Reducing initramfs size and speed up the generation

[Benjamin Drung]

On Mon, 2023-07-10 at 11:09 +0200, Adrien Nader wrote:
> On Sat, Jul 08, 2023, Benjamin Drung wrote:
> > On Sat, 2023-07-08 at 03:49 +0200, Benjamin Drung wrote:
> > > On Sat, 2023-07-08 at 01:25 +0100, Dimitri John Ledkov wrote:
> > > > On Sat, 8 Jul 2023 at 01:19, Benjamin Drung <bdrung at ubuntu.com> wrote:
> > > > > 
> > > > > Hi all,
> > > > > 
> > > > > a year ago we changed the default compression and level for the
> > > > > initramfs to zstd -1. This fixed the very slow creation times on
> > > > > development boards (see bug #1958148), but that leads to bigger
> > > > > initramfs sizes that triggered other bugs (like bug #1842320).
> > > > > Big initramfs sizes can also fill up small sized /boot partitions easily
> > > > > (grooming the 850 initramfs-tools bugs revealed several such reports).
> > > > > 
> > > > > Using xz -9 would give very good compression, but it takes very long
> > > > > (especially on slow development boards) and a lot of memory (good luck
> > > > > on Raspberry Pis with small memory like Pi Zeros).
> > > > > 
> > > > > I propose following approach to address the drawback: Create cpio
> > > > > archives (compressed with xz -9) for the kernel modules and firmware
> > > > > files when building the kernel/firmware Debian package. Then ship those
> > > > > cpio archives in the package (or in a separate binary package). Then the
> > > > > CPU load it put on the builders. The cpio archives would contain the
> > > > > modules for MODULES=most.
> > > > > 
> > > > > mkinitramfs will then look for those cpio archives and uses those in
> > > > > case they are present. Such a initramfs would look like this:
> > > > > 
> > > > > * AMD/Intel microcode cpio archive (on amd64)
> > > > > * main cpio archive compressed with zstd -1
> > > > > * kernel modules from the Debian package compressed with xz -9
> > > > > * firmware files from the Debian package compressed with xz -9
> > > > > 
> > > > 
> > > > Majority of our instances boot without initrd, and there too they
> > > > don't load most of the modules.
> > > > Creating xz -9 compressed archive of all modules, still pays the
> > > > penalty to decompress most of them, and then not modprobe them.
> > > > I was hoping to achieve a similar in spirit approach, but didn't quite
> > > > have the time to implement is:
> > > > 
> > > > 1) change linux-modules and linux-firmware to ship .ko.zst
> > > > firmware.bin.zst compressed with zstd -19 at .deb build time
> > > > 2) this saves install size of the packages, with only slightly
> > > > increased download size
> > > > 3) modify initramfs-tools to include compressed files into a separate
> > > > initrd, which is not compressed (i.e. exclude .zst files from the
> > > > default main compressed cpio archive, and append them in the second
> > > > main cpio archive that is uncompressed)
> > > > 4) this should achieve quick initrd creation, which will be smaller in
> > > > size that current status, and will boot faster as it will only
> > > > decompress modules/firmware it actually needs at boot
> > > > 
> > > > For experimentation locally, you can recompress .ko with zstd in place
> > > > in /lib/modules/; and rerun depmod. To then test initramfs-tools
> > > > changes that skip over .zst compressed files and add them as is in an
> > > > uncompressed appended cpio.
> > > 
> > > That is a very good idea. I created a draft for point 3 in [2]. It moves
> > > the compressed files into a separate directory and creates a separate
> > > cpio archive for that directory without compressing it:
> > > 
> > > * AMD/Intel microcode cpio archive (on amd64)
> > > * main cpio archive (compressed)
> > > * compressed kernel modules / firmware (not compressed)
> > > 
> > > Sadly this does not work (yet). cpio complains with "premature end of
> > > archive" when looking at it and the kernel fails to extract the last
> > > cpio part. I am heading to bed now leaving that bug for another day.
> > > 
> > > [2] https://code.launchpad.net/~bdrung/ubuntu/+source/initramfs-tools/+git/initramfs-tools/+ref/ubuntu/compressed
> > 
> > Okay. It works now. The not-compressed cpio archive must not be the last
> > one. So the order is now:
> > 
> > * AMD/Intel microcode cpio archive (on amd64)
> > * compressed kernel modules / firmware (not compressed)
> > * main cpio archive (compressed)
> > 
> > I'll really stop now. For a first comparison, the firmware files need to
> > be converted correctly. There are symlinks in /lib/firmware. So running
> > following was not correct/enough:
> > 
> > find /lib/firmware -name '*.bin' | while read -r fw; do
> >     sudo zstd -19 -z -o "${fw}.zst" "$fw"
> >     sudo rm "$fw"
> > done
> > 
> > If you want to help, hand me a correct conversion script.
> 
> Some filenames in /lib/firmware contain spaces. There are many more
> files than .bin ones. A number of the files are readmes.
> 
> Recent changes in linux-firmware.git add "install-xz" and "install-zstd"
> targets to make than will do what you want I guess. I haven't checked
> that this was actually merged; it was discussed at least on 2023-03-01
> on the mailing-list. It's probably the best path forward in any case.
> 
> In case it isn't merged, you can have a look at
> https://lore.kernel.org/linux-firmware/20230301-fixes-and-compression-v2-0-e2b71974e842@gmail.com/T/

I agree. The conversion script was just for a quick way to test. The
clean approach would be to patch linux-firmware to produce two
additional binary packages: linux-firmware-zst and linux-firmware-xz.

-- 
Benjamin Drung
Debian & Ubuntu Developer