Linux Performance Tuning Training Kernel Module

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
P
H
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)
4
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)