Linux kernel architecture

Linux Kernel Architecture
Amir Hossein Payberah
payberah@yahoo.com

Contents



What is Kernel ?
Kernel Architecture Overview
 User

Space
 Kernel Space


Kernel Functional Overview
 File

System
 Process Management
 Device Driver
 Memory Management
 Networking
2

What is Kernel ?
Modules or sub-systems that
provide the operating system
functions.
 The Core of OS


3

Types of kernels
Micro kernel (Modular kernel)
 Monolithic kernel


4

Micro kernel


It includes code only necessary to
allow the system to provide major
functionality.
 IPC
 Some

memory management
 Low level process management &
scheduling
 Low level input / output


Such as Amoeba, Mach and …
5

Monolithic kernel
It includes all the necessary
functions.
 Such as Linux and …


6

Micro vs. Monolithic


Micro
 Flexible
 Modular
 Easy



to implement

Monolithic
 Performance

7

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
8

Kernel Architecture
Overview
User Space
 Kernel Space


User Space
System Call
Kernel Space
Hardware

9

User Space




The User Space is the space in memory
where user processes run.
This Space is protected.
 The

system prevents one process from
interfering with another process.
 Only Kernel processes can access a user
process

10

Kernel Space




The kernel Space is the space in memory
where kernel processes run.
The user has access to it only through the
system call.

11

System Call
User Space and Kernel Space are
in different spaces.
 When a System Call is executed,
the arguments to the call are
passed from
User Space to Kernel Space.
 A user process becomes a kernel
process when it executes a system
call.


12

User Space and Kernel Space
Relationship
User
Process

User
Space
Kernel Space

Data Flow

Library Routine
Syscall Dispatch

Kernel
Service
13

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
14

Kernel Functional
Overview
File System
 Process Management
 Device Driver
 Memory Management
 Networking


15


16

Functional Layer &
Architectural Layer
User Space
System Call
MM
PM

FS
Hardware
DM

Net

17

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
18

File System






It is responsible for storing information on
disk and retrieving and updating this
information.
The File System is accessed through
system calls such as : open, read, write, …
Example :
 FAT16,

FAT32, NTFS
 ext2, ext3
…
19

Type of Files


The Unix system has the following types of
files:
 Ordinary


Contain information entered into them by a user, an
application or …

 Directory


Files

Manage the cataloging of the file system

 Special


Files

Files (devices)

Used to access the peripheral devices

 FIFO

Files for Pipes
20

Extended File System
/

/bin /etc
ls

/dev

… /usr

/home

ping

A
data.txt

/root
B

pic.gif
21

File System Structure
Boot Super inode Block
Block Block List
List



Boot Block : information needs to boot the system
Super Block : File System Specifications
Size
 Max. number of files
 Free blocks
 Free inodes





inode List
Block List : The files data
22

Inode
Each file has an inode structure that
is identified by an i-number.
 The inode contains the information
required to access the file.
 It doesn’t contain file name.


23

Directories

File
Name

inode Number

25

Virtual File System
It manages all the different file
system.
 It is an abstraction layer between the
application program and the file
system implementations.


26

Virtual File System (Cont.)


It describes the system’s file in terms
of superblocks and inodes (the same
way as the Ext2).

27

Virtual File System (Cont.)
Inode cache
 Directory Cache


28

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
29

Process Management
The Unix OS is a time-sharing
system.
 Every process is scheduled to run for
a period of time (time slice).
 Kernel creates, manages and deletes
the processes


30

Process Management (Cont.)






Every process (except init) in the system is create
as the result of a fork system call.
The fork system call splits a process into two
processes (Parent and Child).
Each process has a unique identifier (Process ID).

31

Process Structure


Each process is represented by a
task_struct data structure.
 It

contains the specifications of each process
such as:





State
Scheduling information
Identifier
…

32

Process Structure (cont.)


The task_vector is an array of pointers to
every task_struct data structure in the
system.
 This

means that the maximum number of
processes in the system is limited by the size of
the task vector

33

Type of Processes


Running




Waiting




The process is waiting for an event or for a resource.

Stopped




The process is either running or it is ready to run.

The process has been stopped, usually by receiving a
signal.

Zombie


This is a halted process which, for some reason, still has
a task_struct data structure in the task vector.
34

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
35

Device Driver





On of the purpose of an OS is to hide the
system’s hardware from user.
Instead of putting code to manage the HW
controller into every application, the code
is kept in the Linux kernel.
It abstracts the handling of devices.
 All

HW devices look like regular files.

36

Type of devices


Character devices
A

character device is one that can be accessed
as a stream of bytes.
 Example : Keyboard, Mouse, …


Block devices
A

block device can be accessed only as
multiples of a block.
 Example : disk, …


Network devices
 They

are created by Linux kernel.

37

Major Number and Minor Number


Major Number
 The

major number identifies the driver
associated with the device.



Minor Number
 The

minor number is used only by the driver
specified by the major number; other parts of
the kernel don't use it.
 It is common for a driver to control several
devices, the minor number provides a way for
the driver to differentiate among them.
38

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
40

Memory Management



Physical memory is limited.
Virtual memory is developed to overcome
this limitation.

41

Virtual memory






Large Address space
Protection
Memory mapping
Fair physical memory allocation
Shared virtual memory

42

Physical and Virtual memory

43

Swap memory


It is a configurable partition on disk
treated in a manner similar to
memory.

44

Contents



What is Kernel ?
 User

Space
 Kernel Space


 File

System
 Device Driver
 Networking
45

BSD socket layer


It is a general interface (abstract
layer).
 Used



in networking and IPC.

Socket address families:
 UNIX
 INET
 AX25
 IPX
 APPLETALK
 X25
48

What is socket?
main()
{
FILE *fd;
fd = fopen (…);
process (fd);
fclose (fd);
}

main()
{
int sockfd;
sockfd = socket (…);
process (sockfd);
close (sockfd);
}

49

INET socket layer
It supports the Internet address
family.
 Its interface with BSD socket layer is
through a set of operation which is
registered with BSD socket layer.


50

Type of sockets


Stream Socket
 Provide

a reliable, sequenced, two-way
connection (such as TCP).



Datagram Socket
A

connection-less and unreliable connection
(such as UDP).



Raw Socket
 Used

for internal network protocols.

51

Linux kernel architecture

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