en/devices/architecture/kernel/index.html - platform/docs/source.android.com - Git at Google

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     <title>Kernel</title>
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 <p>The Linux kernel is an extremely important part of the software on nearly
 every Android device. This section describes Linux kernel development and
 release models (below),
 <a href="/devices/architecture/kernel/releases.html">stable and long-term
 supported (LTS) kernels</a> (including why all Android devices should use stable
 releases instead of cherry picking patches), kernel
 <a href="/devices/architecture/kernel/config.html">configuration</a> and
 <a href="/devices/architecture/kernel/hardening.html">hardening</a>,
 requirements for
 <a href="/devices/architecture/kernel/reqs-interfaces.html">interfaces</a> and
 the
 <a href="/devices/architecture/kernel/modular-kernels.html">modular kernels</a>
 (introduced in Android O), kernel
 <a href="/devices/architecture/kernel/lldb-debug.html">debugging</a> and
 <a href="/devices/architecture/kernel/network_tests.html">network testing</a>,
 and <a href="/devices/architecture/kernel/squashfs.html">SquashFS</a>.</p>

 <h2 id="linux-kernel-development">Linux kernel development</h2>
 <p>
 The Linux kernel is the largest collaborative software project ever. In 2016,
 over 4,000 different developers from over 450 different companies contributed to
 the project and there were 6 releases, each containing between 12,000 and 16,000
 different changes. At the end of 2016 the size of the Linux kernel was just over
 56 thousand files, consisting of 22 million lines of code, build scripts, and
 documentation (kernel release 4.9). (For full Linux development statistics,
 refer to
 <a href="https://kernelnewbies.org/DevelopmentStatistics" class="external">https://kernelnewbies.org/DevelopmentStatistics</a>.)
 </p>
 <p>
 While the Linux kernel contains code for all the different chip architectures
 and hardware drivers it supports, an individual system runs only a fraction of
 the codebase. An average laptop uses around 2 million lines of kernel code from
 5 thousand files to function properly, while the Pixel phone uses 3.2 million
 lines of kernel code from 6 thousand files (due to the increased complexity of
 an SoC).
 </p>

 <h2 id="linux-kernel-releases">Linux kernel releases</h2>
 <p>The Linux kernel uses a release model that differs substantially from
 standard AOSP releases. With the release of the 2.6 kernel in December of 2003,
 the kernel developer community switched from the previous model of having a
 separate development and stable kernel branch, and moved to a <em>stable
 only</em> branch model. In this model, a new release occurred every 2 to 3
 months, and that release was declared <em>stable</em> and recommended for all
 users to run. This change in development model was due to the very long
 release cycle prior to the 2.6 kernel (almost 3 years), and the struggle to
 maintain two different branches of the codebase at the same time.</p>

 <p>The numbering of the kernel releases began at 2.6.x, where x was an
 incrementing number that changed on every release (the value of the number has
 no meaning, other than it is newer than the previous kernel release). The kernel
 version since then has now moved to 4.x accounting for 2 major version changes.
 These version numbers are chosen by the maintainer(s) only to avoid confusion
 among users caused by higher minor release numbers.</p>

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 </html>
	<html devsite>
	<head>
	<title>Kernel</title>
	<meta name="project_path" value="/_project.yaml" />
	<meta name="book_path" value="/_book.yaml" />
	</head>
	<body>
	<!--
	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<p>The Linux kernel is an extremely important part of the software on nearly
	every Android device. This section describes Linux kernel development and
	release models (below),
	<a href="/devices/architecture/kernel/releases.html">stable and long-term
	supported (LTS) kernels</a> (including why all Android devices should use stable
	releases instead of cherry picking patches), kernel
	<a href="/devices/architecture/kernel/config.html">configuration</a> and
	<a href="/devices/architecture/kernel/hardening.html">hardening</a>,
	requirements for
	<a href="/devices/architecture/kernel/reqs-interfaces.html">interfaces</a> and
	the
	<a href="/devices/architecture/kernel/modular-kernels.html">modular kernels</a>
	(introduced in Android O), kernel
	<a href="/devices/architecture/kernel/lldb-debug.html">debugging</a> and
	<a href="/devices/architecture/kernel/network_tests.html">network testing</a>,
	and <a href="/devices/architecture/kernel/squashfs.html">SquashFS</a>.</p>

	<h2 id="linux-kernel-development">Linux kernel development</h2>
	<p>
	The Linux kernel is the largest collaborative software project ever. In 2016,
	over 4,000 different developers from over 450 different companies contributed to
	the project and there were 6 releases, each containing between 12,000 and 16,000
	different changes. At the end of 2016 the size of the Linux kernel was just over
	56 thousand files, consisting of 22 million lines of code, build scripts, and
	documentation (kernel release 4.9). (For full Linux development statistics,
	refer to
	<a href="https://kernelnewbies.org/DevelopmentStatistics" class="external">https://kernelnewbies.org/DevelopmentStatistics</a>.)
	</p>
	<p>
	While the Linux kernel contains code for all the different chip architectures
	and hardware drivers it supports, an individual system runs only a fraction of
	the codebase. An average laptop uses around 2 million lines of kernel code from
	5 thousand files to function properly, while the Pixel phone uses 3.2 million
	lines of kernel code from 6 thousand files (due to the increased complexity of
	an SoC).
	</p>

	<h2 id="linux-kernel-releases">Linux kernel releases</h2>
	<p>The Linux kernel uses a release model that differs substantially from
	standard AOSP releases. With the release of the 2.6 kernel in December of 2003,
	the kernel developer community switched from the previous model of having a
	separate development and stable kernel branch, and moved to a <em>stable
	only</em> branch model. In this model, a new release occurred every 2 to 3
	months, and that release was declared <em>stable</em> and recommended for all
	users to run. This change in development model was due to the very long
	release cycle prior to the 2.6 kernel (almost 3 years), and the struggle to
	maintain two different branches of the codebase at the same time.</p>

	<p>The numbering of the kernel releases began at 2.6.x, where x was an
	incrementing number that changed on every release (the value of the number has
	no meaning, other than it is newer than the previous kernel release). The kernel
	version since then has now moved to 4.x accounting for 2 major version changes.
	These version numbers are chosen by the maintainer(s) only to avoid confusion
	among users caused by higher minor release numbers.</p>

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	</html>