From: eLinux.org

Kernel Size Tuning Guide

This document describes how to configure the Linux kernel to use a small
amount of memory and flash.

Note: This
document is a work in progress. Please feel free to add material
anywhere you have additional information or data. Sections of this
document which need additional work are denoted with [FIXTHIS] markers.

Contents

• 1 Introduction
• 2 Measuring the kernel
  • 2.1 Measuring the kernel image
    size
  • 2.2 Measuring the kernel text, data and bss
    segments
  • 2.3 Measuring and comparing sub-parts of the
    kernel
    • 2.3.1 Measuring major kernel
      subsystems
    • 2.3.2 Measuring individual kernel
      symbols
    • 2.3.3 Comparing kernel symbols between two kernel
      images
• 3 Kernel Size Tuning features
  • 3.1 Linux-tiny patches
  • 3.2 How to configure the kernel
  • 3.3 Kernel Configuration
    Options
  • 3.4 Special Instructions for some kernel
    options
    • 3.4.1 How to use CONFIG-PRINTK
  • 3.5 Booting without SysFS
  • 3.6 Booting without /proc fs
  • 3.7 Using kernel memory measurement
    features
    • 3.7.1 Kmalloc Accounting
    • 3.7.2 Bootmem Auditing
    • 3.7.3 Counting Inlines
• 4 Outline
• 5 References
• 6 Appendices
  • 6.1 Appendix A - Sample minimum configuration for
    ARM
  • 6.2 Appendix B - Configuration Option
    Details
  • 6.3 Appendix C - Things to
    research

Introduction

One big problem area when using Linux in an embedded project is the size
of the Linux kernel.

Measuring the kernel

There are 3 aspects of kernel size which are important:

the size of the kernel image stored in flash (or other persistent
storage)

the static size of kernel image in RAM (usually, this will be the size
of the uncompressed image)

• This includes the text, data, and BSS segments of the kernel at the
  time it is loaded. The text and BSS segments will stay the same size
  for the kernel throughout it execution. However, the data and stack
  segments may grow according to the needs of the system.

the amount of dynamic RAM used by the kernel.

• This will fluctuate during system execution. However, there is a
  baseline amount of memory which is allocated at system startup.
  Application-specific RAM can be calculated to be above this minimal
  amount of required RAM.

For now, this document ignores Execute-In-Place (XIP) and
Data-Read-In-Place (DRIP) techniques, the use of which have an impact on
the amount of flash and RAM used by the kernel. See the following online
resources for more information about these techniques: Kernel
XIP and Data Read In
Place

Measuring the kernel image size

The compressed kernel image is what is stored in the flash or ROM of the
target device. The size of this image can be obtained by examining the
size of the image file in the host filesystem with the ‘ls -l‘
command:

• for example: ‘ls -l vmlinuz‘ or ‘ls -l bzImage‘ (or whatever the
  compressed image name is for your platform.)

Measuring the kernel text, data and bss segments

Use the size command to determine the size of the text, data, and BSS
segments of a kernel image.

Note that the BSS segment is not stored in the kernel image because
it can be synthesized at boot time by filling a block of memory with
zeros. Note also that portions of the kernel text and data are set aside
in special initialization segments, which are discarded when the kernel
finishes booting. Because of these factors, the size command does not
give you an exactly correct value for the static kernel RAM size.
However, it can be used as a reasonable estimate.

To use the size command, run it with the filename of the uncompressed
kernel image (which is usually vmlinux).

• for example: ‘size vmlinux‘

Example output:

   text    data     bss     dec     hex filename
2921377  369712  132996 3424085  343f55 vmlinux

Measuring and comparing sub-parts of the kernel

In order to find areas where the kernel size can be reduced, it is often
useful to break down the static size of the kernel by sub-system or by
kernel symbol. The following sections describe how to see the size of
each kernel sub-system, how to see the size of individual kernel
symbols, and how to compare the size of symbols between two kernel
versions. This is useful because as you make changes to the kernel
configuration you can determine what part of the kernel is affected by
the change. From this information you may be able to predict what the
affect of the change will be, and decide whether the change is
acceptable.

