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<title>Linux Kernel Documentation</title>

<h2>Linux Kernel Documentation Index</h2>

<p>This page collects and organizes documentation about the Linux kernel, taken
from many different sources.  What is the kernel, how do you build it, how do
you use it, how do you change it...</p>

<p>This is a work in progress, and probably always will be.  Please let us know
on the
<a href=>linux-doc</a> mailing
list (on about any documentation you'd like added to this
index, and feel free to ask about any topics that aren't covered here yet.
This index is maintained by Rob Landley &lt;;, and tracked in
<a href=>this mercurial repostiory</a>.  The
cannonical location for the page is <a href=>here</a>.</p>




<span id="Sources of documentation">

<p>These are various upstream sources of documentation, many of which are linked
into the <a href=>linux kernel documentation index</a>.</p>

<span id="From the kernel">
<li><a href=Documentation>Text files in the kernel's Documentation directory.</a></li>
<li><a href=htmldocs>Output of kernel's "make htmldocs".</a></li>
<li><a href=menuconfig>Menuconfig help</a></li>
<li><a href=readme>Linux kernel README files</a></li>
<li><a href=rfc-linux.html>IETF RFCs referred to by kernel source files.</a></li>
<span id="Out on the web">
<li><a href=xmlman>html version of man-pages package</a></li>
<li><a href=>Linux Weekly News kernel articles</a></li>
<li>Linux Device Drivers book (<a href=>third edition</a>) (<a href=>second edition</a>)</li>
<li><a href=ols>Ottawa Linux Symposium papers</li>
<li><a href=>Linux Journal archives</a></li>
<li><a href=>IBM Developerworks Linux Library</a> (also <a href=>here</a>)
<li><a href=>Linux Kernel Mailing List FAQ</a></li>
<li><a href=>Kernel Planet (blog aggregator)</a></li>
<li><a href=video.html>Selected videos of interest</a></li>
<li><a href=local>Some locally produced docs</a></li>

<span id="Standards">
<li><a href=>Single Unix Specification v3</a> (Also known as Open Group Base Specifications issue 6, and closely overlapping with Posix.  See especially <a href=>system interfaces</a>)</li>
<li>C99 standard (defining the C programming language): <a href=>ISO/IEC C9899 PDF</a> or <a href=>searchable website</a>.</li>
<li><a href=>Linux Foundation's specs page</a> (ELF, Dwarf, ABI...)</li>
</span id="Standards">

<span id="Translations">
<li><a href=>Linux Kernel Translation Project</a></li>
<li><a href=>Kernel Newbies regional pages</a></li>
<li><a href=>Japanese</a></li>
<li><a href=>Chinese</a></li>
</span id="Translations">

</span id="Sources of documentation">

<span id="Building from source">
  <span id="User interface">
<p>Building source packages is usually a three step process: configure, build,
and install.</p>

<p>The Linux kernel is configured with the command "make menuconfig", built
with the command "make", and installed either manually or with the command
"make install".</p>

tar xvjf linux-2.6.??.tar.bz2
cd linux-2.6.??
make menuconfig
make install

<p>For a description of the make options and targets, type <a href=makehelp.txt>
make help</a>.</p>

<span id="Get and extract the source">
<p>The Linux kernel source code is distributed by
<a href=></a> as tar archives.  Grab the most recent
"stable" release (using the tiny little letter F link) to grab a file of the
form "linux-2.6.*.tar.bz2" from the
<a href=>Linux 2.6 releases
directory</a>.  Then extract this archive with the command "tar xvjf
linux-2.6.*.tar.bz2".  (Type the command "man tar" for more information on the
tar command.)  Then cd into the directory created by extracting the archive.</p>

<p>To obtain other Linux kernel versions (such as development releases and
kernels supplied by <a href="#Distibutions">distributions</a>)
see <a href="Following_Linux_development">Following Linux development</a>.</p>

<p>To return your linux kernel source directory to its original (unconfigured)
condition after configuring and building in it, either either delete the
directory with "rm -r" and re-extract it from the tar archive, or run the
command "make distclean".</p>

<span id="Configuring">

<p>Before you can build the kernel, you have to configure it.  Configuring
selects which features this kernel build should include, and specifies other
technical information such as buffer sizes and optimization strategies.
This information is stored in a file named ".config" in the top level directory
of the kernel source code.  To see the various user interfaces to the
configuration system, type "<a href=makehelp.txt>make help</a>".</p>

