** Also affects: linux-oracle (Ubuntu)
Importance: Undecided
Status: New
** Changed in: linux-oracle (Ubuntu)
Status: New => Fix Committed
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https://bugs.launchpad.net/bugs/1931725
Title:
initramfs-tools & kernel: use zstd as the default compression method
Status in Ubuntu on IBM z Systems:
Fix Released
Status in initramfs-tools package in Ubuntu:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in linux-oracle package in Ubuntu:
Fix Committed
Bug description:
Turns out that loading is always the slow part in loading initramfs
into memory and decompressing it since decompression is always the
final 10-20% or so of the task. It therefore makes sense to use a
good compressor that shrinks the initramfs as much as possible with
little decompression overhead.
Benchmarking zstd vs lz4 shows that while zstd can be ~5x slower in
decompression, the image size is much smaller with zstd than lz4, and
since ~80-90% of the boot time is loading the image it makes sense to
use zstd.
Attached is a libreoffice spread sheet showing typical load and
decompression times for a fairly standard 3.4 GHZ intel box with data
for load times for a 5400 RPM, 7200 RPM and SATA SSD drives.
The conclusions from the test results (attached) show:
1. Loading time is always significantly slower than decompression time.
2. ZSTD is 5x slower than LZ4 in decompression speed but produces far
better compressed images
3. Given that loading time is the major factor in loading +
decompression, ZSTD is best for kernel and initramfs boot timings.
(Also refer to https://kernel.ubuntu.com/~cking/boot-speed-
eoan-5.3/kernel-compression-method.txt for some raw data on drive load
speeds for the same UEFI box I did a couple of years ago).
amd64 supports zstd, but s390x does not. Will use this bug to enable
zstd support on s390x.
Upstream submitted patch
https://lore.kernel.org/linux-s390/20210615114150.325080-1-dimitri.led...@canonical.com/T/#u
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