Ch_10_UNIX

8/4/2019 Ch_10_UNIX

1/48

Ceng 334 - Operating Systems 10-1

Chapter 10 : Case Study - UNIX

History of unix

Overview of unix

Processes in unix

Memory management in unix Input/output in unix

The unix file system

Security in unix

Note: This case study covers only UNIX. Please read

chapter 10 of the text book forLINUX.

8/4/2019 Ch_10_UNIX

2/48


History of UNIX

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, nowSCO

BSD (Berkeley Software Distribution)

8/4/2019 Ch_10_UNIX

3/48


Overview of UNIX

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

8/4/2019 Ch_10_UNIX

4/48


UNIX Layers

The layers of a UNIX system.

User

Interface

8/4/2019 Ch_10_UNIX

5/48


UNIX Utility Programs

A few of the more common UNIX utility programs required by POSIX

8/4/2019 Ch_10_UNIX

6/48


UNIX Kernel (1)

Approximate structure of generic UNIX kernel

8/4/2019 Ch_10_UNIX

7/48


UNIX Kernel (2)

Bottom layer

Device drivers for character and block devices

Process dispatcher which stops the current

process, saves its state and starts the appropriate

driver when an interrupt occurs

8/4/2019 Ch_10_UNIX

8/48


Process Creation in UNIX - fork

Process creation in UNIX.

8/4/2019 Ch_10_UNIX

9/48


POSIX Shell

A highly simplified shell

8/4/2019 Ch_10_UNIX

10/48


The ls Command

Steps in executing the command ls type to the shell

8/4/2019 Ch_10_UNIX

11/48


Some UNIX Process Concepts

Daemons (background processes)

Cron daemon which wakes up once a minute to

check scheduled events (eg., disk backup)

Pipes - syncronized channels between

processes to pass byte streams

Signalssoftware interrupts used for

interprocess communication. Choices:

catch, ignore, or kill process

8/4/2019 Ch_10_UNIX

12/48


Signals Required By POSIX

The signals required by POSIX.

8/4/2019 Ch_10_UNIX

13/48


System Calls for Process Management

s is an error code

pid is a process ID

residual is the remaining time from the previous alarm

8/4/2019 Ch_10_UNIX

14/48


Thread Calls in POSIX

The principal POSIX thread calls.

8/4/2019 Ch_10_UNIX

15/48


Thread Calls in POSIX

Threads were not in the first versions of UNIX

There are many thread packages in use which arestandardized in POSIX

Thread calls are the same for user-space or kernel-space

In kernel-space implementation calls are systemcalls

In user-space implementation calls are to a run-time library

8/4/2019 Ch_10_UNIX

16/48


Thread Communication - mutexes

Threads use locks calledmutexes for short-time

locking a resource (say a shared buffer)

A mutex must be firstcreated(and finallydestroyed 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)

8/4/2019 Ch_10_UNIX

17/48


Thread Communicationcondition

variables

For long-term synchronization (such as waiting for

a tape to become free)condition variables are

used Condition variables have to becreatedfirst and

laterdestroyedlike mutexes

A condition variable is used by having one thread

wait on it, and another threadsignalit. If no

thread is waiting when a signal is sent, the signal

is lost

8/4/2019 Ch_10_UNIX

18/48


UNIX Scheduler (1)

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, queue4)

8/4/2019 Ch_10_UNIX

19/48


UNIX Scheduler (2)

Once a second the priority of all processes are recalculatedto avoid starvation using

priority = CPU_usage + nice + base

CPU_usage, represents the average number of clock ticks

per second that the process has had during the past fewseconds

Nice is a value between20 to 20 (default 0). Nice systemcall 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 andinteractive processes are blocked

8/4/2019 Ch_10_UNIX

20/48


Booting UNIX (1)

The first sector of the boot disk (master bootrecord) is read in and executed

This sector loads theboot program

Boot reads root directory, loadskerneland startsits execution

Kernel reads the rest of the operating system(main C-code section)

C code does some initialization, allocates systemdata structures, loads device drivers andhandcrafts the first process,process 0

8/4/2019 Ch_10_UNIX

21/48


Booting UNIX (2)

The sequences of processes used to boot some systems

cp

8/4/2019 Ch_10_UNIX

22/48


Handling Memory

Each process has three segments (shown as one segment in the figure, but ifhardware supports they can be separate):

Text : executable code (which is shared in the figure)

Data : variables, strings, arrays etc.

initialized datavariables which must be initialized to some value when programstarts

Uninitialized data (BSS)not initialized but has value 0 as default

Stack

Text is fixed in length, data and stack can grow and shrink

Process BProcess A

8/4/2019 Ch_10_UNIX

23/48


Paging in UNIX (1)

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 userstructure andpage table. The pages of the

text, data and stack segments are brought in

on demand

8/4/2019 Ch_10_UNIX

24/48


Paging in UNIX (2)Core Map

Main memory: kernel, core map, pages

Core map has an entry for each page and contains

information about the contents of the page frames

Page on Disk

Process

table entry

8/4/2019 Ch_10_UNIX

25/48


Page Replacement Algorithm (1)

The page replacement algorithm is executedby thepage daemon (process 2)

Page daemon wakes up every 250 msec and

transfers pages to disk if the amount of freememory is less than the system parameter

lotsfree (typically set to of memory)

