Understanding Operating Systems Fifth Edition Chapter 16 Linux Operating System.

Understanding Operating Systems Fifth Edition

Chapter 16Linux Operating System

Understanding Operating Systems, Fifth Edition 2

Learning Objectives

• The design goals for the Linux operating system

• The significance of using files to manipulate devices

• The differences between command-driven and menu-driven interfaces

• The roles of the Memory, Device, File, Processor, and Network Managers

• Some strengths and weaknesses of Linux


Overview

• POSIX-compliant• Portable

– Versions for cell phones, supercomputers, and computing systems in between

• Source code: freely available– Configurable: run any device, meet any specification

• User interface– Powerful desktop GUIs attracts users

• Highly modular– Multiple modules load and unload on demand

• Technically robust operating system


History

• Developed by Linus Torvalds (1991)

• Original purpose– Maximize Intel 80386 microprocessor’s limited

capabilities– Roots

• Minix: miniature UNIX with more functionality

• First version meant for small microcomputer– Expensive commercial computer features

• Flexibility and functionality

– Brought UNIX features to small computer


History (continued)

• Open-source program– Updates accepted from anyone

• User interface– Originally typed and cryptic commands– Today

• Command-driven interface (Terminal mode)

• Graphical user interface (GUI)

• Red Hat Linux provided initial primary support– World’s leading Linux distributor (until 2003)

• GNU General Public License


History (continued)


Design Goals

• Three goals– Modularity– Simplicity– Portability

• Numerous standard utilities– Eliminates need to write special code– Used in combination for specific tasks

• Numerous functions• Conforms to IEEE POSIX specifications

– Portable Operating System Interface for Computer Environments


Design Goals (continued)


Memory Management

• Space allocation– Kernel: 1 GB high order memory– Executing processes: 3 GB memory

• Process execution– Segment fixed size– System calls change segment size

• Memory protection– Based on information type stored in address space

region for process


Memory Management (continued)

• Page loading– Least recently used algorithm (LRU)– Maintains a dynamically managed memory area and

page cache (new and old pages inserted and deleted)• System page tables

– Tracks free and busy pages • Virtual memory

– Managed using multiple-level table hierarchy• 64- and 32-bit architectures

– Added flexibility with swap devices • May deactivate without rebooting



• Virtual memory managed using multiple-level table hierarchy– Four fields in virtual address



• Buddy algorithm– Grouping and splitting equal-sized page frames

• Give more contiguous space to job

• Page replacement algorithm– Clock page replacement policy expanded version– Uses eight-bit byte to track page’s activity

• Referred to as “age”


Processor Management

• Uses same parent-child process management design found in UNIX

• “Personality” concept– Allow processes from other operating systems to be

executed


Organization of Table of Processes

• Descriptor: references process• Contains approximately 70 fields

– Describes process attributes • Information needed to manage process

– Dynamically allocated by kernel• Process execution time

– Organized by doubly linked lists• “Next run” field• “Previously run” field

– Scheduler manages and updates descriptors• Macros


Process Synchronization

• Wait queues and semaphores– Synchronize two processes with each other

• Wait queue– Linked circular list of process descriptors– Problems solved

• Mutual exclusion and producers and consumers

• Semaphore structure– Three fields (semaphore counter, number of waiting

processes, list of processes waiting for semaphore)• Counter contains binary values• Except if several units of one resource available


Process Management

• Linux scheduler– Scans processes list in READY state – Chooses process to execute

• Using predefined criteria

• Three scheduling types– Real-time processes (two)– Normal processes (one)

• Process scheduling policy determination– Combination of type and priority


Process Management (continued)



• First type– Highest priority (SCHED_FIFO)

• First in, first out algorithm

– Not preemptible – Runs to completion unless:

• Process goes into WAIT state

• Process relinquishes processor voluntarily

– All FIFO processes complete • Scheduler processes lower priority types



• Second type– Medium priority (SCHED_RR)

• Round robin algorithm with small time quantum– Time quantum expires

• Other higher priority processes (FIFO, RR ) selected and executed

• Before first process allowed to complete

• Third type– Low priority (SCHED_OTHER)– Executed if no higher priority processes in READY

queue


Device Management

• Device independent – Improves portability

• Device drivers– Supervise data transmission

• Between main memory and peripheral unit

• Devices assigned– Name – Descriptors

• Further identify each device

• Stored in device directory


Device Management (continued)


Device Management (continued)

• Device drivers– Comprehensive collection in Linux

• Required driver not available– Obtain from another source

• Install separately

– Manually write the driver• Requires skilled programmer


Device Classifications

• Device identification– Minor device number

• Passed to device driver as an argument• Accesses one of several identical physical devices

– Major device number• Index to array to access appropriate code

• Configuration table for each class– Entry point into driver– Only connection between system code and driver– Importance

• Allows quick creation of device drivers


Device Drivers

• Support for standard classes introduced by UNIX• Allow new device classes supporting new

technology• Device classes not rigid

– Create large, complex, multiple function drivers– Discouraged because:

• Users share code, demand simple drivers • Modular code supports system scalability and

extendibility goal– Encouraged: drivers maximizing system’s effective

device usage


Device Drivers (continued)

• Notable feature– Accept new device drivers on the fly

• System up and running

• No reboot necessary


Device Classes

• Three standard classes


Device Classes (continued)

