The document provides an overview of the Linux architecture including: 1) It discusses the history of Linux from its origins as a free UNIX-like operating system developed by Linus Torvalds to the over 18 million lines of code it contains today. 2) It describes the key components of the Linux system architecture including the hardware layer, kernel, shell, and utilities. The kernel acts as the core of the OS and interacts with hardware to perform low-level services. 3) It outlines several important kernel functions including file system management, process management and scheduling, memory management, and device drivers which allow communication with I/O devices through device files.

1. LINUX ARCHITECTURE

2. Outline
● Linux history
● Linux system architecture
● Linux kernel
● Kernel Architecture overview
● Kernel Functional overview
– File system
– Process management
– Memory management
– Device driver
– Networking

3. History of Linux
● First truly free UNIX like OS
● 1983 , GNU developed by Richard Stallman but kernel was missing
● Personal project by Finnish student Linus Torvalds to create a new
free operating system kernel in year 1991.
● Linus Torvalds began a project that later became the Linux kernel.
● First version of kernel 0.01
● 1991 from few c files to 18 million lines in 2015

4. History of Linux

5. Characteristics of linux OS
● Multiuser Capability
● Multitasking
● Portability – Works on different hardware
● Security
– Authentication
– Authorization
– Encryption

6. Linux system architecture

7. Linux system architecture
● Hardware layer – Consist of all peripherical devices like RAM, CPU, HD
● Kernel
– Core of OS
– Interacts with hardware
– Perform low level services
Shell
– Interface to kernel. Hides complexity of kernel from users.
– Take commands and executes kernel functions.
● Utilities

8. Linux system architecture

9. Linux system architecture

10. Linux kernel
● Monolithic kernel
● Supports
– Preemptive mutitasking
– Virtual memory
– Shared libraries
– Demand loading
– Memory mangement
– Threading
– Loadable kernel modules

11. Kernel Functional overview

12. Linux system architecture
● Virtual address space – 0x0000000 to 0xFFFFFFFF for 32bit linux
● User space - normal programs run
– Contains data, stack
● Kernel space - where kernel run
– Contains kernel code, pagetable..

13. File systems
● In UNIX everything is a file
● Filesystem is a hierarchical storage of data adhering to a specific
structure
● Tree like structure
● Folders named as directories
● Root - '/ '

14. File systems
● Sort of files : directories (d), special files(s), links(l), sockets (s),
named pipes(p)
● Major partitions on a Linux system:
– data partition: system data, including the root partition
containing all the data to start up and run the system
– swap partition: expansion of the computer's physical memory,
extra memory on hard disk(twice amount of RAM)
● Mount point
– Partitions attached through mount point

15. File systems
● Virtual file system

16. File system
● Concepts in VFS
– Super block – filesystem metadata
– Inode – File metadata(permissions, owner, creation time)
● VFS is designed to work with filesystems that understand and
implement such concepts

17. File systems - VFS
● Four object types of VFS
– Superblock object – specific mounted filesystem.
– inode object - a specific file.
– dentry object - directory entry, which is a single component of
a path.
– file object - an open file as associated with a process.

18. Process management and Scheduling
● Illusion of multitasking
● Issues kernel must resolve
– Applications must not interfere.
– CPU time shared fairly.
● Process priorities
– Hard real time process
– Soft real time process
● Allocation of CPU time

19. Process management and Scheduling
● Fork and exec system calls
● Process life cycle
– Running, Waiting, Sleeping, Zombie

20. Process management

21. Process management and Scheduling
● User and kernel mode
● Interrupts and kernel threads
● Pre emptive multitasking
– Scheduler can take global decisions to start and stop a process

22. Process management and scheduling
● I/O bound and processor bound process
– I/O bound : much time on waiting request from I/O
– Process bound : much time on executing code
● Nice values – range from -20 to 19 (default 0)
– Larger nice value low priority
● Time slice (default 20 ms)
– Numeric value showing
how long a task can run

23. Process management and scheduling
● Scheduling policy with example
– Consider two runnable task text editor and video encoder
– Text editor – I/O bound, video encoder – processor bound
– Text editor : Interactive process hence larger timeslice and high
priority

24. Memory management
● Virtual memory – makes system appear to have more memory than it
actually has
– Virtual memory greater than physical memory
– Each process have its own virtual address space
– Sharing of physical memory
● Zones
– ZONE_DMA - This zone contains pages that can undergo DMA.
– ZONE_DMA32 - these pages are accessible only by 32-bit devices
– ZONE_NORMAL - contains normal, regularly mapped, pages.
– ZONE_HIGHMEM - contains “high memory,” which are pages not
permanently mapped into the kernel’s address space.

25. Memory management
● Page table - Data structures used to map virtual to physical address
● Pages – memory divided into chunks of equal size. (Intel x86 4Kbytes)

26. Memory management
● Linux page tables

27. Memory management
● Demand paging
– Creating page faults and bringing image/file/data from disk to
physical memory
– Process should wait in queue until it is fetched
● Swapping
– No free space in RAM swapping done and done by kswapd
– The page selected for swapping has modified (dirty page)it is saved
as swap file.
– Least recently used(LRU) used for swapping.
● Shared virtual memory
– Two process sharing same physical memory

28. Memory management
● Page allocation and deallocation
– mem_map_t : decribes single physical page
– free_area : to find free pages
– Buddy algorithm to allocate and deallocate pages
– Page allocation results in page fragmentation
– Reallocation combiles fragmented pages into large blocks
● Malloc(), vmalloc() and kmalloc()

29. Memory management

30. Devices drivers
● Communicate I/O devices through device files.
● Character and Block devices
– Character - continous stream of data, eg:modem
– Block – BLock of data, eg: hard disc

31. Device drivers
● Dynamic creation of device files

32. Thank you