[Bug 1944082] [NEW] initramfs-tools: zstd uses too much memory in mkinitramfs

[Ko-Zu]

Public bug reported:

Changing the default compression method of initramfs to zstd using
hardcoded compression flag "-19 -T0" takes 200MB memory in return for
40MB reduction of disk IO. I think it is too much memory usage for some
fraction of seconds while booting.

The default compression method of initramfs was changed in Bug #1931725 for impish but the runtime memory usage was not discussed well.
the test results by modifying compression command in /usr/sbin/mkinitramfs on a small VM (amd64, 512MB RAM, 1vCPU, with 5.13.0-16-generic image) 
                | RSSmax | initrd size
zstd -q -19 -T0 |  214MB |  64MB (+-0MB)
zstd -q -3 -T1  |   44MB |  84MB (+20MB)
lz4 -9 -l       |   15MB | 107MB (+43MB)

Since the hardcoded flag uses "-T0", more memory will be used if more CPU threads are available.
Please reconsider the default compression method or patch the hardcoded flag to "-3 -T1" (=zstd default).

** Affects: initramfs-tools (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1944082

Title:
  initramfs-tools: zstd uses too much memory in mkinitramfs

Status in initramfs-tools package in Ubuntu:
  New

Bug description:
  Changing the default compression method of initramfs to zstd using
  hardcoded compression flag "-19 -T0" takes 200MB memory in return for
  40MB reduction of disk IO. I think it is too much memory usage for
  some fraction of seconds while booting.

  The default compression method of initramfs was changed in Bug #1931725 for impish but the runtime memory usage was not discussed well.
  the test results by modifying compression command in /usr/sbin/mkinitramfs on a small VM (amd64, 512MB RAM, 1vCPU, with 5.13.0-16-generic image) 
                  | RSSmax | initrd size
  zstd -q -19 -T0 |  214MB |  64MB (+-0MB)
  zstd -q -3 -T1  |   44MB |  84MB (+20MB)
  lz4 -9 -l       |   15MB | 107MB (+43MB)

  Since the hardcoded flag uses "-T0", more memory will be used if more CPU threads are available.
  Please reconsider the default compression method or patch the hardcoded flag to "-3 -T1" (=zstd default).

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