<p>Note that "make clean" does not delete configuration information, but the
more thorough "make distclean" does.</p>

<span id="Using an existing configuration">

<p>Often when building a kernel, an existing .config file is supplied from
elsewhere.  Copy it into place, and optionally run "make oldconfig" to run
the kernel's diagnostics against it to ensure it matches the kernel version
you're using, updating anything that's out of sync.</p>

<p>Several preset configurations are shipped with the kernel source code.
Run the command <b>find . -name "*_defconfig"</b> in the kernel source
directory to seem them all.  Any of these can be copied to .config and
used as a starting point.</p>

<p>The kernel can also automatically generate various configurations,
mostly to act as starting points for customization:</p>
<li><b>make defconfig</b> - Set all options to default values</li>
<li><b>make allnoconfig</b> - Set all yes/no options to "n"</li>
<li><b>make allyesconfig</b> - Set all yes/no options to "y"</li>
<li><b>make allmodconfig</b> - Set all yes/no options to "y" and all "yes/module/no" options to "m"</li>
<li><b>make randconfig</b> - Set each option randomly (for debugging purposes).</li>
<li><b>make oldconfig</b> - Update a .config file from a previous version of the kernel to work with the current version.</li>

<span id="Creating a custom kernel configuration">
<p>The most common user interface for configuring the kernel is
<b>menuconfig</b>, an interactive terminal based menuing interface invoked
through the makefiles via "<b>make menuconfig</b>".  This interface groups the
configuration questions into a series of menus, showing the current values
of each symbol and allowing them to be changed in any order.  Each symbol
has associated help text, explaining what the symbol does and where to find
more information about it.  This help text is
<a href=menuconfig>also available as html</a>.</p>

<p>The menuconfig interface is controlled with the following keys:</p>

<li><b>cursor up/down</b> - move to another symbol</li>
<li><b>tab</b> - switch action between edit, help, and exit</li>
<li><b>enter</b> - descend into menu (or help/exit if you hit tab first)</li>
<li><b>esc</b> - exit menu (prompted to save if exiting top level menu)</li>
<li><b>space</b> - change configuration symbol under cursor</li>
<li><b>?</b> - view help for this symbol</li>
<li><b>/</b> - search for a symbol by name</li>

<p>Other configuration interfaces (functionally equivalent to menuconfig)
<li><b>make config</b> - a simple text based question and answer interface
(which does not require curses support, or even a tty)</li>
<li><b>make xconfig</b> - QT based graphical interface</li>
<li><b>make gconfig</b> - GTK based graphical interface</li>


    <span id="building">
      <span id="Building out of tree">
    <span id="Installing">
    <span id="running">
    <span id="debugging">
      <span id="QEMU">
    <span id="cross compiling">
<span id="Cross compiling vs native compiling">
<p>By default, Linux builds for the same architecture the host system is
running.  This is called "native compiling".  An x86 system building an x86
kernel, x86-64 building x86-64, or powerpc building powerpc are all examples
of native compiling.</p>

<p>Building different binaries than the host runs is called cross compiling.
<a href=>Cross
compiling is hard</a>.  The build system for the Linux kernel supports cross
compiling via a two step process: 1) Specify a different architecture (ARCH)
during the configure, make, and install stages.  2) Supply a cross compiler
(CROSS_COMPILE) which can output the correct kind of binary code.  An
example cross compile command line (building the "arm" architecture) looks

<pre>make ARCH=arm menuconfig
make ARCH=arm CROSS_COMPILE=armv5l-

<p>To specify a different architecture than the host, either define the "ARCH"
environment variable or else add "ARCH=xxx" to the make command line for each
of the make config, make, and make install stages.  The acceptable values for
ARCH are the names of the directories in the "arch" subdirectory of the Linux
kernel source code, see <a href="#Architectures">Architectures</a> for
details.  All stages of the build must use the same ARCH value, and building a
second architecture in the same source directory requires "make distclean".
(Just "make clean" isn't sufficient, things like the include/asm symlink need
to be removed and recreated.)</p>

