>
>
> On Sat, 8 Jul 2023, 02:49 Benjamin Drung, <bdrung@ubuntu.com> 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@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.
>
>
> I had this before with lz4 and lzo compression and fixed this before
> in the kernel. It is likely a kernel bug mishandling mixed compression
> initrds.
It's not only the kernel. It is also cpio that complains.
> It will likely work if you.... Make compressed-files the third early
> initrd. Such that we have intel, AMD microcode early initrds, followed
> by compressed-files initrd, followed by compressed main initrd.
>
It did.
> > [2]
> > https://code.launchpad.net/~bdrung/ubuntu/+source/initramfs-tools/+git/initramfs-tools/+ref/ubuntu/compressed
> >
--
Benjamin Drung
Debian & Ubuntu Developer
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