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Linux Overview
•Linux is a 32-bit operating system
•Multi-platform, most popular are Intel x86, Sparc, Alpha and
Power PC
•On alpha platform, actually operates as 64-bit operating system
•Did not advance the state of the art
•Linux can and should be considered a full implementation of UNIX,
but cannot be called UNIX because the word UNIX is a trademark
owned by AT&T
•Open Source means that Linux is free to everyone and can even be
sold for a profit, but the all source code must be available as
stated in the Public License Agreement
•Open Source also means that any any problems are found and
updated and redistributed much faster than any commercial
operating system.
•Open Source does not mean that Linux is any less reliable than
commercial operating systems, maybe even more reliable.
Linux Overview Continued
•Linux has been comercially successful, even though it was
not developed by a commercial orgainzation. Every email
sent has up to a 50% chance of being handled by Linux
Sendmail.
•Linux has been economically successful, it has been
packaged and sold for-profit by companies such as Red Hat,
Debian, SuSe, Caldera, Mandrake and Slackware.
•Linux is the first full blown operating system that is open source,
on top that it is very reliable, for this reason it is very technically
successful.
•There is no customer support for Linux. This is because
the companies who sell Linux did not write all of the code
included in the distribution. However Usenet groups such as
www.dejanews.com have answered many questions about
Linux.
Linux Overview Continued
•Usenet groups are how Linus Torvalds announced his project
named Linux to the world, and gave a place for interested
people to look and possibly contribute to the project.
•Linux was originally designed to exploit the new instructions added
to the Intel x86 architecture with the introduction of the 386
processor.
•Linux has evloved to become primarily a network operating system
like UNIX.
•Linux is very scalable and is equally at home on a server or a
workstation. With the recent introduction of Red Hat Package
Management (RPM), it has become even easier to install software
packages to suit specific needs.
•Linux is highly reliable and has been known to run for as long as
five years without rebooting.
•Linux is still very difficult to configure and it is easier to reinstall everything in order to upgrade to a newer version than it is
to patch in new libraries and re-compile the kernel.
Linux Overview Continued
•X-windows has made Linux a graphical operating system
•X-windows is very customizable and the user may change it to look
and act however he/she wants by using different window managers.
•Fvwm95 makes Linux look and act like Microsoft Windows 95/98,
however the underlying operating system is still Linux and it is still
much more stable than Microsoft.
•Recently Linux has been seen media attention due to the Microsoft
Anti-Trust hearing because Microsoft claimed that Linux is a viable
alternative to Microsoft Windows.
•Linux has been gaining popularity due to it’s low cost (free) and it’s
growing support by major software developers such as Oracle,
Corel, Netscape and game developers such as IdSoftware who
makes Quake and Quake II for Linux.
•Many hardware developers are also releasing Linux drivers for
their products. Especially hardware where speed and efficiency is
critical such as 3dfx, Nvidia and sound card Manufacturers.
Linux Overview Continued
•Printers are still very difficult to configure in Linux because they
must use generic drivers. Hp sells it’s own distribution of UNIX
and does not provide driver support for it’s own printers.
•Dell has recently begun selling a computer that comes preconfigured with Linux installed. This is the first time in history
that a major computer company has manufactured a PC that comes
with Linux.
•Some people say that Linux was done right because it is very
scalable, customizable and reliable.
•Some people say that Linux was done wrong because it is not easy
to configure or use.
•Ease of use is something that would limit a system that would limit
a very scalable and customizable system. For this reason, Linux
was done right.
Data Structures
•
An organization of information
•
Data about relative subject put into a form that can be managed and
understood
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Memory Management
Process Management
File Management
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Memory Management
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Basics on how Memory Management Works
– Physical Memory
• Actual memory available on system (hardware)
– Virtual Memory
• Memory created by the system/OS
• Must map to physical memory
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Pages
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Memory is broken up into Pages
– Page Size
• 4 kilobytes in x86 system
• 8 kilobytes in Alpha sytsem
Pages are managed by structure called Page Table
– Main purpose is to translate between physical and virtual memory
addresses
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What a Page Table Looks Like
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Page Table
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Translation from Virtual to Physical Memory
– Each slot holds information about location of actual physical address
– There are several Page Tables for each mapping
• Traverse 2 times on a x86 processor
• Traverse 3 times on an Alpha Processor
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Slots of The Page Table
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Each Slot Contains Information about the Page
– Physical Page Address
– Access Rights
• Can page be written to?
