Operating System Concepts Lecture 1

7/31/2019 Operating System Concepts Lecture 1

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Operating System Concepts

By

Dr. Freddie Acosta9th May 2011


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Definition

An operating system is a suite of program

which takes control over the operation of

the computer. An OS can be viewed as a resource

allocator.


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The Computer System

Applications Programs

Hardware

Operating System Users


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The Hardware

Provides the basic computing resources.

The Central Processing Unit (CPU)

MemoryI/O devices


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Application Programs

Define the ways in which these resources

are used to solve the computing problems

of the users.Compilers

Database Systems

Games

Business programs


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Users

People

Machines

Other computers


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The Purpose of OS

To make the computer to operate in a

convenient and efficient manner.

To allow/process as many jobs as possiblein a given period of time.


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OS Services

Program Execution

The system must be able to load a program

into memory and to run it. The program must

be able to end its execution, either normally or

abnormally (indicating error).


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Services - Cont.

I/O Operations

A running program may require I/O. This I/O

may involve a file or an I/O device. For

specific devices, special functions may be

desired.


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Servicescont.

File System manipulations

The file system is of particular interest. It

should be obvious that programs need to read

and write files. They also need to create and

delete files by name.


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Servicescont.

Communications

between processes executing on the same

computer; between processes executing on

different computer systems that are tied

together by a computer network.


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Servicescont.

Resource Allocation

CPU times, memory and file storage, I/O

devices etc must be allocated when there are

multiple users or multiple jobs running at the

same time.


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Servicescont.

Accounting

To keep track of which users use how much

and what kinds of computer resources.


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Servicescont.

Protection

Involves ensuring that all access to system

resources is controlled.

Security of the system outsiders is also

important through authentication.


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Process Management

A process can be thought of as a program

in execution.

A process will need certain resourcessuch as CPU time, memory, files and I/O

devicesto accomplish its tasks.


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Process State

New : the process is being created

Running: Instructions are being executed

Waiting: the process is waiting for someevent to occur (I/O completion)

Ready: the process is waiting to beassigned to a processor

Terminated: the process has finishedexecution


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Process Scheduling

The purpose of multiprogramming 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.


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Context Switch

Switching the CPU to another process

requires saving the state of the old process

and loading the save state for the newprocess.

This task is known as context switch.


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CPU Scheduling

Scheduling is a fundamental OS function.

The idea of multiprogramming is that, a

process is executed until it must wait. With multiprogramming, we try to use this

idle time productively.


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eria

CPU Utilization

We want the CPU as busy as possible.

For lightly loaded system, it should be about40 percent.

For heavily loaded system, it should be about

90 percent.


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Criteriacont.

Throughput

If the CPU is busy, then work is being done.

One measure of work is the number of

processes that are completed per time unit.


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Criteriacont.

Turn Around Time

The interval from the time of submission to

the time of completion is the TAT.

TAT 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.


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Criteriacont.

Waiting Time

CPU scheduling algorithm does not affect the

amount of time during which a process

executes or does I/O;

It affects only the amount of time that a

process spends waiting in the ready queue

Waiting time is the sum of the periods spentwaiting in the ready queue.


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Criteriacont.

Response Time

It is the amount of time it takes to start

responding, but not the time that it takes to

output that response.


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Scheduling Algorithms

Non-Preemptive

Once CPU is allocated, it cant be taken back.

The process has to voluntarily release the

CPU.

Preemptive

The CPU can be taken away from a process if

a process with higher priority has arrived or

after a time quantum.


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FCFS

The process that requests the CPU first is

allocated the CPU first.

When the CPU is free, it is allocated to theprocess at the head of the queue.

The Convoy Effect


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Example

Job No CPU Burst

Time

Arrival Time

1 24 0

2 3 0

3 3 0


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SJF

When the CPU is available, it is assigned

to the process that has the smallest next

CPU burst.

If two processes have the same length next

CPU burst, FCFS scheduling is used to

break the tie.


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Example

Job No CPU Burst

Time

Arrival Time

1 6 0

2 8 0

3 7 0

4 3 0


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Priority

The SJF is a special case of the general

priority scheduling algorithm.

Where the priority (p) is the inverse of thenext CPU burst (t): p = 1/t.

The larger the CPU burst, the lower the

priority, and vice versa.


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Priorities

Internal Time limits

Memory requirements

The number of open files

Ratio of I/O Burst with CPU burst

External Importance of the process

Type and amount of funds being paid for computer

use, The department sponsoring the work

Political


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Example

Process Burst Time/

Arrival Time

Priority

1 10/0 3

2 1/1 1

3 2/2 3

4 1/3 4

5 5/4 2


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SRTF (Preemptive Algorithm)

Process AT BT

1 0 8

2 1 4

3 2 9

4 3 5


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Problem with SRTF

Indefinite blocking or starvation


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Round Robin

Was designed for time sharing.

It is similar to FCFS but preemption is

added to switch between processes. A small unit of time, called time quantum

(q) time slice, is defined. A q is generally

from 10 to 100 ms.


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Example (q=4)

Process CPU AT

1 24 0

2 3 0

3 3 0


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RR

If q positive infinity (very large), the

algorithm results to FCFS.

If q 0 (very small), RR approach is called

processor sharing and appears to the user as

though each n processes has its own processor

running at 1/n the speed of the real processor.

Rule of thumb, 80% of the CPU burst is lowerthan q.


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Multi Level Queue

Partitions the ready queue into several

separate queues.

The processes are assigned permanently toa queue based on some properties such as

memory size, priority, process type.

Each queue has its own schedulingalgorithm.


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Example 1

System Process

Interactive Processes

Interactive Editing processes

Batch processes

Student processes

Highest priority

Lowest priority


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Example 2

System Process

Student processes

High Priority

Low Priority

80% of CPU Time

20% of CPU TIme


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Multi Level Feedback Queue

Q = 8

Q = 16

FCFS


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Algorithm Evaluation

FCFS, SJF, RR (q=10)

Process CPU BT AT

1 10 02 29 0

3 3 0

4 7 05 12 0

Solve for Average TAT and Average WT.

Operating System Concepts Lecture 1

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