Download Ceng 334 - Operating Systems

Chapter 10 : Case
Study - UNIX
• History
• Overview
• Processes
• Memory management
• Input/output in
• The unix file system
• Security
Note: This case study covers only UNIX. Please read
chapter 10 of the text book for LINUX.
1
History of UNIX
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Originated from the MULTICS (Multiplexed
Information and Computing Service) operating
system by M.I.T, Bell Labs and GE
There are two main versions:
 AT&T System V Release 4 (SVR4)
 Originally developed by AT&T, now SCO
 BSD (Berkeley Software Distribution)
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Overview of UNIX
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Supports various architectures
Structure varies
Supports preemptive multitasking
Multiuser environment - generally secure
Supports multithreaded applications
Protection/Security is high on modern versions
Supports symmetric multiprocessing
Highly scalable/portable to various systems
Many types/flavours of UNIX exist
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UNIX Layers
User layers of a UNIX system.
The
Interface
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UNIX Utility Programs
A few of the more common UNIX utility programs required by POSIX
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UNIX Kernel (1)
Approximate structure of generic UNIX kernel
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UNIX Kernel (2)
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Bottom layer
Device drivers for character and block devices
 Process dispatcher which stops the current
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process, saves its state and starts the appropriate
driver when an interrupt occurs
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Process Creation in UNIX - fork
Process creation in UNIX.
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POSIX Shell
A highly simplified shell
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The ls Command
Steps in executing the command ls type to the shell
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Some UNIX Process Concepts
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Daemons (background processes)
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Cron daemon which wakes up once a minute to
check scheduled events (eg., disk backup)
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Pipes - syncronized channels between
processes to pass byte streams
Signals – software interrupts used for
interprocess communication. Choices: catch,
ignore, or kill process
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Signals Required By POSIX
The signals required by POSIX.
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System Calls for Process
Management
s is an error code
pid is a process ID
residual is the remaining time from the previous alarm
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Thread Calls in POSIX
The principal POSIX thread calls.
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Thread Calls in POSIX
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Threads were not in the first versions of UNIX
There are many thread packages in use which are
standardized in POSIX
Thread calls are the same for user-space or kernelspace
In kernel-space implementation calls are system calls
In user-space implementation calls are to a run-time
library
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Thread Communication mutexes
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Threads use locks called mutexes for short-time
locking a resource (say a shared buffer)
A mutex must be first created (and finally destroyed)
Mutual exclusion is implemented by locking a mutex
before accessing a resource and unlock it when they
are done (like binary semaphores which is “0” or “1”, a
mutex is either locked or unlocked)
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Thread Communication –
condition variables
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For long-term synchronization (such as waiting for a
tape to become free) condition variables are used
Condition variables have to be created first and later
destroyed like mutexes
A condition variable is used by having one thread wait
on it, and another thread signal it. If no thread is
waiting when a signal is sent, the signal is lost
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UNIX Scheduler (1)
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The UNIX scheduler is based on a multilevel queue structure
(highest priority queue first, round-robin in each queue)
In this scheme, a process which was blocked and waiting for an
event joins the appropriate queue when blocking is over (a process
whose disk I/O is finished joins,say, queue –4)
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UNIX Scheduler (2)
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Once a second the priority of all processes are recalculated to
avoid starvation using
priority = CPU_usage + nice + base
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CPU_usage, represents the average number of clock ticks per
second that the process has had during the past few seconds
Nice is a value between –20 to 20 (default 0). Nice system
call can be used to set this value 0-20
Base is a system parameter in UNIX source code
The scheduler forces CPU bound (on positive queues) get
any service that is left over when all I/O bound and
interactive processes are blocked
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Booting UNIX (1)
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The first sector of the boot disk (master boot
record) is read in and executed
This sector loads the boot program
Boot reads root directory, loads kernel and starts its
execution
Kernel reads the rest of the operating system (main
C-code section)
C code does some initialization, allocates system
data structures, loads device drivers and handcrafts
the first process, process 0
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Booting UNIX (2)
cp
The sequences of processes used to boot some systems
21
Handling Memory
Process A
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Process B
Each process has three segments (shown as one segment in the figure,
but if hardware supports they can be separate):
 Text : executable code (which is shared in the figure)
 Data : variables, strings, arrays etc.