<p>To specify a cross compiler prefix, define the CROSS_COMPILE environment
variable (or add CROSS_COMPILE= to each make command line).  Native compiler
tools, which output code aimed at the environment they're running in, usually
have a simple name ("gcc", "ld", "strip").  Cross compilers usually add a
prefix to the name of each tool, indicating the target they produce code for.
To tell the Linux kernel build to use a cross compiler named "armv4l-gcc" (and
corresponding "armv4l-ld" and "armv4l-strip") specify "CROSS_COMPILE=armv4l-".
(Prefixes ending in a dash are common, and forgetting the trailing dash in
CROSS_COMPILE is a common mistake.  Don't forget to add the cross compiler
tools to your $PATH.)</p>

      <span id="User Mode Linux">
  <span id="Infrastructure">
    <span id="kconfig">
<p>The Linux configuration system is called Kconfig.  The various
configuration front-ends (such as menuconfig) parse data files
written in the <a href=Documentation/kconfig-language.txt>Kconfig language</a>,
which define the available symbols and provide default values, help entries,
and so on.</p>

<p>The source code for the front ends is in scripts/kconfig.  The
Makefile in this directory defines the make targets for the configuration

    <span id="kbuild">
    <span id="build and link (tmppiggy)">

<span id="Installing and using the kernel">
  <span id="Installing">
    <span id="Kernel image">
    <span id="Bootloader">
  <span id="A working Linux root filesystem">
<p><a href=ols/2002/ols2002-pages-176-182.pdf>Advanced Boot Scripts</a></p>
    <span id="Finding and mounting /">
      <span id="initramfs, switch_root vs pivot_root, /dev/console">
    <span id="Running programs">
      <span id="init program and PID 1">
        <span id="What does daemonizing really mean?">
      <span id="Executable formats">
<p>The Linux kernel runs programs in response to the
<a href=xmlman/man3/exec.html>exec</a> syscall, which is called on a
file.  This file must have the
executable bit set, and must be on a filesystem that implements mmap() and which
isn't mounted with the "noexec" option.  The kernel understands
several different <a href="#executable_file_formats">executable file
formats</a>, the most common of which are shell scripts and ELF binaries.</p>
        <span id="Shell scripts">
<p>If the first two bytes of an executable file are the characters "#!", the
file is treated as a script file.  The kernel parses the first line of the file
(until the first newline), and the first argument (immediately following
the #! with no space) is used as absolute path to the script's interpreter,
which must be an executable file.  Any additional arguments on the first
line of the file (separated by whitespace) are passed as the first arguments
to that interpreter executable.  The interpreter's next argument is the name of
the script file, followed by the arguments given on the command line.</p>

<p>To see this behavior in action, run the following:</p>
<pre>echo "#!/bin/echo hello" > temp
chmod +x temp
./temp one two three

<p>The result should be:</p>
<blockquote>hello ./temp one two three</blockquote>

<p>This is how shell scripts, perl, python, and other scripting languages
work.  Even C code can be run as a script by installing the
<a href=>tinycc</a> package,
adding "#!/usr/bin/tcc -run" to the start of the .c file, and setting the
executable bit on the .c file.</p>
        <span id="ELF">
          <span id="Shared libraries">

      <span id="C library">
<p>Most userspace programs access operating system functionality through a C
library, usually installed at "/lib/*".  The C library wraps system
calls, and provides implementations of various standard functions.</p>

<p>Because almost all other programming languages are implemented in C
(including python, perl, php, java, javascript, ruby, flash, and just about
everything else), programs written in other languages also make use of the
C library to access operating system services.</p>

<p>The most common C library implementations for Linux are
<a href=>glibc</a>
and <a href=>uClibc</a>.  Both are full-featured
implementations capable of supporting a full-featured desktop Linux

<p>The main advantage of glibc is that it's the standard implementation used by the
largest desktop and server distributions, and has more features than any other
implementation.  The main advantage of uClibc is that it's much smaller and
simpler than glibc while still implementing almost all the same functionality.
For comparison, a "hello world" program statically linked against glibc is half
a megabyte when stripped, while the same program statically linked against
uClibc strips down to 7k.</p>

<p>Other commonly used special-purpose C library implementations include
<a href=>klibc</a> and
<a href=>newlib</a>.</p>

<span id="Exporting kernel headers">
<p>Building a C library from source code requires a special set
of Linux kernel header files, which describe the API of the specific version
of the Linux kernel the C library will interface with.  However, the header
files in the kernel source code are designed to build the kernel and contain
a lot of internal information that would only confuse userspace.  These
kernel headers must be "exported", filtering them for use by user space.</p>

<p>Modern Linux kernels (based on and newer) export kernel headers via
the "make headers_install" command.  See
<a href=Documentation/make/headers_install.txt>exporting kernel headers for
use by userspace</a> for more information.</p>
      <span id="Dynamic loader">
    <span id="FHS directories">
      <p>FHS spec</p>
      <a href="pending/hotplug.txt">populating /dev from sysfs</a>.

