Understanding Operating Systems
Seventh Edition
Chapter 15
Linux Operating Systems
Learning Objectives
After completing this chapter, you should be able to
describe:
• The design goals for the Linux operating system
• The flexibility offered by using files to manipulate
devices
• The roles of the Memory, Device, File, Processor,
and Network Managers
• The impact of open source software
• Some strengths and weaknesses of Linux
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Introduction
• Linux
– Originally based on a version of UNIX
– Powerful
– Inexpensive or free to use
• Portable
– Versions for cell phones, supercomputers, and
computing systems in between
• Source code: freely available
– Configurable: runs any device; meets any
specification
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Introduction (cont'd.)
• Open source
– Source code freely available to anyone for
improvement
– Under constant development
– Contributors around the world: not paid for their work
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Brief 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
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Brief History (cont'd.)
• User interface
– First Linux operating systems: command-driven
interface
• Sometimes cryptic commands
– Recent years
• Graphical user interfaces (GUI)
• Written and distributed under the GNU General
Public License
• Fedora Project responsible for the open-source
development of the Linux kernel
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(table 15.1)
Selected releases of Ubuntu Linux. New releases are scheduled every 6
months. Normal Ubuntu releases are supported for 18 months; LTS releases
are supported longer. (adapted from https://wiki.ubuntu.com/Releases)
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Design Goals
• Three goals
– Modularity
– Simplicity
– Portability
• Numerous standard utilities
– Eliminate need to write special code
– Used in combination for specific tasks
• Numerous functions
• IEEE POSIX (Portable Operating System Interface)
specifications conformity
– Programs’ portability
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(table 15.2)
Select system functions supported by Linux.
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(table 15.2) (cont’d.)
Select system functions supported by Linux.
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Memory Management
• Space allocation
– Kernel: 1 GB high order memory
– Executing processes: 3 GB memory
• Process execution
– Fixed-size segment
– System calls change segment size
• Memory protection
– Based on information type stored in address space
region for process
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Memory Management (cont'd.)
• 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
– Track free and busy pages
– Added flexibility with swap devices
• Virtual memory
– Managed using multiple-level table hierarchy
• 64- and 32-bit architectures
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Memory Management (cont'd.)
(table 15.3)
The four fields that make up the virtual address for Line 214 in Figure 15.2.
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(figure 15.2)
Virtual memory management uses three levels of tables (Main, Middle, and
Page Table Directories) to locate the page frame with the requested
instruction or data within a job.
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Memory Management (cont'd.)
• Buddy algorithm
– Groups and splits into equal-sized page frames
• Gives more contiguous space to job
• Page replacement algorithm
– Clock page replacement policy: expanded version
– Uses eight-bit byte to track page’s activity: “age”
– Page swap based on form of least frequently used
(LFU) replacement policy
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(figure 15.3)
Main memory is divided to accommodate jobs of different sizes. In (a), the
original group of 32 page frames is divided to satisfy the request of Job 1 for
16 page frames. In (b), another group of 16 page frames is divided to
accommodate Job 2, which needs eight page frames. In (c), after Job 2
finishes, the two groups of eight page frames each are recombined into a
group of 16, while Job 1 continues processing.
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Processor Management
• Uses same parent-child process management
design found in UNIX
• Supports “personality” concept
– Allows processes from other operating systems to be
executed
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Organization of Process Table
• Descriptor: referencing process
– Contains approximately 70 fields: describe process
attributes
– Includes 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 using
macros
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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 only binary values: except if several
units of one resource available
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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
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Process Management (cont'd.)
(table 15.4)
Linux has three process types, each signaling a different level of priority.
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Process Management (cont'd.)
• First type
– Highest priority (SCHED_FIFO)
• First in, first out algorithm
– Cannot be preempted
– Runs to completion unless:
• Process goes into WAIT state
• Process relinquishes processor voluntarily
– All FIFO processes complete
• Scheduler processes lower priority types
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Process Management (cont'd.)
• 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
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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
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Device Management (cont'd.)
(figure 15.4)
Details about each device can be accessed via the Device Manager.
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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 programmers quick creation of device drivers
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Device Drivers
• Support for standard classes introduced by UNIX
• Allow new device classes supporting new
technology
• Device classes not rigid
– Creating large, complex, and multiple function drivers
is discouraged because:
• Users share code: wider demand for simple drivers
• Modular code supports system scalability and
extendibility goals
– Encouraged: drivers maximizing system’s effective
device usage
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Device Drivers (cont'd.)
• Notable feature
– Accept new device drivers on the fly
• System up and running
• No reboot necessary
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Device Drivers (cont'd.)
