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Re: Evaluation (Re: Proposal: A new approach to differential debs)

On Tue, Aug 22, 2017 at 10:53:47PM +0200, Sebastian Andrzej Siewior wrote:
> On 2017-08-16 00:21:09 [+0200], Julian Andres Klode wrote:
> > libreoffice-core (size only):
> > 
> >   -rw-r--r-- 1 jak jak 29M Jul 22 20:02 libreoffice-core_5.3.5~rc1-3_amd64.deb
> >   -rw-r--r-- 1 jak jak 31M Jul 16 00:10 libreoffice-core_5.4.0~rc2-1_amd64.deb
> >   -rw-r--r-- 1 jak jak 31M Jul 28 18:29 libreoffice-core_5.4.0-1_amd64.deb
> >   -rw-r--r-- 1 jak jak  18M Aug 15 23:44 libreoffice-core_5.3.5~rc1-3_5.4.0-1_amd64.pdeb
> >   -rw-r--r-- 1 jak jak 4.5M Aug 15 23:42 libreoffice-core_5.4.0~rc2-1_5.4.0-1_amd64.pdeb
> > 
> > For 5.4~rc2 to 5.4 it made a huge difference, for 5.3.5~rc1 to 5.4 not so much,
> > so it probably is a good fit for stable updates, but not for unstable and testing.
> I would say it depends on the software. Some differs more than other.


> > firefox (size & performance):
> > 
> >  -rw-r--r-- 1 jak jak 2.3M Aug 15 20:59 firefox_55.0-1_55.0-2_amd64.debdelta
> >  -rw-r--r-- 1 jak jak 2.4M Aug 15 22:13 firefox_55.0-1_55.0-2_amd64.pdeb
> and bsdfiff of old data.tar vs new data.dat 2.3MiB which is probably
> what debdelta does. So I am not sure if it is worth the extra mile to
> diff file by file. It might be faster and less memory hungry however.

debdelta also does some file thing, but not sure. We definitely want
to go per file, as we will not generate a new tar but directly patch
the installed files (basically, changing the code that unpacks a file
to .dpkg-new to apply the delta to the old file and write the result
to .dpkg-new) - you might not even have to read (all) the old files.

> > # Further extensions
> > 
> > If you put a pristine-tar delta into the delta file, you can fully
> > reconstruct debs.
> There is deltarpm [0] which does the same thing for rpm/fedora. I tried
> to find a design document but it seems that there is none. It seems
> however they grab all the binaries and make a delta via in-tree copy of
> bsdiff (delta.c). They also have intree-zlib but I didn't look why…
> I was mostly interrested if they have some filters to replace the
> offsets in binaries with something else but didn't find anything yet.

Right. We do have our own fork of bsdiff as well now, it's temporarily
known as jkdiff (https://github.com/julian-klode/jkdiff). The goal is
of course to turn this into a re-usable library and frontend programs,
and not limit it to "just" dpkg.

I did two changes compared to bsdiff:

* Instead of storing three different control (tuples of delta length,
  extra length, and an amount to seek), delta (sumwise difference
  of bytes), and extra (extra bytes to write); it now stores the tuple
  directly followed by the diff data and the extra data.

  This makes the patches streamable - we don't have to read from
  three different positions at the same time. Which is important for
  us because we can now apply the deltas without even extracting
  the individual diffs (which is important due to the next point).

* jkdiff files are uncompressed, bsdiff uses bzip2 on the control,
  delta, and extra blocks. We will store the files in an xz compressed
  tarball, so not compressing them first saves time and space. Also,
  xz is faster than bzip2 (or even gzip) when decompressing AFAICT.

The diff tool is still based on bsdiff, but jkpatch was completely
written from scratch and should be really easy to read. The jkpatch
tool applies diffs using SIMD instructions (using GCCs vector_size
attribute), which improved user time by about 50%.

Also, I applied the following changes for the generator:

* Debian: Do not support large files (> 2GB) - For large files, we need
  (9 * old + 1 * new) memory (essentially because we build a suffix
  offset array, so sizeof(off_t) * input size), so at 2GB that would
  be about 20GB of  RAM needed already. By only supporting small files,
  we reduce the memory overhead to 5 * old + new, so for the maximum of
  2 GB we only need 12 GB of RAM.
  This only affects generation though. The patches still use 64 bit
  values, and the patch tool supports large files.

* Chromium changes:
  - Replace embedded qsufsort with linking to libdivsufsort - much
    faster diff generation (for libreoffice it went from 93 seconds
    to 5 seconds), especially in some pathological
  - Some more fixes for pathological cases
  (See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=409664#44)

I'll be writing a blog post and/or email with more details

Debian Developer - deb.li/jak | jak-linux.org - free software dev
                  |  Ubuntu Core Developer |
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