some time ago, the default compressor for initramfs was changed
from lz4 -9 to zstd -19. This caused significant problems:
- it is very slow
- it uses a lot of memory
The former is a problem for everyone, the latter means that
zstd just crashes on a Pi Zero.
This is an analysis of what we have in terms of time spent,
memory spent, and file size achieved, and where we can
go from here.
# Comparison of different compression levels
## Desktop (ThinkPad T480s, jammy)
level usertime elapsed memory fileSize
lz4 9.65s 11.09s 12M 64M
-1 5.69s 6.99s 24M 57M
-6 12.59s 8.58s 99M 47M
-12 19.85s 10.82s 249M 41M
-19 71.29s 26.95s 519M 35M
-> I believe that somewhere around -12 is a decent
compromise between size and speed.
## Pi 4 (arm64, focal)
Times have been measured for mkinitramfs only. A full
update-initramfs call spends much more time copying
some firmware bits to boot partition with flash-kernel
level usertime elapsed memory fileSize
lz4 21.10s 64.85s 21M 29M
-1 13.73s 44.55s 21M 27M
-6 26.07s 49.09s 91M 24M
-12 48.18s 54.67s 203M 22M
-19 130.07s 92.80s 350M 20M
-> 6 is essentially free if the Pi 4 is idle. Nice.
-> -6 is still 20% of total RAM of a Pi 0
-> There's no meaningful difference between -6 and -12
in terms of time elapsed. -6 uses 116% CPU, -12 uses
145% CPU.
## Adaptive compression
zstd also supports adaptive compression, compressing as hard as
it can while not impacting I/O speed. So hardware with slow I/O
like a Pi would compress harder to avoid idling.
This is somewhat suboptimal with recent update-initramfs though,
as it first writes the cpio archive to the disk and then compresses
it rather than doing it in a pipe where that would be more
meaningful.
Question: Does zstd --adapt adapt to memory available?
# Way(s) forward
To remedy the issue the proposal is to build with
- zstd -1 on hardware with 512 MB or less memory
- zstd between -1 and -19 on other hardware
- zstd -19 during image building
Finding the right level between -1 and -19 is hard. The more
cores you have, the less penalty you pay for higher level.
Going for adaptive compression would remove the guess work, but
will result in larger images on faster machines. Maybe that's
fine, though - they probably have more space on /boot anyway?
If we want to aim for 5% of total memory, we should probably
aim for something like:
-1 on <= 512MB
-6 on <= 2 GB (or --adapt=min=1,max=6)
-12 on the rest (or --adapt=min=12)
It's clear that in all cases, zstd -1 is at least better than the
lz4 -9 we used before; both in terms of space used, and time spent.
# Concerns
Lowering the compression level will reduce the boot speed by fractions
of a second on hardware with fast I/O.
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