<span id="Reading the source code">
  <span id="Source code layout">
    <span id="Following the boot process">
    <span id="Major subsystems">
    <span id="Architectures">
  <span id="Concept vs implementation">
    <p>Often the first implementation of a concept gets replaced.
       Journaling != reiserfs, virtualization != xen, devfs gave way to udev...
       Don't let your excitement for the concept blind you to the possibility
       of alternate implementations.</p>
  <span id="Concepts">
    <span id="rbtree">
    <span id="rcu">
<p>RCU stands for "Read Copy Update".  The technique is a lockless way to manage data structures
(such as linked lists or trees) on SMP systems, using a specific sequence of reads and updates,
plus a garbage collection step, to avoid the need for locks in both the read and the update

<p>RCU was invented by Paul McKenney, who maintains an excellent page of
<a href=>RCU documentation</a>.
The Linux kernel also contains some <a href=Documentation/RCU>additional RCU

<p>RCU cannot be configured out of the kernel, but the kconfig symbol
<a href=menuconfig/lib-Kconfig.debug.html#RCU_TORTURE_TEST>CONFIG_RCU_TORTURE_TEST</a> controls the
<a href=Documentation/RCU/torture.txt>RCU Torture test module</a>.</p>

<li><a href=ols/2001/read-copy.pdf>Read-Copy Update</a> (OLS 2001)</li>


<span id="Kernel infrastructure">
  <span id="Process Scheduler">

<span id="History of the Linux Process Scheduler">
<p>The original Linux process scheduler was a simple design based on
a goodness() function that recalculated the priority of every task at every
context switch, to find the next task to switch to.  This served almost
unchanged through the 2.4 series, but didn't scale to large numbers of
processes, nor to SMP.  By 2001 there were calls for
change (such as the OLS paper <a href=ols/2001/elss.pdf>Enhancing Linux
Scheduler Scalability</a>), and the issue
<a href=>came to a head</a> in December 2001.</p>

<p>In January 2002, Ingo Molnar
<a href=>introduced the "O(1)" process scheduler</a> for the 2.5 kernel series, a design
based on separate "active" and "expired" arrays, one per processor.  As the name
implied, this found the next task to switch to in constant time no matter
how many processes the system was running.</p>

<p>Other developers (<a href=>such as Con Colivas</a>) started working on it,
and began a period of extensive scheduler development.  The early history
of Linux O(1) scheduler development was covered by the website Kernel

<p>During 2002 this work included
<a href=>preemption</a>,
<a href=>User Mode Linux support</a>,
<a href=>new drops</a>,
<a href=>runtime tuning</a>,
<a href=>NUMA support</a>,
<a href=>cpu affinity</a>,
<a href=>scheduler hints</a>,
<a href=>64-bit support</a>,
<a href=>backports to the 2.4 kernel</a>,
<a href=>SCHED_IDLE</a>,
discussion of <a href=>gang scheduling</a>,
<a href=>more NUMA</a>,
<a href=>even more NUMA</a>).  By the end of 2002, the O(1) scheduler was becoming
the standard <a href=>even in the 2.4 series</a>.</p>

<p>2003 saw support added for
<a href=>hyperthreading as a NUMA variant</a>,
<a href=>interactivity bugfix</a>,
<a href=>starvation and affinity bugfixes</a>,
<a href=>more NUMA improvements</a>,
<a href=>interactivity improvements</a>,
<a href=>even more NUMA improvements</a>,
a proposal for <a href=>Variable Scheduling Timeouts</a> (the first rumblings of what
would later come to be called "dynamic ticks"),
<a href=>more on hyperthreading</a>...</p>

