for the CPU can be considered

blocked. A priority scheduling algorithm can leave some low priority processes waiting

indefinitely. 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.

Round Robin

The process may either have a CPU burst of less than 1 time quantum. Then, the

process itself will release the CPU voluntarily. The scheduler will then proceed to the next

process in the ready queue. Else if the CPU burst of the currently running process is longer than

1 time quantum, the timer will go off and will cause an interrupt to the operating

:

The round robin scheduling algorithm is designed for time sharing systems. It is similar to FCFS

scheduling but preemption is added to switch between processes. A small unit of time

called a time quantum or time slice is defined. A time quantum is generally from 10 to 100

milli seconds. The ready queue is treated as a circular queue. The CPU scheduler goes around

the ready queue allocating the CPU to each process for a time interval of up to 1

time quantum. To implement RR scheduling, ready queue is implemented as a FIFO

queue. New processes are added to the tail of ready queue. The CPU scheduler

picks the first process from the ready queue, sets the timer to interrupt after 1 time

quantum and dispatches the process.

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system. A context switch will be executed and the process will be put at the tail of the

ready queue. The CPU scheduler will then select the next process in the ready queue.

In the RR scheduling algorithm, no process is allocated the CPU for more than 1 time quantum

in a row. If a process’s CPU burst exceeds 1 time quantum, that process is preempted and is

put back in the ready queue. Thus this algorithm is preemptive. The performance of the RR

algorithm depends heavily on the size of time quantum. If the time quantum is extremely

large, the RR policy is same as the FCFS policy. If the time quantum is extremely

small, the RR approach is called processor sharing.

Algorithm evaluation

Selection of an algorithm is difficult. The first problem is defining the criteria to be

used in selecting an algorithm. Criteria are often defined in terms of CPU utilization,

response time or through put. Criteria includes several measures such as-

a) Maximizing CPU utilization under the constraint that the maximum response time is

1 second.

b) Maximizing through put such that turnaround time is linearly proportional to total

execution time.

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