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From: klondike <>
Cc: P J P <>, Paul Bolle <>,
	Andrew Morton <>
Subject: [PATCH v3 2/2] initramfs: Allow again choice of the embedded initram compression algorithm
Date: Tue, 27 Sep 2016 22:32:59 +0200
Message-ID: <> (raw)
In-Reply-To: <>

Choosing the appropriate compression option when using an embeded initramfs
can result in significant size differences in the resulting data.

This is caused by avoiding double compression of the initramfs contents.
For example on my tests, choosing CONFIG_INITRAMFS_COMPRESSION_NONE when
compressing the kernel using XZ) results in up to 500KiB differences (9MiB to
 8.5MiB) in the kernel size as the dictionary will not get polluted with
uncomprensible data and may reuse kernel data too.

Despite embedding an uncompressed initramfs, a user may want to allow for a
compressed extra initramfs to be passed using the rd system, for example to
boot a recovery system. Commit 9ba4bcb645898d562498ea66a0df958ef0e7a68c
("initramfs: read CONFIG_RD_ variables for initramfs compression") broke
that behavior by making the choice based on CONFIG_RD_* instead of adding
choose the supported RD compression algorithms by the kernel and a user may
want to suppport more than one.

This patch also reverses 3e4e0f0a8756dade3023d1f47d50fbced7749788
("initramfs: remove "compression mode" choice") restoring back the
"compression mode" choice and includes the CONFIG_INITRAMFS_COMPRESSION_LZ4
option which was never added.

As a result the following options are added or readed affecting the embedded
initramfs compression:

These depend on the corresponding CONFIG_RD_* option being set (except NONE
which has no dependencies).

This patch depends on the previous one (the previous version didn't) to
simplify the way in which the algorithm is chosen and keep backwards
compatibility with the behaviour introduced by commit

Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <>
Cc: P J P <>
Cc: Paul Bolle <>
Cc: Andrew Morton <>
I'm sorry for the noise. I'm resending this as a new version because
Thunderbird likes to word wrap things it shouldn't be word wrapping.
diff --git a/usr/Kconfig b/usr/Kconfig
index bf8e8f1..6278f13 100644
--- a/usr/Kconfig
+++ b/usr/Kconfig
@@ -99,8 +99,125 @@ config RD_LZ4
 	  Support loading of a LZ4 encoded initial ramdisk or cpio buffer
 	  If unsure, say N.
+	prompt "Built-in initramfs compression mode"
+	depends on INITRAMFS_SOURCE!=""
+	optional
+	help
+	  This option allows you to decide by which algorithm the builtin
+	  initramfs will be compressed.  Several compression algorithms are
+	  available, which differ in efficiency, compression and
+	  decompression speed.  Compression speed is only relevant
+	  when building a kernel.  Decompression speed is relevant at
+	  each boot. Also the memory usage during decompression may become
+	  relevant on memory constrained systems. This is usually based on the
+	  dictionary size of the algorithm with algorithms like XZ and LZMA
+	  featuring large dictionary sizes.
+	  High compression options are mostly useful for users who are
+	  low on RAM, since it reduces the memory consumption during
+	  boot.
+	  Keep in mind that your build system needs to provide the appropriate
+	  compression tool to compress the generated initram cpio file for
+	  embedding.
+	  If in doubt, select 'None'
+	bool "None"
+	help
+	  Do not compress the built-in initramfs at all. This may sound wasteful
+	  in space, but, you should be aware that the built-in initramfs will be
+	  compressed at a later stage anyways along with the rest of the kernel,
+	  on those architectures that support this. However, not compressing the
+	  initramfs may lead to slightly higher memory consumption during a
+	  short time at boot, while both the cpio image and the unpacked
+	  filesystem image will be present in memory simultaneously
+	bool "Gzip"
+	depends on RD_GZIP
+	help
+	  Use the old and well tested gzip compression algorithm. Gzip provides
+	  a good balance between compression ratio and decompression speed and
+	  has a reasonable compression speed. It is also more likely to be
+	  supported by your build system as the gzip tool is present by default
+	  on most distros.
+	bool "Bzip2"
+	depends on RD_BZIP2
+	help
+	  It's compression ratio and speed is intermediate. Decompression speed
+	  is slowest among the choices. The initramfs size is about 10% smaller
+	  with bzip2, in comparison to gzip. Bzip2 uses a large amount of
+	  memory. For modern kernels you will need at least 8MB RAM or more for
+	  booting.
+	  If you choose this, keep in mind that you need to have the bzip2 tool
+	  available to be able to compress the initram.
+	bool "LZMA"
+	depends on RD_LZMA
+	help
+	  This algorithm's compression ratio is best but has a large dictionary
+	  size which might cause issues in memory constrained systems.
+	  Decompression speed is between the other choices. Compression is
+	  slowest. The initramfs size is about 33% smaller with LZMA in
+	  comparison to gzip.
+	  If you choose this, keep in mind that you may need to install the xz
+	  or lzma tools to be able to compress the initram.
+	bool "XZ"
+	depends on RD_XZ
+	help
+	  XZ uses the LZMA2 algorithm and has a large dictionary which may cause
+	  problems on memory constrained systems. The initramfs size is about
+	  30% smaller with XZ in comparison to gzip. Decompression speed is
+	  better than that of bzip2 but worse than gzip and LZO. Compression is
+	  slow.
+	  If you choose this, keep in mind that you may need to install the xz
+	  tool to be able to compress the initram.
+	bool "LZO"
+	depends on RD_LZO
+	help
+	  It's compression ratio is the second poorest amongst the choices. The
+	  kernel size is about 10% bigger than gzip. Despite that, it's
+	  decompression speed is the second fastest and it's compression speed
+	  is quite fast too.
+	  If you choose this, keep in mind that you may need to install the lzop
+	  tool to be able to compress the initram.
+	bool "LZ4"
+	depends on RD_LZ4
+	help
+	  It's compression ratio is the poorest amongst the choices. The kernel
+	  size is about 15% bigger than gzip; however its decompression speed
+	  is the fastest.
+	  If you choose this, keep in mind that most distros don't provide lz4
+	  by default which could cause a build failure.
+	default ".bz2"  if INITRAMFS_COMPRESSION_BZIP2
+	default ".xz"   if INITRAMFS_COMPRESSION_XZ
+	default ".lzo"  if INITRAMFS_COMPRESSION_LZO
+	default ".lz4"  if INITRAMFS_COMPRESSION_LZ4
 	default ".gz"   if RD_GZIP
 	default ".lz4"  if RD_LZ4
 	default ".lzo"  if RD_LZO

  parent reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-25  4:47 [PATCH] initramfs: allow again choice of the embedded " klondike
2014-09-29  8:08 ` P J P
2014-09-29 23:42   ` klondike
2016-09-27 19:30 ` [PATCH v2 1/2] " klondike
2016-09-27 19:31   ` [PATCH v2 2/2] " klondike
2016-09-27 20:32   ` [PATCH v3 1/2] initramfs: Select builtin initram compression algorithm on KConfig instead of Makefile klondike
     [not found]   ` <>
2016-09-27 20:32     ` klondike [this message]
2016-10-21 21:21       ` [PATCH v3 2/2] initramfs: Allow again choice of the embedded initram compression algorithm Andrew Morton
2016-10-21 21:29         ` Francisco Blas Izquierdo Riera (klondike)
2017-05-20 22:30       ` [v3, " Florian Fainelli
2017-05-21  2:48         ` Florian Fainelli

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