<p>In 2004 there was work on <a href=>load balancing and priority handling</a>, and
<a href=>still more work on hyperthreading</a>...</p>

<p>In 2004 developers proposed several extensive changes to the O(1) scheduler.
Linux Weekly News wrote about Nick Piggin's
<a href=>domain-based scheduler</a>
and Con Colivas' <a href=>staircase scheduler</a>.  The follow-up article <a href=>Scheduler tweaks get serious</a> covers both.  Nick's scheduling domains
were merged into the 2.6 series.</p>

<p>Linux Weekly News also wrote about other scheduler work:</p>

<li><a href=>Filtered wakeups</a></li>
<li><a href=>When should a process be migrated</a></li>
<li><a href=>Pluggable and realtime schedulers</a></li>
<li><a href=>Low latency for audio applications:</a></li>
<li><a href=>Solving starvation problems in the scheduler:</a></li>
<li><a href=>SMPnice</a></li>

<p>In 2007, Con Colivas proposed a new scheduler, <a href=>The Rotating Staircase Deadline Scheduler</a>, which
<a href=>hit a snag</a>.  Ingo
Molnar came up with a new scheduler, which he named the
<a href=>Completely Fair Scheduler</a>,
described in the LWN writeups
<a href=>Schedulers: the plot thickens</a>,
<a href=>this week in the scheduling discussion</a>, and
<a href=>CFS group scheduling</a>.</p>

<p>The CFS scheduler was merged into 2.6.23.</p>

    <span id="fork, exec">
    <span id="sleep">
    <span id="realtime">
<p><a href=ols/2001/rtai.pdf>The Real-Time Application Interface</a> (OLS 2001, obsolete)</a></p>
  <span id="Timers">
    <span id="Interrupt handling">
  <span id="memory management">
    <li><a href="gorman">Understanding the Linux Virtual Memory Manager</a>, online book by Mel Gorman.</li>
    <li> What every programmer should know about memory, article series by Ulrich Drepper,
<a href=>one</a>,
<a href=>two</a>,
<a href=>three</a>,
<a href=>four</a>,
<a href=>five</a>.
    <li>Ars technica ram guide, article series by Jon "Hannibal" Stokes, parts
<a href=>one</a>,
<a href=>two</a>,
<a href=>three</a></li>
    <span id="mmap, DMA">
  <span id="vfs">
    <span id="Pipes, files, and ttys">
<p>A pipe can be read from or written to, transmitting a sequence of bytes
in order.</p>

<p>A file can do what a pipe can, and adds the ability to seek to a location,
query the current location, and query the length of the file (all of which are
an integer number off bytes from the beginning of the file).</p>

<p>A tty can do what a pipe can, and adds a speed (in bits per second)
and cursor location (X and Y, with the upper left corner at 0,0).  Oh, and
you can make it go beep.</p>

<p>Note that you can't call lseek() on a tty and you can't call termios
(man 3 termios) functions on a file.  Each can be treated as a pipe.</p>
    <span id="Filesystems">
      <span id="Types of filesystems (see /proc/filesystems)">
        <span id="Block backed">
<span id="ext2">
  <li><a href=ols/2002/ols2002-pages-117-129.pdf>Online ext2 and ext3 Filesystem Resizing</a> (OLS 2002)</li>
<span id="jffs2">
  <li><a href=ols/2001/jffs2.pdf>JFFS: The Journalling Flash Filesystem</a> (OLS 2001)</li>
<span id="vxfs">
  <li><a href=menuconfig/fs-Kconfig.html#VXFS_FS>CONFIG_VXFS_FS</a></li>
  <li><a href=ols/2002/ols2002-pages-191-196.pdf>Reverse engineering an advanced filesystem</a></li>
        <span id="Ram backed">
          <span id="ramfs">
          <span id="tmpfs">
        <span id="Synthetic">
          <span id="proc">
          <span id="sys">
          <span id="internal (pipefs)">
          <span id="usbfs">
          <span id="devpts">
          <span id="rootfs">
          <span id="devfs (obsolete)">
<p>Devfs was the first attempt to do a dynamic /dev directory which could change
in response to hotpluggable hardware, by doing the seemingly obvious thing of
creating a kernel filesystem to mount on /dev which would adjust itself as
the kernel detected changes in the available hardware.</p>

