Update on reducing initramfs size and speed up

[Benjamin Drung]

Hi all,

A few weeks ago, I posted an idea how to reduce the initramfs size and
speed up the generation:

https://lists.ubuntu.com/archives/ubuntu-devel/2023-July/042652.html

This post sparked a lively discussion. The initial idea was ditched for
a better solution: mkinitramfs will put all compressed files (kernel
modules and firmware files) into a cpio archive that is not compressed
(because compressing compressed files makes no sense). All other files
will be added to a cpio archive that gets compressed. As next steps, the
kernel modules and firmware files need to be shipped compressed.

After several iterations for the implementation and review by Daves
Jones, I just uploaded initramfs-tools 0.142ubuntu8 to mantic that puts
compressed kernel modules and firmware files in an uncompressed cpio
(https://launchpad.net/bugs/2028567).

I created/updated the follow-up tickets and added my patches to them:

Ship kernel modules Zstd compressed
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2028568

compress firmware in /lib/firmware
https://bugs.launchpad.net/ubuntu/+source/linux-firmware/+bug/1942260

And without further ado, here come the benchmark results:

The benchmarks were done either on an AMD Ryzen 7 5700G with schroot and
overlay on tmpfs or on the hardware mentioned. All tests were running
the latest Ubuntu mantic development release.

* minimal: schroot with linux-image-generic initramfs-tools zstd
* full: minimal + busybox-initramfs cryptsetup-initramfs
  isc-dhcp-client kbd lvm2 mdadm ntfs-3g plymouth plymouth-theme-spinner
* nvidia: full + linux-headers-generic nvidia-driver-525
* nvidia fw: nvidia + compressed /lib/firmware/nvidia/525.125.06/
* VisionFive 2: VisionFive 2 RISC-V board
* RPi Zero 2: Raspberry Pi Zero 2 ARM board (running armhf)

"next" means using kernel/firmware/initramfs from ppa:bdrung/ppa i.e.
* initramfs-tools 0.142ubuntu7bd4
* linux 6.3.0-7.7bd2
* linux-firmware 20230629.gitee91452d-0ubuntu1bd1

|                | build   | size               | uncompressed size  |
| test           | time    | in bytes  | in MiB | in bytes  | in MiB |
|----------------|---------|-----------|--------|--------------------|
| minimal        | 4.30 s  |  66701818 |  63.6  | 224087608 | 213.7  |
| minimal next   | 4.54 s  |  59935186 |  57.2  |  67738810 |  64.6  |
| full           | 7.15 s  | 118007038 | 112.5  | 387976843 | 370.0  |
| full next      | 7.29 s  | 106937908 | 102.0  | 128610985 | 122.7  |
| nvidia         | 7.04 s  | 209200523 | 199.5  | 513554279 | 489.8  |
| nvidia next    | 7.21 s  | 195246287 | 186.2  | 235288174 | 224.4  |
| nvidia fw next | 7.16 s  | 191329102 | 182.5  | 213078234 | 203.2  |
| VisionFive 2   | 142.9 s | 121895035 | 116.2  | 411160836 | 392.1  |
| VF 2 next      | 126.7 s | 111651453 | 106.5  | 134120804 | 127.9  |
| RPi Zero 2     | 109.5 s |  39803044 |  40.0  |  69592789 |  66.4  |
| RPi Zero 2 ²   | 103.5 s |  39804499 |  40.0  |  69592789 |  66.4  |
| RPi Zero 2 next| 101.2 s |  31463352 |  30.0  |  41145762 |  39.2  |

² Updated initramfs-tools (but no compressed modules or firmware)

The build time was averaged over five runs.

| improvement  | build time | size   | uncompressed size |
|--------------|------------|--------|-------------------|
| minimal      |  105.6 %   | 89.9 % |      30.2 %       |
| full         |  102.0 %   | 90.6 % |      33.1 %       |
| nvidia       |  101.7 %   | 91.5 % |      41.5 %       |
| VisionFive 2 |   88.7 %   | 91.6 % |      32.6 %       |
| RPi Zero 2   |   92.4 %   | 79.0 % |      59.1 %       |

Building the initramfs takes more CPU cycles (see tests on tmpfs), but
saves time on disk IO. Daves Jones saw much bigger time savings on his
Raspberry Pis but his tests were on lunar.

Build time influence:
+ add_directories plus uniq take several milliseconds
+ depmod on compressed kernel modules take hundreds of
  milliseconds longer
- copying smaller kernel modules (due to compression) is faster
- cpio archive that needs to be compressed is smaller
- not storing intermediate cpio archives saves time

Saving 10 to 20 percent on the initramfs size and only needing half or a
third of the size when unpacked (i.e. needed memory during boot) is a
good improvement.

-- 
Benjamin Drung
Debian & Ubuntu Developer