Page daemon uses thetwo-handed clockalgorithm

8/4/2019 Ch_10_UNIX

26/48


Basic Clock Algorithm (1)

The pointer (hand) points to the oldest page

When a page fault occurs,

if theR bit of the pointed page is 0 (page not referred), this page isevicted (the new page replaces this page - written to disk first if itis dirty)

if theR bit is 1 (page accessed),R bit is cleared and the hand isadvanced to the next page

8/4/2019 Ch_10_UNIX

27/48


Two-handed Clock Algorithm (2)

Page daemon has to do two passes withone core mappointer. Pass 1 clears allR bits, second pass removespages (R bits are set between pass 1 and 2)

Page daemon maintains two pointers into the coremap to speed up the process (one pass instead of two)for large memories

When page daemon runs, it first clears theR bit at thefront hand, and then checks theR bit at thebackhand, after which it advances both hands

Each time the page daemon runs, the hands rotate lessthan a full revolution, the amount depending on thenumber of pages needed to reach lotsfree

8/4/2019 Ch_10_UNIX

28/48


I/O in UNIX

All I/O devices are integrated into the file

system asspecial files

These special files are accessed likeordinary files (ie., file operations such as

read, write, open are the same for special

files

8/4/2019 Ch_10_UNIX

29/48


Networking in UNIX (1)

Sockets are used to establish a connection

between network nodes

8/4/2019 Ch_10_UNIX

30/48


Networking in UNIX (2) Sockets are created and destroyed dynamically

Creating a socket returns a file descriptor, which isneeded 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 aconnect call giving asparameters the file descriptor of the local socket and

the address of a remote socket (a sockets has anaddress in the network like the internet)

Once a connection is established, a socket functionslike apipe

8/4/2019 Ch_10_UNIX

31/48


UNIX I/O (1)

When a user accesses a special file, the file systemdetermines the major and minor device numbers andwhether it is a block or character special file

Major device number is used to index into eitherbdevswarray for block special orcdevsw for character special files

These structures contain pointers to the procedures to openthe device, read, write etc., Some of the fields of a typical

cdevsw table are shown below

8/4/2019 Ch_10_UNIX

32/48


UNIX I/O (2)

The UNIX I/O system in BSD

C-list

8/4/2019 Ch_10_UNIX

33/48


UNIX I/O (3)

For block special files (eg., disks) the blocks arecached in abuffer cache

The buffer cache works for both reads and writes

Usually dirty (modified) blocks are written to thedisk in every 30 seconds

For character special devices, data is buffered in achain ofC-lists. A C-list block is 64 characterslong, plus a count and a pointer to the next block(BSD method of character buffering)

8/4/2019 Ch_10_UNIX

34/48


The UNIX File System (1)

Some important directories found in most

UNIX systems

8/4/2019 Ch_10_UNIX

35/48



Before linking.

After linking.

(a) Before linking. (b) After linking

8/4/2019 Ch_10_UNIX

36/48



Separate file systems

After mounting

(a) Before mounting (b) After Mounting

8/4/2019 Ch_10_UNIX

37/48


Locking Files in UNIX (1)

Accessing a file by several processes need somecritical section management

This is done by locks

A lock is defined by a file name, the starting byteand the number of bytes

When placing a lock, the process specifies to

Block: when the existing block is removed, the process is

unblocked and the lock is placed

Not to block: the system call returns with a status code

telling whether the lock succeeded or not

8/4/2019 Ch_10_UNIX

38/48


Locking Files (2)

(a) File with one lock

(b) Addition of a second lock

(c) A third lock

8/4/2019 Ch_10_UNIX

39/48


System Calls for File Management

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

8/4/2019 Ch_10_UNIX

40/48


The statSystem Call

Fields returned by the stat system call

8/4/2019 Ch_10_UNIX

41/48


System Calls for Directory Management

s is an error code

dir identifies a directory stream

dirent is a directory entry

8/4/2019 Ch_10_UNIX

42/48


UNIX File System (1)

Disk layout in classical UNIX systems

Block 0 is the boot block

Block 1 is thesuperblock which contains information

about the layout of the file system, including thenumber of i-nodes, number of disk blocks, and startof the list of free disk blocks

8/4/2019 Ch_10_UNIX

43/48



Directory entry fields.

Structure of the i-node in System V

8/4/2019 Ch_10_UNIX

44/48



The relation between the file descriptor table, the open file description

File descriptor table is

indexed by thefd

parameter and has oneentry for each file

8/4/2019 Ch_10_UNIX

45/48



A BSD directory with three files.The same directoryafter the file voluminous has been removed

File name can be 255 characters long The first 4 fields are fixed length

8/4/2019 Ch_10_UNIX

46/48


Security in UNIX (1)

Each UNIX user has a UID (User ID). AUID 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 thesuperuser

Users can be organized in groups, which are

also numbered with 16-bit GIDs (GroupID)

8/4/2019 Ch_10_UNIX

47/48


Security in UNIX (2)

Some examples of file protection modes

8/4/2019 Ch_10_UNIX

48/48

System Calls for File Protection

s is an error code uid and gid are the UID and GID, respectively

Ch_10_UNIX

Documents

Transcript of Ch_10_UNIX