Prepared By:

Santosh Kumar Rath

GANDHI INSTITUTE FOR EDUCATION & TECHNOLOGY Baniatangi, Bhubaneswar

FOR 6TH SEM (CSE, ECE, EEE)

Operating System

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.

System Components

An operating system is an important part of almost every computer

system. A computer system can be divided roughly into four components:

(i) Hardware

(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.

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Operating System 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

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.

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Operating System 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.

Advantages:-

Multiprogramming increases CPU utilization .

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

Throughput of the CPU may also increase.

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

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:

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Operating System Increases computation speed

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.

Advantages:-

Increases throughput.

Increases reliability.

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.

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Operating System Real-Time Embedded Systems

A real-time operating system (RTOS) is an 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.

SYSTEM STRUCTURES

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Operating SystemOperating-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 –

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.

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.

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Operating System 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.

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.

System Calls

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

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Operating System

An example to illustrate how system calls are used:

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

another file-

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

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Operating Systemprotected 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.

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

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Operating Systemsystem 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.

Types of system calls

Five major categories:

1) Process control

end, abort

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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Operating System 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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Operating System

System Programs

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.

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Operating System Communications: These programs provide the mechanism for creating

virtual connections among processes, users and computer systems.

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.

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Operating SystemA 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

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.

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Operating SystemProcess 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:

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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Operating System

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

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Operating Systemprocess for program execution on the CPU. For a single processer

system, there will never be more than one running process.

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.

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Operating SystemQueuing 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.

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.

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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Operating SystemIn 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.

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.

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Operating SystemContext 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.

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.

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.

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Operating SystemResource 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.

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

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:

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Operating SystemMany 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 multi processor. Also, when a thread performs a blocking system

call, the kernel can schedule another thread for execution.

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Operating System

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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Operating System

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:

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Operating SystemCPU scheduling decisions may take place under the following four

conditions-

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:

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Operating SystemDifferent 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.

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

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Operating Systemrequesting I/O. It is simple, fair, but poor performance. Average

queuing time may be long.

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.

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Operating SystemPriority:

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. 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 round robin scheduling algorithm is designed for time sharing

systems. It is similar to FCFS scheduling but preemption is added to

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Operating Systemswitch 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.

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 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.

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Operating System

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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