• Can page be executed?
– Valid Flag
• If not valid passed on to operating system for error control
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Other Memory Data Structures
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Mem_map
– Contains information about each page
– Sections of mem_map
• Count – Used only if there are more then one processor using
the page
• Age – Keeps track of how long page has been in memory, when
it was last accessed, etc.
• Map_nr – Holds the physical address location
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Free_Area Data Structure
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Hold information about blocks of memory
Main use is for finding and releasing pages in memory
Free_area is an array
– 1st Element – Sets of 1 page
– 2nd Element – Sets of 2 pages
– 3rd Element – Sets of 4 pages
– Etc.
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Process Data Structures
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Goal of Linux Process Management is to have a process on a processor
at all times
Each process has its own data structure
– Task_struct
• Task vector array contains pointers to every task_struct on the
system
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Task_Struct Areas
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State
– The current running state of the process
• Running, Waiting, Stopped, Zombie
Scheduling Data
– When the process will run
Identifiers
– Every process has a unique identifier number
– Each process also contains a User and Group identifier Number
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Task_Struct (Cont.)
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Links
– Holds pointers to the parent processes
– No such thing as an independent process in Linux except for very
first process
Time
– When process created and how much CPU time used (measured in
jiffies)
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Task_Struct (Cont.)
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File System
– Pointers to every open file descriptor
Virtual Memory
– Tracks Virtual Memory mappings
Processor Context
– When the process is put on hold, the context is stored here
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File System Data Structures
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2 main file system structures
– Fs_struct
– Files_struct
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Fs_struct
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Hold pointers to the process virtual file system nodes (explained later)
and the umask
– The umask is the mode that new files are in when they are first
created.
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Files_struct
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Manages information about all of the files a process is using
Includes pointers for up to 256 files per file_struct
– Every time a file is opened three descriptors are opened by default
• Standard Input – programs read from
• Standard Output – programs write to
• Standard Errors – all errors go here
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File_struct (Cont.)
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F_mode area tells what what mode file is in
– Read only, read/write, write only
F_inode area points at the VFS inode which describes the file
F_ops area is a pointer to address vector
– Contains functions that can be performed on files
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Memory Management in Linux
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Virtual Memory Addressing
– Uses a three level page table structure
– Three types of tables
• Page Directory
– Each active process has one page directory in main
memory.
– Each entry in the page directory points to one page in the
page middle directory
• Page Middle Directory
– Each entry in the page middle directory points to one page
of the page table.
• Page Table
– Each entry points to one virtual page of the process.
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Virtual Memory Addressing Cont.
– 4 fields in a virtual address starting from the left.
• Index of the page directory
• Index into the page middle directory
• Index to the page table
• Offset within the selected page of memory
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Memory Management Cont.
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Page allocation
– Mechanism for dealing with contiguous blocks of pages mapped to
contiguous blocks of page frames.
• Buddy system is used where the kernel keeps a list of
contiguous page frame groups of a fixed size.
• Groups can have 1, 2, 4, 8, 16, or 32 page frames.
• Groups split and merged using buddy algorithm as pages are
allocated and deallocated.
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Memory Management Cont.
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Page Replacement
– Modified version of Simple Clock Algorithm
• Use bit replaced with an 8 bit age variable.
• Age variable incremented every time page is accessed.
• Age variable decremented periodicall by Linux
• Larger the page variable, less likely it will be replaced
Kernel memory allocation
– Based on the page allocation mechanism used for virtual memory.
– Kernel memory can be allocated and deallocated in units of one or
more pages using buddy algoritm
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Kernel memory allocation cont.
– Minimum amount of memory that can be allocated using this system
is one page.
– Kernel also requires small short-term memory chunks in odd sizes
• Linux uses a scheme called slab allocation to take care of these
chunks
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File Management
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File Systems
– Linux has a large advantage in that it has many available file
systems
• VFAT/FAT – DOS/Windows
• NTFS – Windows 2000/NT 4.0
• Ext2 – Extended File System – Linux
• Many more available (see mount manpage for list)
Virtual File System (VFS) – Reason for so many available file systems
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Virtual File System
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The virtual file system is a layer between the target file system and
the system
– Allows modular transparency for different file systems
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Ext2 File System
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Data kept in data blocks
Trade off of space for less CPU usage
– 1240 bytes takes up 2 blocks totaling 2048 bytes even though total
space is 1240 bytes
Inodes
– Holds which blocks file data will occupy
– Access Rights of files
– File modification type
– Type of File
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Ext2 (Cont.)
