linux-kernel - Re: [PATCH] initramfs: don't double-compress built-in initramfs if the kernel is compressed

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Date:   Tue, 4 Feb 2020 11:29:17 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] initramfs: don't double-compress built-in initramfs if
 the kernel is compressed

On Tue, Feb 04, 2020 at 11:22:19AM +0900, Masahiro Yamada wrote:
> Hi.
> 
> On Tue, Feb 4, 2020 at 6:16 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > If the kernel is going to be compressed anyway, there is no point in
> > double-compressing the built-in initramfs. Hide the built-in initramfs
> > compression choice in this case.
> >
> > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> 
> 
> Double-compression slightly reduces the data size.
> And, keeping the bare vmlinux small is beneficial, IMHO.
> 

Ok. In my setup (kernel is compressed by xz), none for the builtin
initramfs is optimal by a measurable amount, but I that will depend on
config/initramfs contents/compression methods, so might well not be true
in general. Maybe add a note to the help text?

none    7030816 arch/x86/boot/bzImage
gz      7382144 arch/x86/boot/bzImage
bz2     7353696 arch/x86/boot/bzImage
lzma    7118176 arch/x86/boot/bzImage
xz      7126400 arch/x86/boot/bzImage
lzo     7379360 arch/x86/boot/bzImage
lz4     7335712 arch/x86/boot/bzImage


diff --git a/usr/Kconfig b/usr/Kconfig
index a6b68503d177..b68910b225fc 100644
--- a/usr/Kconfig
+++ b/usr/Kconfig
@@ -125,6 +125,12 @@ choice
 	  low on RAM, since it reduces the memory consumption during
 	  boot.
 
+	  Note that if the kernel itself is compressed, the builtin initramfs
+	  will be compressed twice, first by the algorithm selected here, and
+	  then by the kernel compression method chosen. This may increase the
+	  final image size, so you should test whether compressing the builtin
+	  initramfs provides any savings.
+
 	  Keep in mind that your build system needs to provide the appropriate
 	  compression tool to compress the generated initram cpio file for
 	  embedding.