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initialized data – variables which must be initialized to some value when
program starts
Uninitialized data (BSS) – not initialized but has value 0 as default
Stack
Text is fixed in length, data and stack can grow and shrink
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Paging in UNIX (1)
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Prior to 3BSD, UNIX systems used swapping
(if memory is full, swap processes to disk)
To run a process, all that is needed is the user
structure and page table. The pages of the
text, data and stack segments are brought in on
demand
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Paging in UNIX (2) – Core Map
Page on Disk
Process
table entry
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Main memory: kernel, core map, pages
Core map has an entry for each page and contains
information about the contents of the page frames
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Page Replacement Algorithm (1)
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The page replacement algorithm is executed by
the page daemon (process 2)
Page daemon wakes up every 250 msec and
transfers pages to disk if the amount of free
memory is less than the system parameter
lotsfree (typically set to ¼ of memory)
Page daemon uses the two-handed clock
algorithm
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Basic Clock Algorithm (1)
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The pointer (hand) points to the oldest page
When a page fault occurs,
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if the R bit of the pointed page is 0 (page not referred), this page is
evicted (the new page replaces this page - written to disk first if it is dirty)
if the R bit is 1 (page accessed), R bit is cleared and the hand is advanced
to the next page
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Two-handed Clock Algorithm (2)
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Page daemon has to do two passes with one core map
pointer. Pass 1 clears all R bits, second pass removes
pages (R bits are set between pass 1 and 2)
Page daemon maintains two pointers into the core map
to speed up the process (one pass instead of two) for
large memories
When page daemon runs, it first clears the R bit at the
front hand, and then checks the R bit at the back
hand, after which it advances both hands
Each time the page daemon runs, the hands rotate less
than a full revolution, the amount depending on the
number of pages needed to reach lotsfree
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I/O in UNIX
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All I/O devices are integrated into the file
system as special files
These special files are accessed like ordinary files
(ie., file operations such as read, write, open are
the same for special files
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Networking in UNIX (1)
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Sockets are used to establish a connection
between network nodes
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Networking in UNIX (2)
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Sockets are created and destroyed dynamically
Creating a socket returns a file descriptor, which is
needed for establishing a connection, reading data,
writing data, and releasing the connection
One party makes a listen call on a local socket, which
creates a buffer and blocks until data arrive
The other party makes a connect call giving as
parameters the file descriptor of the local socket and
the address of a remote socket (a sockets has an
address in the network like the internet)
Once a connection is established, a socket functions
like a pipe
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UNIX I/O (1)
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When a user accesses a special file, the file system determines
the major and minor device numbers and whether it is a
block or character special file
Major device number is used to index into either bdevsw
array for block special or cdevsw for character special files
These structures contain pointers to the procedures to open
the device, read, write etc., Some of the fields of a typical
cdevsw table are shown below
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UNIX I/O (2)
C-list
The UNIX I/O system in BSD
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UNIX I/O (3)
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For block special files (eg., disks) the blocks are
cached in a buffer cache
The buffer cache works for both reads and writes
Usually dirty (modified) blocks are written to the disk
in every 30 seconds
For character special devices, data is buffered in a
chain of C-lists. A C-list block is 64 characters long,
plus a count and a pointer to the next block (BSD
method of character buffering)
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The UNIX File System (1)
Some important directories found in most UNIX
systems
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The UNIX File System (2)
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Before linking.
After linking.
(a) Before linking. (b) After linking
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The UNIX File System (3)
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Separate file systems
After mounting
(a) Before mounting
(b) After Mounting
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Locking Files in UNIX (1)
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Accessing a file by several processes need some critical
section management
This is done by locks
A lock is defined by a file name, the starting byte and
the number of bytes
When placing a lock, the process specifies to
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Block: when the existing block is removed, the process is
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unblocked and the lock is placed
Not to block: the system call returns with a status code
telling whether the lock succeeded or not
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Locking Files (2)
(a) File with one lock
(b) Addition of a second lock
(c) A third lock
38
System Calls for File
Management
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s is an error code (-1 if an error has occured)
fd is a file descriptor (a positive number: 0 standard input)
position is a file offset
39
The stat System Call
Fields returned by the stat system call
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System Calls for Directory
Management
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s is an error code
dir identifies a directory stream
dirent is a directory entry
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UNIX File System (1)
Disk layout in classical UNIX systems
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Block 0 is the boot block
Block 1 is the superblock which contains information
about the layout of the file system, including the
number of i-nodes, number of disk blocks, and start of
the list of free disk blocks
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UNIX File System (2)
Directory entry fields.
Structure of the i-node in System V
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UNIX File System (3)
File descriptor table is
indexed by the fd
parameter and has one
entry for each file
The relation between the file descriptor table, the open file description
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UNIX File System (4)
A BSD directory with three files.The same directory
after the file voluminous has been removed
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File name can be 255 characters long
The first 4 fields are fixed length
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Security in UNIX (1)
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Each UNIX user has a UID (User ID). A
UID is an integer between 0 and 65536. Files,
processes and other resources are marked
with the UID of their owner
The user with UID 0 is the superuser
Users can be organized in groups, which are
also numbered with 16-bit GID’s (Group ID)
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Security in UNIX (2)
Some examples of file protection modes
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System Calls for File Protection
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s is an error code
uid and gid are the UID and GID, respectively
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