Optimize Embedded Linux Boot Time Performance · The core Linux OS has a simple micro-kernel architecture. It provides a modular block to build a custom embedded system. The File

Optimize Embedded Linux Boot Time Performance

WHITE PAPER

©2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author. All rights reserved.

TABLE OF CONTENTS

Abstract ............................................................................................. 3

What is “Embedded Linux”? ............................................................. 4

What does Embedded Linux offer? .................................................. 4

What is Boot Time? ........................................................................... 5

Boot Sequence .................................................................................. 6

Optimizing Boot Time in an Embedded Linux System ...................... 6

Reducing Kernel Boot Time Thereby Increasing Speed ................... 7

Increasing System Startup Time and Application Speed .................. 8

Reducing the Disk Footprint and RAM Size of the Linux Kernel ....... 9

Reducing Application Size and RAM Usage..................................... 10

Reducing Power Consumption ........................................................ 10

Conclusion ....................................................................................... 11

Reference ........................................................................................ 12

Author Info .................................................................................... 112


3

Abstract

Almost everyone has heard of Linux. It is open source, and has

been much in discussion in the software development and system

administration communities. Linux was originally targeted

specifically for desktop PCs running an Intel 80x86 or compatible

microprocessor.

Linus Torvalds, the inventor of Linux, had said, “Linux will never run

on anything but a PC with an IDE hard disk since that is all that I

have”. At present, Linux is ported to different microprocessors and

runs on multiple platforms that don’t have a hard disk. Most of

these devices are not general purpose computer systems and

include devices such as network routers and data collection units

for tracking tornadoes. It is these types of systems that have

collectively been labeled as “Embedded Linux”.

Linux has won the race in the embedded device segment. By

offering enormous advantages, Linux has crossed every hurdle to

enter embedded systems across a wide variety of processor

architectures. This has led developers to raise questions about how

they could get more performance out of their embedded Linux

design, which made “reducing boot time” one of the hot topics of

discussion in the embedded Linux community. It is not necessary to

invent an engineering resource to save boot time. It is possible to

substantially reduce boot time by applying certain system

configurations.

This whitepaper presents an insight into embedded Linux and a

typical boot sequence. It also covers various techniques that can be

used to optimize boot time.


4

What is “Embedded Linux”?

Linux is itself referred to as a kernel but in daily usage it rarely

means so. Embedded Linux generally refers to a complete Linux

distribution aimed at embedded devices. In reference to

embedded development, there is a Linux kernel and other software

or an embedded Linux distribution which contains a package of

applications and development tools meant for embedded systems.

Linux was originally designed to be a general-purpose operating

system (OS). Many approaches have commenced to convert the

Linux OS into an embedded OS.

There are two ways to switch to embedded Linux:

1. Solutions provided and supported by vendors like

MontaVista, Wind River or TimeSys contain their own

development tools and environments, and use a few open-

source components and proprietary tools.

2. Using community solutions that are open and supported

by the community.

With Linux, the dimension of computing applications increases, and

this includes IBM's tiny Linux wrist watch, hand-held devices (PDAs

and cell phones), Internet appliances, robotics, telephony

infrastructure equipment, and much more.

What does Embedded Linux offer?

Though most Linux systems run on PC platforms, it can be a reliable

OS for embedded systems. Linux is easier and more flexible to

install and administer. Its “back-to-basics” approach is an added

advantage.

Though a fully featured Linux kernel requires about 1 MB of

memory, the Linux micro-kernel actually consumes very little of

http://www.engineersgarage.com/contribution/what-is-kernel-programming


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this memory - around 100K on a Pentium CPU, which also includes

a virtual memory and all core operating system functions. It is

considered a lightweight operating system.

The core Linux OS has a simple micro-kernel architecture. It

provides a modular block to build a custom embedded system. The

File system is on the top of the micro-kernel. Drivers can be either

compiled or added to the kernel at run-time.

Linux supports multiprocessor systems and is best-suited for

embedded Internet devices. It offers scalability that opens an

option of running a real-time application on a dual processor

system.

In various market segments like consumer electronics, products are

such that the device needs to be available for immediate use after

power on. An OS for desktops offers boot times in the range of 20

seconds to a few minutes. This is highly unacceptable for various

market segments. For a device to qualify for an embedded market,

it must be available for use as soon as it is turned on.

What is Boot Time?

Boot time is the time taken by the system to make it available from

the time the power button is pressed. Boot time is one of the

major factors in defining the usability of the system.

The definition of system availability is different for different

devices:

For display devices like cellphones – the appearance of

home

For display of shell prompt on development systems with

consoles

There is no set criterion to measure boot time. In fact, boot time

must be measured in the context of the device and its usage.


6

Boot Sequence

Optimizing Boot Time in an Embedded Linux

System

Many factors are responsible for boot time performance. This calls

for a set of techniques to be applied to optimize the boot time of a

Linux system.

We are using the term 'optimizing' - instead of 'reducing'. as

‘reducing’ means achieving less boot time by taking custom

shortcuts, which are difficult to maintain across component

versions. Optimization methods are generic in nature and are easy

to maintain across different versions.

Power Applied

Pwr/CLK/Reset

Load and execute BootLoader

Load and execute Kernel

Kernel Init

Locate/Load/Mount RootFS

Init User Space

Application start


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Areas of optimization

Optimization methods fall under the following categories:

1. Size

Reducing the size of binaries for each component

loaded

Removing features that are not required

2. Speed

Optimizing the target processor

Reducing the number of steps for booting

Removing features that are not required

To begin with optimizing the boot time of a device:

Identify the current boot-time

Set the target environment (processor)

Define boundary conditions that are specific to the

end-product

Optimization is an iterative process. To measure the boot time, a

reliable source is required. We have listed a set of techniques that

can be applied in order to achieve an optimized boot-time for an

embedded Linux system.

