How can a linux kernel be so small?

Question

An ordinary stable 3* kernel is about 70 mb now. But there are little linux distributions of 30-10 mb with software and other stuff running out of the box.

How can the linux kernel be so small in these distros? Can stripping down only unneeded modules and drivers dramatically reduce the size? What else is done to reduce the kernel size?

Answer 1

Yes. If you strip them down (compile/make modules only things that you need), compress the kernel (optimize for size)... it'll result in such a small kernel. You can make an even smaller ones, for targeted system. Like I know my PC, I know my stuff. So I'll just compile what I need. Like my own SATA drivers, driver for USB, etc. Nothing else. No webcam, no old IDE drives, nothing.

Also: The bigger ones you encounter are the WHOLE kernel (which one never needs to use). Even the desktop ones contains (most of the times, as modules) a LOT OF unneeded stuff. But newbies expect that every kind of hardware they plug-in will just work.

(Note that, the modules won't get loaded until they are needed. There are 3 states:
[ ] - Won't get compiled, won't be part of your kernel.
[M] - Will be compiled as a module. (You can modprobe it, or your system will auto-load it when it's needed. Depends on the system, userland.)
[X] - It'll be compiled AND always loaded.)

Distros with small kernel:
http://distro.ibiblio.org/tinycorelinux/welcome.html
http://puppylinux.org/main/Overview%20and%20Getting%20Started.htm

Answer 2

Early Linux distributions such as Tom's Root-Boot, MuLinux, Diet-router, (the now defunct) LOAF and many others now departed, all fitted a working Linux system on to one or two 1.44 MB diskettes.

The Linux kernel has grown but don't forget it is modular, kernel modules are loaded as needed. Thus it is still possible to produce a Linux distribution with a very small footprint.

See http://elinux.org/Kernel_Size_Tuning_Guide

If you read http://elinux.org/images/7/72/Linux_tiny.pdf you will see this

• historic 0.99pl15 kernel: Slackware 1.1.2, 1994 301K
• Fedora Core 2 1.2M
• SuSE 9.1 1.5M
• 2.6.5-tiny1 test config: IDE, ext2, TCP, NIC 363K

Answer 3

Most major distros configure their kernels with maximum hardware support so it will work out of the box with most computers and anything you plug in later on. They also use an initrd system which is not needed in most cases. There are also options in the kernel configuration that make the build process optimize for size, and a better (though slower) compression method can be selected as well. Lastly, if you are looking at the size of the kernel source code, keep in mind that it also includes support for several architectures beyond x86/amd64, network protocols, file systems, and other features that you'll probably never use and that can be disabled.

Answer 4

It is quite simple, a minimal system just need bootloader + kernel though it is almost useless. Most of commands used everyday, are optional binary for the system, like sh, ls, etc. If you are interesting in internals, http://www.linuxfromscratch.org/ is a good start point.

Answer 5

The default kernel configuration is configured to support as many hardware as possible. A non-stripped kernel with default configuration resulted in a size of 1897996 kB (including kernel + modules). When stripping many unnecessary drivers and options (why would I need a HP module if I own a Clevo notebook), it resulted in a size of 892892 kB which is a size reduction of 53% compared to the stock kernel.

When installing the kernel modules, append the INSTALL_MOD_STRIP=1 option. This will strip all debugging symbols and reduced the size by 92% for me (from 892892 kB to 69356 kB). Note this will only affects modules to be installed and not the kernel (vmlinuz) itself:

make INSTALL_MOD_STRIP=1 modules_install

From Documentation/kbuild/kbuild.txt:

INSTALL_MOD_STRIP
--------------------------------------------------
INSTALL_MOD_STRIP, if defined, will cause modules to be
stripped after they are installed.  If INSTALL_MOD_STRIP is '1', then
the default option --strip-debug will be used.  Otherwise,
INSTALL_MOD_STRIP value will be used as the options to the strip command.