<p>Devfs was an interesting learning experience, but turned out to be the wrong
approach, and was replaced by sysfs and udev.  Devfs was removed in kernel
version 2.6.18.  See
<a href=local/hotplug-history.html>the history of hotplug</a> for details.</p>

        <span id="Network">
          <span id="nfs">
<p><a href=ols/2001/nfsv4_ols.pdf>Linux NFS Version 4: Implementation and Administration</a> (OLS 2001)</a></p>
          <span id="smb/cifs">
          <span id="FUSE">
      <span id="Filesystem drivers">
        <span id="Using">
        <span id="Writing">

  <span id="Drivers">
    <span id="Filesystem">
    <span id="Block (block layer, scsi layer)">
      <span id="SCSI layer">
	<li><a href="Documentation/scsi">Documentation/scsi</a> scsi.txt scsi_mid_low_api.txt scsi-generic.txt scsi_eh.txt</li>
        <li><a href="">SCSI Generic (sg) HOWTO</a></li>
	<li><a href="xmlman/man4/sd.html">man 4 sd</a></li>
        <li><a href="">SCSI standards</a></li>
        <li><a href=ols/2002/ols2002-pages-40-49.pdf>Incrementally Improving the Linux SCSI Subsystem</a> (OLS 2002)</li>
    <span id="Character">
      <span id="serial">
      <span id="keyboard">
      <span id="tty">
        <span id="pty">
      <span id="audio">
      <span id="null">
      <span id="random/urandom">
      <span id="zero">
    <span id="DRI">
    <span id="Network">

  <span id="Hotplug">
<p><a href=>Hotpluggable devices and the Linux kernel</a> (OLS 2001)</p>
<p><a href=local/hotplug-history.html>The history of hotplug</a></p>
  <span id="Input core">
  <span id="Network">
<p><a href=ols/2001/mipl.pdf>MIPL Mobile IPv6 for Linux in HUT Campus Network MediaPoli</a> (OLS 2001)</p>
<p><a href=ols/2001/sctp.pdf>Linux Kernel SCTP: The Third Transport</a> (OLS 2001)</p>
<p><a href=ols/2002/ols2002-pages-8-30.pdf>TCPIP Network Stack Performance in Linux Kernel 2.4 and 2.5</a></p>
  <span id="Modules">
    <span id="Exported symbols">
      <p>List of exported symbols.</p>
  <span id="Busses">
  <span id="Security">
    <span id="Traditional Unix security model">
Users, groups, files (rwx), signals.
    <span id="More complicated security models">
<p>The traditional Unix security model is too simple to satisfy the
certification requirements of large corporate and governmental organizations,
so several add-on security models have been implemented to increase
complexity.  There is some debate as to which of these (if any) are actually an

      <span id="Posix capabilities">
      <span id="SELinux">
<p><a href=ols/2001/selinux.pdf>Meeting Critical Security Objectives with Security-Enhanced Linux</a> (OLS 2001)</p>
<p><a href=ols/2002/ols2002-pages-65-72.pdf>SE Debian: how to make NSA SE LInux work in a distribution</a> (OLS 2002)</p>
    <span id="Encryption">
<p><a href=ols/2002/ols2002-pages-73-92.pdf>The Long Road to the Advanced Encryption Standard</a></p>
  <span id="API (how userspace talks to the kernel)">
    <span id="Syscalls">
    <span id="ioctls">
    <span id="executable file formats">
      <span id="a.out">
      <span id="elf">
        <span id="css, bss, etc.">
      <span id="scripts">
      <span id="flat">
      <span id="misc">
    <span id="Device nodes">
    <span id="Pipes (new pipe infrastructure)">
    <span id="Synthetic filesystems (as API)">

<span id="Hardware">
  <span id="Architectures">
  <a href=ols/2002/ols2002-pages-130-145.pdf>Running Linux on a DSP: Exploiting the Computational Resources of a programmable DSP Micro-Processor with uClinux</a> (OLS 2002)
  <a href=ols/2002/ols2002-pages-183-190.pdf>Porting Drivers to HP ZX1</a>
  <a href=ols/2001/iseries.pdf>The Linux Kernel on iSeries</a> (OLS 2001)
  <a href=ols/2001/ppc64.pdf>PowerPC 64-bit Kernel Internals</a> (OLS 2001)
  <a href=>PowerPC implementation reference for QEMU</a>
  <a href=ols/2001/uml.pdf>User-Mode Linux</a> (OLS 2001)
  <a href=ols/2002/ols2002-pages-107-116.pdf>Making Linux Safe for Virtual Machines</a> (OLS 2002)
  <a href=ols/2001/x86-64.pdf>Porting Linux to x86-64</a> (OLS 2001)