Measuring major kernel subsystems

The major sub-systems of the kernel are put into library object files
named built-in.o in the corresponding sub-directory for that
sub-system within the kernel build directory. The major sub-directories,
at the time of this writing (for kernel 2.6.17) are:
init, user, kernel, mm, fs, ipc, security, crypto, block, ltt, drivers, sound, net, lib

To see the size of the major kernel sections (code, data, and BSS), use
the size command, with a wildcard for the first level of
sub-directory:

• size */built-in.o

You can pipe this output through sort to sort by the largest
libraries:

• size */built-in.o | sort -n -r -k 4

Example output:

 731596   53144   33588  818328   c7c98 drivers/built-in.o
 687960   24972    2648  715580   aeb3c fs/built-in.o
 547844   19508   28052  595404   915cc net/built-in.o
 184072    6256   32440  222768   36630 kernel/built-in.o
 141956    3300    2852  148108   2428c mm/built-in.o
  68048    1804    1096   70948   11524 block/built-in.o
  26216     768       0   26984    6968 crypto/built-in.o
  17744    2412    2124   22280    5708 init/built-in.o
  20780     292     124   21196    52cc ipc/built-in.o
  18768      68       0   18836    4994 lib/built-in.o
   2116       0       0    2116     844 security/built-in.o
    134       0       0     134      86 usr/built-in.o
   text    data     bss     dec     hex filename

To see even greater detail, you can examine the size of built-in.o
files even deeper in the kernel build hierarchy, using the find
command:

• find . -name "built-in.o" | xargs size | sort -n -r -k 4

Example output:

 731596   53144   33588  818328   c7c98 ./drivers/built-in.o
 687960   24972    2648  715580   aeb3c ./fs/built-in.o
 547844   19508   28052  595404   915cc ./net/built-in.o
 260019    9824    4944  274787   43163 ./net/ipv4/built-in.o
 184072    6256   32440  222768   36630 ./kernel/built-in.o
...

Note: Please be careful interpreting the results from the size of the built-in.o files in sub-directories. In general, the object files are aggregated into the libraries of parent directories, meaning that many object files will have their size counted twice. You cannot simply add the columns for an indication of the total kernel size

Measuring individual kernel symbols

You can measure the size of individual kernel symbols using the ‘nm’
command. Using the nm --size -r vmlinux

[tbird@crest ebony]$ nm --size -r vmlinux | head -10
00008000 b read_buffers
00004000 b __log_buf
00003100 B ide_hwifs
000024f8 T jffs2_garbage_collect_pass
00002418 T journal_commit_transaction
00002400 b futex_queues
000021a8 t jedec_probe_chip
00002000 b write_buf
00002000 D init_thread_union
00001e6c t tcp_ack

Legend: The columns of this output are:

1. size in bytes (in hexadecimal)
2. symbol type
3. symbol name.

The symbol type is usually one of:

• ‘b’ or ‘B’ for a symbol in the BSS segment (uninitialized data),
• ‘t’ or ‘T’ for a symbol in the text segment (code), or
• ‘d’ or ‘D’ for a symbol in the data segment.

Use ‘man nm‘ for additional information on the ‘nm‘ command.

Comparing kernel symbols between two kernel images

Use the bloat-o-meter command, found in the kernel source scripts
directory, to compare the symbol sizes between two kernel images.

• \/scripts/bloat-o-meter vmlinux.default vmlinux.altconfig

If you get an error: ‘chmod a+x \/scripts/bloat-o-meter’

Example output, comparing a baseline kernel to one configured with
CONFIG_PRINTK=n:

[] $ ../../linux/scripts/bloat-o-meter vmlinux.baseline vmlinux.no-printk
add/remove: 5/23 grow/shrink: 8/1541 up/down: 1141/-199824 (-198683)
function                                     old     new   delta
proc_ioctl_default                             -     610    +610
proc_reapurb                                   -     296    +296
proc_disconnectsignal                          -      88     +88
proc_releaseinterface                          -      72     +72
proc_claiminterface                            -      36     +36
xprt_adjust_cwnd                             169     182     +13
do_timer                                    1052    1063     +11
i8042_controller_reset                        78      84      +6
serio_init                                   167     172      +5
usb_exit                                      80      81      +1
early_uart_console_init                       45      46      +1
console_unblank                              103     104      +1
console_conditional_schedule                  21      22      +1
parse_early_param                            102     101      -1
machine_emergency_restart                    249     248      -1
console_callback                             239     238      -1
arch_align_stack                              45      44      -1
quirk_p64h2_1k_io                            183     181      -2
printk_time                                    4       -      -4
printk_cpu                                     4       -      -4
oops_timestamp.7                               4       -      -4
neigh_resolve_output                         733     729      -4
msg_level.4                                    4       -      -4
...
de_dump_status                             1586     313    -1273
decode_getfattr                             3156    1748   -1408
ext3_fill_super                             5980    4545   -1435
usbdev_ioctl                                6476    4846   -1630
usb_get_configuration                       4001    1878   -2123
proc_submiturb                              2294       -   -2294
__log_buf                                 131072       - -131072

