Re: Git packs friendly to block-level deduplication

[Jeff King <peff@peff.net>]

On Wed, Jan 24, 2018 at 07:10:15PM -0500, Jeff King wrote:

> On Thu, Jan 25, 2018 at 01:03:25AM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> > > You may also want to use "--threads=1" to avoid non-determinism in the
> > > generated packs. In theory, both repos would then produce identical base
> > > packs, though it does not seem to do so in practice (I didn't dig in to
> > > what the different may be).
> > 
> > ..and north of 20% with --threads=1.
> >
> > [...]
> >
> > Thanks. I'll look into that, although the above results (sans hacking on
> > the core pack-objects logic) suggest that even once I create an island
> > I'm getting at most 20%.
> 
> I think it may be worth figuring out where the two differ. With
> --no-reuse-object and --no-reuse-delta, I'd think that the pack
> generated for a particular apex commit would be totally deterministic,
> regardless of other objects available in the repo. But it's not for some
> reason.

I think I see it. If I compare the objects in pack-order of the two
packs:

  diff -u <(git show-index <$one | sort -n) <(git show-index <$two | sort -n)

I get:

  6281 fac64e011f1b1ecabcccf7ad2511efcac3e26bdc (9381a4f9)
  6585 59c276cf4da0705064c32c9dba54baefa282ea55 (1b02a1c0)
  6869 8279ed033f703d4115bee620dccd32a9ec94d9aa (ca7acf33)
 -7042 298d861208d71089dd308761ae96738e81ad3e68 (135aefd7)
 -7222 ea7b5de1c1187294d3d4dca93b129e049ca7ca76 (3708bbfc)
 -7512 b6947af2294ea0c814f5b4cb8737c782895519b2 (719fad14)
 -8004 e26f7f19b6c7485f04234946a59ab8f4fd21d6d1 (59127876)
 -8826 2512f15446149235156528dafbe75930c712b29e (fc72468e)
  [...]
 +7042 2512f15446149235156528dafbe75930c712b29e (fc72468e)
 +7215 c6c75c93aaeb97bab6fd25b672a641c84dd85d59 (68fe0b8d)
 +7391 36438dc19dd2a305dddebd44bf7a65f1a220075b (c424e726)
 +7565 1eaabe34fc6f486367a176207420378f587d3b48 (4a27450b)

So the write order differs; one pack stuck 2512f15446 much earlier.

That's probably due to this line from compute_write_order():

     /*
      * Mark objects that are at the tip of tags.
      */
     for_each_tag_ref(mark_tagged, NULL);

If I remove that line, then your manual pack (with --threads=1) gives me
identical packs for each repo.

And if I then mark each with a .keep, then "git repack -ad" to remove
duplicates, your block-dedup check yields:

   17742 2
    1939 1

which is 90%, and about what I'd expect between those two repos. So I
think the idea of computing a "base" pack (either as a separate pack, or
as a layer in the pack) is a viable strategy.

-Peff