dev-manual: Applied review comments to poky-tiny section
Fixes YOCTO #2568 Applied Darren's review comments where I could for his review. Not all questions are answered but this represents the third draft of the section. (From yocto-docs rev: da0bc9542259238caf7b474bb15157d80a2b3651) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Scott Rifenbark
Richard Purdie
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@ -2197,12 +2197,13 @@
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<para>
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Very small distributions have some significant advantages such
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as requiring less on-die memory (cheaper), better performance
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through efficient cache usage, lower power requirements due to less
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memory, faster boot times, and reduced development overhead.
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as requiring less on-die or in-package memory (cheaper), better
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performance through efficient cache usage, lower power requirements
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due to less memory, faster boot times, and reduced development
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overhead.
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Some real-world examples where a very small distribution gives
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you distinct advantages are digital cameras, medical devices,
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small headless systems, and partitioned flash.
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and small headless systems.
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</para>
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<para>
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@ -2217,10 +2218,9 @@
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<para>
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The following list presents the overall steps you need to
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consider and perform to create very small systems that
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have a kernel and root filesystem under 4 Mbytes, boot to the
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shell in under two seconds, maintain <filename>ipv4</filename>
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functionality, and avoid an initial RAM disk:
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consider and perform to create distributions with smaller
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filesystems, faster boot times, maintain your critical
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functionality, and avoid initial RAM disks:
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<itemizedlist>
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<listitem><para>Determine your goals and guiding
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principles.</para></listitem>
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@ -2254,7 +2254,7 @@
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</para></listitem>
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<listitem><para>Do not create any difficult "hacks"
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to achieve your goals.</para></listitem>
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<listitem><para>Leverage off the device-specific
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<listitem><para>Leverage the device-specific
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options.</para></listitem>
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<listitem><para>Work in a separate layer so that you
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keep changes isolated.
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@ -2290,11 +2290,10 @@
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Memory consists of static, dynamic, and temporary memory.
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Static memory is the TEXT (the code), DATA (initialized data
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in the code), and BSS (uninitialized data) sections.
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Dynamic memory contains memory that is allocate at runtime,
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Dynamic memory contains memory that is allocated at runtime,
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stacks, hash tables, and so forth.
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Temporary memory is decompressed from the expanded kernel
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after boot.
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Temporary memory also includes the <filename>__init__</filename>
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after boot and also includes the <filename>__init__</filename>
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functions.
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</para>
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@ -2305,9 +2304,9 @@
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the <filename>scripts</filename> directory:
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<itemizedlist>
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<listitem><para><filename>ksize.py</filename>: Reports
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the size of the kernel files.</para></listitem>
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component sizes for the kernel files.</para></listitem>
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<listitem><para><filename>dirsize.py</filename>: Reports
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the size of the root filesystem.</para></listitem>
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component sizes for root filesystem.</para></listitem>
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</itemizedlist>
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This next tool and command helps you organize configuration
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fragments and view file dependencies in a human-readable form:
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@ -2332,12 +2331,12 @@
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section of the Yocto Project Linux Kernel Development
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Manual and the "<link linkend='creating-config-fragments'>Creating Configuration Fragments</link>"
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section, which is in this manual.</para></listitem>
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<listitem><para><filename>bitbake -u depexp -g core-image-*</filename>:
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<listitem><para><filename>bitbake -u depexp -g <bitbake_target></filename>:
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Using the BitBake command with these options brings up
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a Dependency Explorer from which you can view file
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dependencies.
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Understanding these dependencies allows you to make
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sane decisions when cutting out various pieces of the
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informed decisions when cutting out various pieces of the
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kernel and root filesystem.</para></listitem>
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</itemizedlist>
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</para>
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@ -2357,7 +2356,7 @@
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<para>
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First, check out what is hogging your file system running the
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<filename>dirsize.sh</filename> script from your root directory:
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<filename>dirsize.py</filename> script from your root directory:
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<literallayout class='monospaced'>
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$ cd <root-directory-of-image>
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$ dirsize.py 100000 > dirsize-100k.log
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@ -2366,6 +2365,9 @@
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You can apply a filter to the script to ignore files under
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a certain size.
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This example filters out anything below 100 Kbytes.
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The sizes reported by the tool are uncompressed and thus,
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will be smaller by a relatively constant factor in a
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compressed root filesystem.
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When you examine your log file, you can focus on areas of the
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root filesystem that take up large amounts of memory.
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</para>
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@ -2388,7 +2390,8 @@
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result in minimal impact on the feature set.
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For example, you might not need a VGA display.
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Or, you might be able to get by with <filename>devtmpfs</filename>
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and <filename>mdev</filename>.
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and <filename>mdev</filename> instead of
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<filename>udev</filename>.
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</para>
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<para>
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@ -2402,10 +2405,14 @@
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</para>
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<para>
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Some other examples of where you can look to eliminate size
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from the root filesystem are using <filename>ext2</filename>
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rather than <filename>ext3</filename>, which requires a
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1 Mbyte journal.
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Finally, you should consider exactly what filesystem you need
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to meet your needs while also reducing size.
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For example, consider <filename>cramfs</filename>,
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<filename>squashfs</filename>, <filename>ubifs</filename>,
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<filename>ext2</filename>, or an <filename>initramfs</filename>
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using <filename>initramfs</filename>.
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Be aware that <filename>ext3</filename> requires a 1 Mbyte
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journal.
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If you are okay with running read-only you don't need this
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journal.
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</para>
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@ -2446,6 +2453,9 @@
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taken up with the built-in <filename>.o</filename> files for
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drivers, networking, core kernel files, filesystem, sound,
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and so forth.
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The sizes reported by the tool are uncompressed and thus,
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will be smaller by a relatively constant factor in a compressed
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kernel image.
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Look to reduce the areas that are large and taking up around
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the "90% rule".
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</para>
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