Dimitri John Ledkov: Ubuntu 23.10 significantly reduces the installed kernel footprint

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Ubuntu systems typically have up to 3 kernels installed, before they are auto-removed by apt on classic installs. Historically the installation was optimized for metered download size only. However, kernel size growth and usage no longer warrant such optimizations. During the 23.10 Mantic Minatour cycle, I led a coordinated effort across multiple teams to implement lots of optimizations that together achieved unprecedented install footprint improvements.

Given a typical install of 3 generic kernel ABIs in the default configuration on a regular-sized VM (2 CPU cores 8GB of RAM) the following metrics are achieved in Ubuntu 23.10 versus Ubuntu 22.04 LTS:

2x less disk space used (1,417MB vs 2,940MB, including initrd)
3x less peak RAM usage for the initrd boot (68MB vs 204MB)
0.5x increase in download size (949MB vs 600MB)
2.5x faster initrd generation (4.5s vs 11.3s)
approximately the same total time (103s vs 98s, hardware dependent)

For minimal cloud images that do not install either linux-firmware or modules extra the numbers are:

1.3x less disk space used (548MB vs 742MB)
2.2x less peak RAM usage for initrd boot (27MB vs 62MB)
0.4x increase in download size (207MB vs 146MB)

Hopefully, the compromise of download size, relative to the disk space & initrd savings is a win for the majority of platforms and use cases. For users on extremely expensive and metered connections, the likely best saving is to receive air-gapped updates or skip updates.

This was achieved by precompressing kernel modules & firmware files with the maximum level of Zstd compression at package build time; making actual .deb files uncompressed; assembling the initrd using split cpio archives – uncompressed for the pre-compressed files, whilst compressing only the userspace portions of the initrd; enabling in-kernel module decompression support with matching kmod; fixing bugs in all of the above, and landing all of these things in time for the feature freeze. Whilst leveraging the experience and some of the design choices implementations we have already been shipping on Ubuntu Core. Some of these changes are backported to Jammy, but only enough to support smooth upgrades to Mantic and later. Complete gains are only possible to experience on Mantic and later.

The discovered bugs in kernel module loading code likely affect systems that use LoadPin LSM with kernel space module uncompression as used on ChromeOS systems. Hopefully, Kees Cook or other ChromeOS developers pick up the kernel fixes from the stable trees. Or you know, just use Ubuntu kernels as they do get fixes and features like these first.

The team that designed and delivered these changes is large: Benjamin Drung, Andrea Righi, Juerg Haefliger, Julian Andres Klode, Steve Langasek, Michael Hudson-Doyle, Robert Kratky, Adrien Nader, Tim Gardner, Roxana Nicolescu – and myself Dimitri John Ledkov ensuring the most optimal solution is implemented, everything lands on time, and even implementing portions of the final solution.

Hi, It’s me, I am a Staff Engineer at Canonical and we are hiring https://canonical.com/careers.

Lots of additional technical details and benchmarks on a huge range of diverse hardware and architectures, and bikeshedding all the things below:

For questions and comments please post to Kernel section on Ubuntu Discourse.

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