On Thu, 27 Jul 2023 at 09:21, Benjamin Drung <bdrung@ubuntu.com> wrote:
On Wed, 2023-07-26 at 17:53 +0200, Benjamin Drung wrote:
> 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.
The smaller initramfs overall size (less to load into memory and unpack)
and the smaller compressed cpio (less to decompress) have a positive
effect on the boot speed, especially on systems with slow CPU and/or
slow IO.
When looking at the "kernel" time from systemd-analyze, the improvement
ranges from 1.62s - 1.36s = 0.26s in a VM on my desktop to a heavily
noticeable 37.9s - 16.5s = 21.4s on the VisionFive 2 RISC-V board.
This is good stuff. It's a bit of a shame that the build time for the initramfs hasn't improved much. I guess it's not as dominated by compression time as I thought?
Do you have any thoughts about making it faster? I know I once ran 'strace -ff mkinitramfs' and ended up with tens or hundreds of thousands of trace files so not having everything done by a billion tiny shell scripts would help, but I don't know how much.
I wonder if we can make depmod incremental somehow?
Cheers,
mwh
