This document discusses the Linux kernel architecture. It describes the Linux kernel as a monolithic kernel that supports system calls, loadable modules, preemptive multitasking, virtual memory, and other features. It outlines the kernel's layers and function libraries. It also discusses how the kernel handles processes and context switching, and how it uses a CPU scheduler to allocate processing time between tasks. Finally, it covers inter-process communication techniques like message passing, shared memory, and pipes that allow processes to exchange data.

1. Linux Kernel

2. + Linux Kernel Architecture
What is Linux ?
What is the Linux Kernel ?
How does the Kernel effect system
performance ?

3. + Kernel
P
P
P P P
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H H H
Kernel

4. + Linux Kernel Architecture
Monolithic Kernel (diigo.com/085lp2)
Supports for System Calls (diigo.com/085lq5)
Support for Loadable Modules
Implements Preemptive Multitasking
Virtual Memory / Filesystem
Demand Paging / Loading
etc..

5. + Linux Kernel Architecture
Kernel Function Libraries
– System (/proc, generic drivers)
– Networking (TCP, sockets, drivers)
– Storage (VFS, block devices)
– Memory (VM, PM)
– Processing (scheduler, fork, exec)
– Human Interface (HI devices)

6. + Linux Kernel Architecture
Kernel Layers
– User space interface
– Virtual subsystem
– L2V bridge / transformer
– Logical subsystem
– Hardware interface

7. + Linux Kernel Architecture
www.makelinux.net/kernel_map/

8. + Linux Kernel and Processes
Linux Kernel and Processes
The Linux Kernel supports preemptive multitasking
(diigo.com/085lqo) which allows multiple processes
to run individually at the same time while prioritising
these with higher importance
What is a “Context Switch” ?

9. + Linux Kernel and Processes
Context Switch
In order to support multi tasking a context switch
is being handled
Context switch stores or restores the CPU state
to where is 'stopped' before the last CS

10. + Linux Kernel and Processes
Context switch can be forced by an interrupt
timer in case of a low latency operation
e.g: Disk I/O
Context Switch is also being used in order to
handle Interrupt requests
e.g: Completed read request

11. + Linux Kernel and Processes
Context switch is being handled by the OS
Each process stack has a Process Control Block
which holds the relevant meta data for the CS
e.g: Program Counter, Registers Address List
Some CPUs (all x86 variants included) has a
Task State Segment (TSS) which is used to
store the PCB

12. + Linux Kernel and Processes
In case a process needs to transite to Kernel
mode (during a system call) a Mode Switch is
being initiated
Price
Context switch is the most resource consuming
task of the OS.

13. + Linux Kernel and Processes
To review Context Switch statistics use
– 'sar -w'
– 'vmstat'
Exercise:
 - Review the reciprocal relations between
User / System CPU utilization and CS

14. + Linux Kernel and Processes
To review an individual process structure use
– 'pmap'
Exercise
 - Review the structure of several
processes. Can you identify the process
stack ?

15. + Linux Kernel and Processes
CPU Scheduler
The Linux Kernel has a power full and high
performance CPU scheduler.
The CUP scheduler is designed to provide the
most appropriate amount of CPU [time]slices
– Scheduling is based on a dynamic
process priority rank
– Priority is changed to make sure no
process is 'hogging' the CUP nor starving

16. + Linux Kernel and Processes
CPU Scheduler
– For each CPU the scheduler maintain
two priority ordered arrays for active and
expired tasks. After the active one is
done arrays are swapped
(Exception: diigo.com/085lrk)
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3
2
1
4
3
2
1

17. + Linux Kernel and Processes
CPU Scheduler
– The scheduling policy involves assessing
the priority for each task
– There are two classifications for tasks in
terms of scheduling
• CPU bound
• I/O bound
 These can be either Interactive, Batch or
RT processes

18. + Linux Kernel and Processes
CPU Scheduler
– Process pre-emption occurs, when a
process receives a hardware interrupt
– Pre-emptive kernel, can ‘prefer’ a user
mode process over a kernel mode
process (even during an exception
handling), in order to support interactivity

19. + Linux Kernel and Processes
CPU Scheduler
– When setting ‘nice’ value to a process, one
set a static portion of the scheduler priority
for that task
– Exercise
 ‘grep a /etc/services >/dev/null’
 Can you ‘tune’ this command group to
run faster ?