Reducing Kernel Boot Time and thereby Increasing

Speed

1. Disable IP auto-config – Instead of initializing the IP address

on the kernel command line, use the /etc/init.d/rcS script.

2. The output of kernel bootup messages to the console

consumes time. This is normally not needed in the

production environment. Disable unwanted console output

messages using the “quiet” argument in the Linux kernel

bootloader settings.


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Example:

root=/dev/ram0 rw init=/startup.sh quiet

3. Directly boot from the bootstrap code to directly load and

execute the Linux kernel image instead of the Uboot.

4. Reduce the kernel size by removing things like features,

debugging facilities, etc., which are not from the dedicated

system. A smaller kernel loads fast and also executes fast.

This reduces the time spent on initializing drivers.

5. Spare memory at boot time by skipping the memory

allocation part and manage it yourself.

Example: there is 32 MB of RAM

Boot your kernel with mem=30

The remaining 2 MB can be claimed by the driver code

buf = ioremap (

0x1e00000, /* Start: 30 MB */

0x200000 /* Size: 2 MB */

);

This saves a critical amount of time allocating memory.

Increasing System Startup Time and Application

Speed

1. SysV vs Init – SysV starts the services sequentially. It waits

for the current startup script to be completed before

starting the next one. In case of Init, if the pre-requisites

are fulfilled, it starts the services in parallel. System

utilization is also better in the case of Init.

2. Compiler speed optimizations – Most tools are compiled

using compiler optimizations. “O2” is the most common

optimization switch of gcc. “O3” can also be used to

optimize speed for critical executables.


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3. Prelink – For executables using shared libraries, it takes a

lot of time to load and start an executable. The dynamic

linker has to perform a significant amount of work mainly

in terms of address relocation. It has to perform the same

steps over and over again each time the executable is

loaded. In order to reduce this, “prelink” modifies

executables and shared libraries. This reduces the memory

relocation task of the dynamic linker. This, in turn reduces

the loading and startup time for large applications using

many shared libraries. This also reduces the memory

consumed by relocations. Prelink needs to be run every

time the executable is updated.

4. Choose appropriate file systems to run fast – Use

SquashFS which is a compressed read-only file system. For

NAND flash storage, use UBIFS - the successor of JFFS2.

5. Avoid system calls which are heavy – Because of heavy

system calls, calls to executables from the shell becomes

slow. fork / exec system calls are very heavy. The

underlying implementation of Pipes is done using fork /

exec. To reduce the delay, avoid their usage in scripts.

Example:

cat /etc/bin | grep sysfile

Replace it with: grep sysfile /etc/bin

Reduce the Disk Footprint and RAM Size of the

Linux Kernel

1. Tiny Linux – Select the features in kernel configuration

(CONFIG_EMBEDDED). Remove unnecessary kernel

messages (printk, BUG). Certain features that are not

needed in some systems can be removed. Use the


10

compiling optimizations for size. A smaller kernel

executable also saves RAM.

2. Replace initrd by initramfs – This reduces overheads and

RAM waste. It allows no block and file system overhead

and also no duplication in RAM.

Reduce the Application Size and RAM Usage

1. Static vs Dynamic linking – In static linking, all shared

library code is duplicated in the executables whereas in

dynamic linking, shared library code is not duplicated in the

executables. This makes the executable smaller in size.

Static linking is useful when there are few executables.

Synamic linking is best for medium to big systems (> 500 KB

1MB)

2. Using a light C library – Use uClibc instead of glibc. uClibc

exists in embedded Linux systems.

3. “strip” executables – Executables and libraries contain

extra information needed to identify problems in a

debugger. This is generally used by the developer but is of

no use to an end user. The “Strip” command is used to

remove debugging information. This helps in saving a

significant amount of space.

Example :

gcc -o test test.c (output size: 4740 bytes)

strip test (output size: 2760 bytes)

Reducing Power Consumption

1. Tickless system – The processor receives a timer interrupt

at a given frequency (every 4 ms by default on Linux 2.6).

This is the way multitasking is achieved. If the system is


11

idle, disable the timer interrupt and re-enable it when an

event occurs. This saves power in embedded devices.

2. Cpufreq – This enables the user to change the CPU

frequency on the fly. Supported architectures (2.6.20):

i386, sh, ia64, sparc64, x86_64, powerpc, arm (i.MX only).

This is done from userspace through /sys by a user

configurable governor process.

Conclusion

There are many techniques that developers and system designers

can employ in order to meet early boot requirements. To start with,

it is good to get a stable baseline measurement of system boot

speed. Then start with applying any of the techniques described

here. This way, one can be sure that applying techniques helps in

achieving real progress towards the goal.


12

References

http://www.ibm.com/developerworks/library/l-embl/

http://www.embedded.com/electronics-blogs/open-

mike/4420567/Learning-Linux-for-embedded-systems

Author Info

Harsha Vachher

Harsha has been working with HCL’s ERS Practice as a Senior

Technical Lead, helping to drive forward a number of new internal

initiatives.

http://www.ibm.com/developerworks/library/l-embl/

http://www.embedded.com/electronics-blogs/open-mike/4420567/Learning-Linux-for-embedded-systems

http://www.embedded.com/electronics-blogs/open-mike/4420567/Learning-Linux-for-embedded-systems

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Optimize Embedded Linux Boot Time Performance · The core Linux OS has a simple micro-kernel architecture. It provides a modular block to build a custom embedded system. The File