  <span id="DMA, IRQ, MMU (mmap), IOMMU, port I/O">
  <span id="Busses">
    <span id="PCI, USB">
<p><a href=ols/2001/pci.pdf>PCIComm: A Linux Device Driver for Communication over PCI Shared Memory</a> (OLS 2001)</p>
<p><a href=ols/2001/powertweak.pdf>Linux performance tuning using Powertweak</a> (OLS 2001)</p>

<span id="Following Linux development">
  <span id="Distibutions">
  <span id="Releases">
    <span id="Source control">

<p>Linux releases from 0.0.1 through 2.4.x used no source control system, just
release tarballs.  Releases 2.5.0 through 2.6.12-rc2 used a proprietary
source control system called BitKeeper.  Releases 2.6.12-rc2 through the
present use a source control system called git.</p>

<p>Early Linux development didn't use source control.  Instead Linus would
apply patches to his copy of the source, and periodically release tarball
snapshots of his development tree with a hand-edited changelog file noting who
contributed each patch he'd applied.  Most of these patches were posted to the
Linux Kernel Mailing List, and with a little effort could be fished out of the
mailing list archives.</p>

<p>This worked for many years, but didn't scale as Linux development grew.
Eventually the issue came to a head [link], and after some discussion Linus
decided to use a proprietary distributed source control system called
BitKeeper for the 2.5 development branch.  Linux releases v2.5.0 through
v2.6.12-rc2 were put out this way.</p>

<p>Linux development no longer uses BitKeeper, due to the sudden
<a href=>expiration of the
"Don't piss off Larry license"</a> under which BitKeeper was made available
to the Linux community (<a href=>more here</a>).
This prompted Linus to take a month off from Linux development to write his own
distributed source control system, git.  This is why the current source control
history in the main git development repository goes back to 2.6.12-rc2.
(The revision history from the BitKeeper era was later
<a href=;a=summary>converted to git</a>, but remains separate for historical reasons.)</p>

<p>Linus initially chose BitKeeper because he wanted a distributed source
control system, and the open source alternatives available at the time were
all centralized source control systems.</p>

<p>In a distributed source control
system, every user has a complete copy of the project's entire revision
history, which they can add their own changes to locally.  A centralized source
control system requires a single central location, with user accounts to
control access and either locking the tree or rejecting attempts to apply out
of date patches.  A distributed source control system is instead designed to
download and merge changes from many different repositories after they're
checked in to those other repositories.  The source control system handles
almost all of this merging automatically, because it can trace the changes in
each repository back to a common ancestor, and then use three-way merge
algorithms to better understand the changes.  (Patches don't indicate
which version they apply to.  A distributed source control system has
more information avialable to it, and uses that information to automatically
merge changes more effectively.)</p>

<p>This allows Linux subsystem maintainers to develop
and test their own local versions, then send changes to Linus in large batches
(without smearing together the individual patches they committed), and finally
resync with Linus's repository to get everyone else's changes.  Open source
development is already distributed, so distributed source control is a better
fit.  In this development model, Linus's repository serves as a coordination
point, but not a development bottleneck for anything except putting out
releases (which come from Linus's repository).</p>

<p>Linus described the appeal of distributed source control, and his reasons
for developing git, in the Google Video tech talk
<a href=>Linus Torvalds on git</a>.</p>

<p>To get started with git, see <a href=local/git-quick.html>git quickstart</a>.</p>

<p>The linux kernel source is also available as a
<a href=>mercurial repository</a>, another
popular open source distributed source control system.</p>

<p>This paper still serves as a decent introduction to distributed source
control: <a href=>BitKeeper
for Kernel Development</a> (OLS 2002, obsolete)</p>

  <span id="community">
  lwn, kernel traffic, kernelplanet.
  The four layer (developer, maintainer, subsystem, linus) model.
    Stable API nonsense
    Why reiser4 not in.
  </span id="community">
  <span id="Submitting Patches">

<span id="Glossary">