[Bug 1931725] Comment bridged from LTC Bugzilla

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------- Comment From cborntra at de.ibm.com 2021-06-16 07:19 EDT-------
FWIW, with Z15 and later we do have a zlib HW accelerator on the chip. So zlib would also be an option - assuming that z15 will be the most common Z pretty soon.
>From my checks the HW decompression this was really fast.
How does zlib compression ratio (maybe try level 6 and 9) compare here?

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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:
  New
Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in linux package in Ubuntu:
  New

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.ledkov@canonical.com/T/#u

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