20. + Linux Kernel and Processes
Multi Threading
Multi Threading is being used in order to have
several processing objects within the same
process
– Each Thread is being scheduled
separately by the Kernel
– Multiple threads share the same Memory
segment

21. + Linux Kernel and Processes
Symmetric Multi Process systems can run
threads of the same process simultaneously
Linux implements 1-1 (1 scheduling object per
thread) threading via the Native Posix Thread
Library (NTPL)
* Just like Solaris and FreeBSD

22. + Linux Kernel and Processes
To review Threads information use
– 'ps -eLf'
Exercise
– Can you identify multi threaded
processes on your system ?
– Can you find any difference between PID
and Thread ID ?

23. + Inter Process Communication
IPC
IPC is an implementation of techniques to
exchange data between different threads
IPC can be used internally or across a computer
network

24. + Inter Process Communication
IPC has 4 techniques
– Message passing
– Synchronization
– Shared memory
– Remote procedure call

25. + Inter Process Communication
Message passing
Message can be a signal or data packet
Passing can be synchronous or asynchronous
Synchronisation
Used in order to keep several threads
synchronized and able to provide the same /
parallel service(s)

26. + Inter Process Communication
Shared memory
Simply a shared form of memory
No control from of OS side
RPC
Method of running a procedure (subroutine) on a
remote process

27. + Inter Process Communication
IPC implementations
PIPE – pass data between hierarchy related
threads
FIFO – pass data between non-related threads
Socket – pass large amount of data. Can
implement TCP internally
Shared Memory (SHM) – share defined amount
of storage between threads

28. + Inter Process Communication
IPC implementations
Semaphore (SEM) – shared abstract data (like a
class) used to control access on a shared
resource

29. + Inter Process Communication
IPC
Use the following commands to review IPC
information
– 'ipcs' – Review SHM, SEM, MSG
– 'ipcrm' – clean up IPC objects
– 'rpcinfo' – Review RPC
– 'popen' – Create a PIPE
– 'mkfifo' – Create a FIFO pipe
 *Try 'man ipc'

30. + Inter Process Communication
IPC
Exercise
– Review IPC information on your system
Are there any SHMs ?
– Review RPC Services available on your
system
– Create a named pipe. Try to use it to pass a
message between two processes
(e.g: bash)
Can you name any advantages of using
fifo ?

31. + Linux Kernel
Kernel Tune able parameters
Changing the kernel's behaviour when working
with processes, IPC, etc
Tune able parameters can be used via the '/proc'
filesystem or the 'syscall' command

32. + Linux Kernel
Kernel Tune able parameters
General parameters
– 'ctrl-alt-del': sets the kernel reaction to
'ctrl-alt-del' key sequence
– 'domainname': sets machine's domain
name
– 'hostname': sets machine's host name
– 'panic': sets timer before restart after panic
– 'pid_max': sets maximum PID number

33. + Linux Kernel
Kernel Tune able parameters
IPC parameters
– 'shmax': maximum size of shared memory
segment (bytes)
– 'shmall': total amount of shared memory
pages system wide
– 'shmmni': maximun number of shared
memory segments system wide

34. + Linux Kernel
Kernel Tune able parameters
IPC parameters
– 'sem': holds 4 parameters
• maximum semaphores per array
• total maximum semaphores
• maximum operations per semaphore
operation call
• total maximum arrays

35. + Linux Kernel
Kernel Tune able parameters
IPC parameters
– 'msgmni': maximun message queues
system wide
– 'msgmax': maximum size of message
(bytes)
– 'msgmnb': maximum size of queue (bytes)