Kernel Size Tuning features

The Linux kernel includes a number of options for to control the
features and options it supports. The kernel, over time, has accumulated
a large set of features and capabilities. But many features are not
needed in Consumer Electronics products. By carefully tuning the kernel
options, you can omit many parts of the kernel and save memory in your
product.

Linux-tiny patches

The Linux-tiny patch set is a set of patches maintained by Matt Mackall
developed with the intent to help a developer reduce the size of the
Linux kernel.

These patches are described at: Linux Tiny

The Linux-tiny patch set includes a number of different patches to allow
the kernel to be reduced in size. Sometimes, the size reductions are
accomplished by reducing the number of objects for a particular features
(like the number of possible swap areas, or the number of tty discipline
structures). Sometimes, the size reductions are achieved by removing
features or functions from the kernel.

Here is a list of the individual Linux-tiny patches that are available
for the 2.6.22 kernel at Linux Tiny Patch
Details

Please note that the last patch in this list (“do-printk”) is available
separately from the main Linux-tiny patch set. Please find this patch
at: Do Printk

The patches listed in this table represent patches that can be applied
to a 2.6.16 Linux kernel. However, as of version 2.6.16, many options
for reducing the kernel were already available in Linux. A list of
options, both from these patches and from existing code, which are
interesting for tuning the kernel size is provided in the section:
“Kernel configuration Options”

How to configure the kernel

[FIXTHIS - need detailed kernel configuration instructions]

• use ‘make menuconfig’
• perform thorough testing of your library and applications with the
  smaller config
• development vs. deployment configurations
• describe all_no config - most times it won’t boot.

Kernel Configuration Options

Here is a table of kernel configuration options, including a
description, the default value for a kernel, and the recommended value
for a smaller configuration of the kernel:

Legend:

• “Y *“ - Set to ‘Y’ for measurement during development, and set to
  ‘N’ for deployment.
• “N +” - Whether you can set this to ‘N’ depends on whether this
  feaure is needed by your applications.
• “Y -“ - You probably need this, but it might we worth checking to
  see if you don’t.

Special Instructions for some kernel options

How to use CONFIG_PRINTK

If the “do-printk” patch is applied, there are two options which control
the compilation of printk elements in the kernel: CONFIG_PRINTK_FUNC
and CONFIG_PRINTK. You can use these options to control how much printk
support the kernel provides, and to control on a global basis whether
any printk messages at all are compiled into the kernel. Another special
preprocessor variable is also available, called DO_PRINTK, which
provides the ability to enable printk messages inside a single C
compilation unit, even if printk messages are disabled globally.

This section explains how to use these features to reduce the kernel
size, while still enabling sufficient printk messages to be useful
during development and deployment.

The CONFIG_PRINTK option disables all of the kernel printk calls. By
setting this option to ‘N’ in your kernel configuration, all uses of
“printk” throughout the kernel source are turned into empty statements,
and omitted when the program is compiled. This provides a substantial
size savings, since the kernel messages often account for more than 100
kilobytes of space in the kernel image. Setting this option to ‘N’ will
not, however, remove the actual

printk

code itself (just the calls to

printk

). The CONFIG_PRINTK_FUNC option controls whether the

printk

function and various helper functions are compiled into the Linux
kernel. When this is set to ‘N’, CONFIG_PRINTK is automatically set to
‘N’, and no printk messages are compiled into the kernel. This usually
saves about another 4K of size in the kernel image.

By using both CONFIG_PRINTK and CONFIG_PRINTK_FUNC, you can reduce
the size of the kernel image (and that flash and RAM it requires).
However, there is a drawback to eliminating all the messages. Obviously,
it is then not possible to get any status, diagnostic or debug messages
from the kernel! Another mechanism is available, which allows you to
control on a per-file basis which printk calls are compiled into the
kernel. This is the pre-processor variable DO_PRINTK.