• Open and release: functions performed when
opening a device
– Verify that the device is available and in working order
– Increase usage counter for the device by 1: alert the
subsystem that the module cannot be unloaded until
its file is appropriately closed
– Initialize the device: old data is removed and the
device is ready to accept new data
– Identify the minor number and update the appropriate
pointer if necessary
– Allocate any appropriate data structure
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Device Drivers (cont'd.)
• Open and release: functions (device_close or
device_release) performed when releasing a device
– Deallocate any resources allocated with the open
function
– Shut down the device
– Reduce usage counter by 1 so the device can be
released to another module
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Device Classes
• Three standard classes
– Character devices
– Block devices
– Network devices
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(figure 15.5)
This example of the three primary classes of device drivers shows how
device drivers receive direction from different subsystems of Linux.
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Device Classes (cont'd.)
• Character (char) devices
– Accessed as a stream of bytes
• Communications port, monitor, or other byte-stream-fed
device
– Implement open, release, read, and 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
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Device Classes (cont'd.)
• Block devices
– Host a file system (e.g., 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)
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Device Classes (cont'd.)
• Network interfaces
– Function
• Send and receive information packets
• Directed by network subsystem
– Network device functions
• Relate to packet transmission
• System device handled by device driver
– Under Linux subsystem’s direction
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File Management
• Very similar to UNIX
– Easy transition for programmers and administrators
who are familiar with one operating system to move to
the other
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Data Structures
• Files organized in directories
– Connected in treelike structure
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(table 15.5)
The file type indicates how each file is to be used.
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Filename Conventions
• Case sensitive
– Recognizes uppercase and lowercase letters
• Up to 255 characters long
• Contain alphabetic characters, underscores, and
numbers
• File suffixes: optional
• Can include space
– Complications if running command-line programs
• File hierarchy
– First slash indicates an absolute path name
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(figure 15.6)
A sample file hierarchy. The forward slash (/ ) at the top represents the root
directory.
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Filename Conventions (cont'd.)
• Path name rules
1. Path name starting with slash: begins at root
directory
2. Path name
• One name or list of names separated by slashes
• Last name on list: name of file requested
3. Two periods (..) in path name
• Move upward in hierarchy: closer to root
• Only way to go up the hierarchy
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Filename Conventions (cont'd.)
• 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 the file
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New Versions
• Linux operating systems: patched between version
releases
– Downloaded on request or
– System setup checks for available updates
• Patch management
– Replaces or changes parts of the operating system
that need to be enhanced or replaced
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New Versions (cont'd.)
• Reasons for operating system patches
– Security precautions: constantly changing system
threats
– System compliance: government regulations
regarding privacy and financial accountability
– Peak efficiency
• System manager must:
– Maintain vigilance: security vulnerabilities
– Assess patch’s criticality
• If important, apply immediately
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User Interface
• Early Linux versions
– Required typed commands
• Thorough knowledge of valid commands required
• Current versions
– Include powerful and intuitive menu-driven interfaces
– Can still use Terminal mode
• Type commands similar to those used for UNIX
– Several graphical user interfaces available
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User Interface (cont'd.)
• Many Linux versions
– Equipped with Windows-compatible word processors,
spreadsheet, and presentation applications
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System Monitor
• System Monitor window
– Moment-by-moment system status
• Immediate history: CPUs, memory, and network usage
– Other information
• Supported file systems
• Currently running processes information
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(figure 15.9)
This Linux system monitor shows the recent history of both CPUs, shown in
the top graph.
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System Logs
• Provide detailed description of activity on system
• Invaluable to administrators
– Tracking system malfunction
– Firewall failure
– Disabled device
• Found in /var/log directory
• Seen using a log viewer
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(figure 15.10)
The system log viewer shows details of all activity.
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(table 15.6)
A few of the many available Linux log files. See the documentation for your
system for specifics.
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File Listings
(figure 15.11)
Directories are shown in the left-most column, and the contents of the
currently highlighted folder are displayed in the window on the right.
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File Listings (cont'd.)
• Explore directory’s or file’s contents
– Double click its icon
• Establish the directory’s or file’s access control
settings
– Right click on item’s icon
• Terminal window
– Display directory details: type command ls-l
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(figure 15.12)
Directory details shown in Terminal mode, which is another way to explore
Linux.
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Setting Permissions
• Network administrators
– Full control: manage access to files and directories
• Three levels of access
– Owner
– Group
– Others
• Access level settings: under properties option
• Access types
– Create and delete files, list files only, access files, and
none
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(figure 15.13)
User activity can be restricted by using document properties options.
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Conclusion
• Originally designed to gain more power from a
1990s computer
– Evolved into powerful, flexible operating system: runs
supercomputers, cell phones, and many other
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
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Conclusion (cont'd.)
• Large organizations
– Commercial Linux products available
• Viable marketplace player
– Popularity expected to grow for many years
– Android operating system foundation
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