Describe the Linux Kernel internal core mechanism;

1. Linux Internals
Core / Kernel
mailto:shayco@gmail.com

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

3. Kernel
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H H H
Kernel

4. Linux Kernel Architecture
Monolithic Kernel
http://en.wikipedia.org/wiki/Monolithic_kernel
Supports for System Calls
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
http://www.makelinux.net/kernel_map/

8. Linux Kernel and Processes
Linux Kernel and Processes
The Linux Kernel supports pre-emptive multitasking
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 transit 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
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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)