[Bug 2059976] Re: unmkinitramfs is very slow on initrd from ubuntu 23.10+, solution proposed

[Adam Vodopjan]

> I'll change unmkinitramfs to use 3cpio in case it is installed.

It does not solve the problem reported in this bug. How would one know
unmkinitramfs is dead slow because 3cpio is not installed? Mby you
should make initramfs-tools-core depend on rust-threecpio?

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Title:
  unmkinitramfs is very slow on initrd from ubuntu 23.10+, solution
  proposed

Status in initramfs-tools package in Ubuntu:
  Triaged

Bug description:
  Since 23.10 the initrd consists of 4 embedded cpio:
  - amd microcode
  - intel microcode
  - firmware + kernel modules
  - rest (compressed)

  Previously it was:
  - amd microcode
  - intel microcode
  - rest (compressed)

  There is no archive size header for cpio, so unmkinitramfs has to
  parse headers of each entry in uncompressed archives.

  > time unmkinitramfs 23.04/{initrd,unpack}

  real    0m3.625s
  user    0m1.595s
  sys     0m3.467s

  > time unmkinitramfs 23.10/{initrd,unpack}

  real    0m15.982s
  user    0m20.174s
  sys     0m6.995s

  > for dir in 23.04/unpack/early*; do printf '%-20s %s\n' "$dir" "$(find "$dir" | wc -l)"; done
  23.04/unpack/early   5
  23.04/unpack/early2  6

  > for dir in 23.10/unpack/early*; do printf '%-20s %s\n' "$dir" "$(find "$dir" | wc -l)"; done
  23.10/unpack/early   5
  23.10/unpack/early2  6
  23.10/unpack/early3  2874

  
  Here https://github.com/slowpeek/unmkinitramfs-turbo I've made it a lot faster with two changes:
  - bash instead of sh for bash's "read -N"
  - xxd dump of initrd instead of raw binary data

  
  I wonder, if the developers are interested in accepting my improvements into the code base?

  
  > time unmkinitramfs-classic-turbo 23.10/{initrd,unpack}

  real    0m2.209s
  user    0m2.161s
  sys     0m1.341s

  > time unmkinitramfs-turbo 23.10/{initrd,unpack}
   early .. +
  early2 .. +
  early3 .. +
    main .. +

  real    0m1.973s
  user    0m2.094s
  sys     0m1.244s

  
  All tools in the repo, even the classic one, feature -s/--scan option. With it, the tools only dump offset + size for embedded archives, nothing is unpacked. It can be used to measure parsing speed.

  In the repo there are three tools:
  - unmkinitramfs-classic: the original tool
  - unmkinitramfs-classic-turbo: the improved tool with minimal changes
  - unmkinitramfs-turbo: the improved tool with many changes to my tastes, extra dependency on "file" and some additional features (-ss option, decompression with 7za)

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