• Character (char) devices– Accessed as a stream of bytes

• Communications port, monitor, other byte-stream-fed device

– Implement open, close, read, write system calls– Accessed by file system nodes

• Look like ordinary data area

– Drivers treated as ordinary files • Exception: drivers are data channels accessed

sequentially



• Block devices– Host a file system (hard disk)– Accessed by file system nodes in /dev directory

• Transfer in blocks of data

– Similarity to char driver• Appear as ordinary files

– Dissimilarity to char driver• Access file system in connection with device (not

possible with char device)



• Network interfaces – Function

• Send and receive information packets

• Directed by network subsystem

– Network device functions• Relate to packet transmission

• Not read and write calls

• Dissimilar from block and char

• System device handled by device driver– Under direction of Linux subsystem



• Open and release– Allocate and deallocate the appropriate device– Open operation example

• Verify device available and working• Increase device usage counter by one (subsystem

knows module cannot be unloaded until file appropriately closed)

• Initialize device so old data is removed and device ready to accept new data

• Identify minor number and update appropriate pointer (if necessary)

• Allocate appropriate data structure



• Open and release (continued)– Release operation example

• Deallocate resources allocated with open function

• Shut down device

• Reduce usage counter by one (device released to another module)


File Management

• Data structures

• Filename conventions

• Directory listings


Data Structures

• Files organized in directories– Connected in treelike structure

• Five file types


Filename Conventions

• Case sensitive– Recognizes uppercase and lowercase letters

• Up to 255 characters long• Contain alphabetic characters, underscores,

numbers• File suffixes: optional• Can include space

– Complications if running command-line programs• Uses file hierarchy

– First slash indicates an absolute path name


Filename Conventions (continued)



• Path name rules– Path name starting with slash (at root directory)– Path name

• One name or list of names separated by slashes– Last name on list

• Name of file requested• Preceding names must be directory names

– Two periods (..) in path name• Move upward in hierarchy (closer to root)• Only way to go up hierarchy• Other path names go down tree



• Data structures: Virtual File System (VFS)– Kernel

• Allows processes to access files in a consistent manner

• Maintains interface between file related system calls and file management code

– Virtual file system layer• Receives process-initiated system call to files

• Performs file operations

• Independent of file system format

• Redirects request to module managing file


Directory Listings

• Creation– ls or ls -l command– GUI interface

• Displays:– File or directory name– Size– Modification date and time

• Permissions column– Code: file’s type and access privileges– Order of letters indicates the specific access granted


Directory Listings (continued)



• First column character: nature of folder entry– Dash (-) indicates a file– d indicates a directory file– l indicates a link– b indicates a block special file– c indicates a character special file

• Next three characters (rwx): file owner privileges – r indicates read access– w indicates write access– x indicates execute access



• Next three characters– Group access privileges

• Group: set of users, excluding owner, having something in common (project, class, department)

• System-wide group of users: “world”

• Last three characters– Access privileges granted to “others”

• Others: users at large (excluding owner and group member)



• Change file security– Owner (and only the owner) opens file properties to

be protected • File-Properties from the File menu

– Click on Permissions tab– Choose the appropriate access

• For owner, group, others


User Interface

• Early Linux versions – Required typed commands

• Thorough knowledge of valid commands required

• Current versions– Include powerful and intuitive GUI desktops

• Novice user can use successfully

– Navigate operating system– Can still use Terminal mode

• Type commands similar to those used for UNIX


Command-Driven Interfaces

• Typed command general syntax– command arguments filename

• Command: legal operating system command• Arguments: required or optional • Filename: filename

– Relative or absolute path name• Shell (bash shell)

– Command interpreter– Interprets and executes command– Key to system program coordination and combination


Command-Driven Interfaces (continued)


Graphical User Interfaces

• Multiple graphical user interfaces (often free)– Allowing choice for end users – Different GUIs used by different users on same

system (certain environments)– Flexibility – Spurring Linux acceptance

• Sophisticated Windows-compatible word processors, spreadsheet, presentation applications (some at no cost)– Spurring Linux popularity


System Monitor

• System Monitor window – System well-being information– Immediate history

• CPU, memory, network usage

• Other information– Supported file systems – Currently running processes information


System Monitor (continued)


Service Settings

• Variety of services help manage system– Linux distribution dependent (see documentation)


System Logs

• System logs – Provide detailed description of activity on system– Invaluable to administrators

• Tracking system malfunction

• Firewall failure

• Disabled device

– Found in /var/log directory– System log viewer to see data


System Logs (continued)


Keyboard Shortcuts

• Users easily switch from one task to another

• Keyboard shortcuts– Many identical to commonly used Windows operating

systems’ shortcuts• Ease operating system transition

– Example: CTRL-V • Quick way to issue PASTE command

• Linux, UNIX, and Windows


Keyboard Shortcuts (continued)


Summary

• Originally designed to gain more microcomputer chip power– Evolved into powerful, flexible operating system

• Runs supercomputers, cell phones, many devices

• Unparalleled popularity among programmers– Contribute standard code set enhancements

• Supports broad range of applications– Available for minimal cost and easy to install– Growing acceptance among non-programmers

• Large organizations– Commercial Linux products available

Understanding Operating Systems Fifth Edition Chapter 16 Linux Operating System.

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