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Each file has one inode with a unique number identifier
Files that have access paths are directories
Superblocks
– Holds information about the file system size and shape
– Read when the file system is mounted
– Holds vital information about starting blocks and block size and
groups
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Processor Modes
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Processor Modes
– User Mode
• Mode where user programs run
• Cannot run privileged instructions
– Kernel Mode
• Mode that allows access to all kernel functions
• Can run privileged instructions
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Privileged Instructions
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“An instruction that can be executed only in a specific mode, usually by a
supervisory program” – Stallings
In the case of Linux the mode that runs privileged instructions is the kernel mode
Privileged instructions are privileged so that the user cannot accidentally change
something that could cause the system to crash
Examples of privileged instructions are I/O, process and memory instructions
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Uniprocessor or Multiprocessor?
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Linux supports both uniprocessor and multiprocessor systems
Multiprocessors
– Linux can run on any or all of the available processors, sharing
memory between them
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Symmetric Multiprocessing
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Linux uses symmetric multiprocessing
Allows Linux to run on any available processor or on multiple processors
simultaneously
Current kernel supports up to 16 processors using SMP
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Process States in Linux
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5 process states in Linux
– TASK_RUNNING
– TASK_ZOMBIE
– TASK_STOPPED
– TASK_UNINTERRUPTABLE
– TASK_INTERRUPTABLE
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Process States Cont.
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TASK_RUNNING
– Corresponds to both the running and ready states listed in the
textbook.
TASK_ZOMBIE
– Corresponds to the exit state in textbook.
– Process is terminated but still must have its task structure in the
process table.
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Process States Cont.
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TASK_STOPPED
– Process has been halted
– Can only be resumed by an action from another process
– Used in Debugging
– No corresponding state in the textbook
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Process States Cont.
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TASK_INTERRUPTABLE
– Corresponds to Blocked in the textbook
– Process is waiting for an event
• I/O
• Resource to become available
• Signal from another process.
TASK_UNINTERRUPTABLE
– Also corresponds to blocked state
– Waiting directly on hardware conditions
• Cannot accept signal from another process
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Deadlock
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What is it?
– “An impasse that occurs when multiple processes are waiting for the
availability of a resource that will not become available because it
is being held by another process that is in a similar wait state” –
Stallings
Depending on the version of Linux deadlock can be handled different
ways
Three ways to deal with deadlock
– Deadlock prevention
– Deadlock avoidance
– Deadlock detection
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Deadlock Prevention
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Tries to prevent deadlock from occurring by not allowing all three of the
conditions for deadlock (mutual exclusion, hold and wait, no preemption)to occur or
by checking for a circular wait
Advantages
– Works well with processes that perform a single burst of activity
Disadvantages
– Inefficiency
– Preempts without much use and more often than necessary
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Deadlock Avoidance
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Makes resource assignments based on potential for deadlock. If a process that has not started
and might lead to deadlock then it is not started and if a process makes an incremental
resource request that could lead to deadlock that request is denied.
Can result in initiation denial or allocation denial to prevent deadlock
Advantages
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No preemption necessary
Disadvantages
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Process can block for long periods of time
Future resource requirements must be known
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Deadlock Detection
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Grants resources if possible and checks for a circular wait periodically
This strategy does not prevent deadlock but determines if deadlock
exists
Advantages
– No delays in process initiation
Disadvantages
– Preemptive losses
– Slower system performance depending on how often a check for
circular wait is made
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Threads in Linux
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Older versions of Linux used both Kernel-space threads
and User-Space threads.
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POSIX-threads are now a standard part of all modern
Linux distributions. (multi-threaded)
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LinuxThreads
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Scheduling in Linux
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Based on the time-sharing technique
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Scheduling is also based on a ranking process (priority)
• Process priority is dynamic
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Processes are preemptive
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Supports symmetric multiprocessor architecture
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Scheduling limitations
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Algorithm doesn’t scale well
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Predefined time interval too large
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