To use DO_PRINTK, set CONFIG_PRINTK to ‘N’ and CONFIG_PRINTK_FUNC to
‘Y’ in your kernel configuration. This will globally disable all printk
calls in the kernel. Now, determine the C files where you wish to enable
printk messages, and add the line:

#define DO_PRINTK 1

at the top of each file. Now, the printk calls in those files will be
compiled normally. Printk calls in other modules will be omitted.

- Important
Note: The DO_PRINTK variable controls how the preprocessor will treat
printk statements in the code. For this reason, this statement MUST
appear at the top of the file, before any

#include

lines. In order to change the set of printk messages preserved in the
code, you will need to modify the

DO_PRINTK

lines, and recompile the kernel. (There is no runtime control of the
printk calls.) This is a simple mechanism, but it does provide a way to
omit most of the printk messages from the kernel while still
preserving some messages that may be useful during

development or on a deployed product.

In review, there are basically 3 different settings combinations for
CONFIG_PRINTK_FUNC and CONFIG_PRINTK that make sense:

Booting without SysFS

(copied from linux-tiny wiki)

Turning off sysfs support can save a substantial amount of memory in
some setups. One big downside is that it breaks the normal boot process
because the kernel can no longer mapa symbolic device name to the
internal device numbers.

Thus, you will need to pass a numeric device number in hex. For example,
to boot off /dev/hda1, which has major number 3 and minor 1, you’ll need
to append a root== option like this:

/boot/vmlinuz root==0x0301 ro

Booting without /proc fs

It is also possible to boot with

/proc

fs, but many programs expect this psuedo-filesystem to be present and
mounted. For example,

free

and

ps

are two commands which retrieve information from

/proc

in order to run.

list some workarounds here

Using kernel memory measurement features

FIXTHIS - need
instruction on bootmem auditing and counting inlines - need more detail
for kmalloc accounting

Kmalloc Accounting

This is a features of Linux-tiny, which tracks callers of kmalloc and
kfree, and produces summary statistics for kernel memory allocations, as
well as detailed information about specific kmalloc callers.

This was first published by Matt Mackall in February of 2005, but was
not mainlined at that time.

To see results for kernel allocations, follow these steps:

• turn on the CONFIG_KMALLOC option. This will show up on the kernel
  configuration menus as “Enabled accounting of kmalloc/kfree
  allocations?”
• recompile your kernel
• boot the kernel
• periodically, examine the accounting stats
  • cat /proc/kmalloc

See http://lwn.net/Articles/124374/

Bootmem Auditing

Counting Inlines

Outline

FIXTHIS - need
to review outline and fill in missing material

• Tuning the kernel

• how to measure kernel size

• in-kernel size reporting - kmalloc accounting

• bloat-o-meter

• kernel configuration options

• mainline options

• optional features
• minimal config

• sufficient API?

• POSIX compliance

• LSB compliance

• LTP compliance

• file systems

• comparison of file system sizes

• compiler options for reducing size

• gcc -os

• gcc -whole-program

• online resources:

• bloatwatch

• kconfigsize

References

• Linux-tiny project web site:
  [linux-tiny]
• eLinux wiki Linux-tiny page: Linux Tiny
• Matt Mackall’s
  http://elinux.org/images//8/83/Pdf.gif
  Linux-tiny
  presentation
  http://elinux.org/images/d/da/Info_circle.png
• CE Linux Forum resources for reducing system size: System
  Size

Appendices

Appendix A - Sample minimum configuration for ARM

[FIXTHIS - need ARM minimum config.]

Appendix B - Configuration Option Details

Want to fill in
this section with details about configuration options.

For each option, would like to document:

• what is size affect for different option values

• This page & Kernel Size Tuning Guide Config Option
  Impact
  describe kernel size and RAM usage impact affected by each
  configuration option listed in “Kernel Configuration Options” above,
  on i386.

• what is affect of performance, functionality, etc.

• what programs (if any) will stop working if option is turned off (or
  reduced)

Appendix C - Things to research

• miniconfigs
• how to use an initramfs (to avoid using NFS-mounted rootfs)
• how to use a local fs (to avoid using NFS-mounted rootfs)

• Eric Biederman’s turning off CONFIG_BLOCK - will any FS work after
  this??

• he got a 2.6.1 kernel (presumably all_no) to: “191K bzImage and a
  323K text segment”. See
  here.

• why is networking so big??

• why are file systems so big??
• capture serial output from kernel for size measurement (see
  grabserial program)

Categories:

• Kernel Size Tuning
  Guide
• System Size