Re: [PATCH 7/7] builtin/merge-tree.c: implement support for `--write-pack`

[Jeff King <peff@peff.net>]

On Sun, Oct 08, 2023 at 09:37:42PM -0400, Taylor Blau wrote:

> Very interesting, thanks for running (and documenting!) this experiment.
> I'm mostly with you that it probably doesn't make a huge difference in
> practice here.
> 
> One thing that I'm not entirely clear on is how we'd treat objects that
> could be good delta candidates for each other between two packs. For
> instance, if I write a tree corresponding to the merge between two
> branches, it's likely that the resulting tree would be a good delta
> candidate against either of the trees at the tips of those two refs.
> 
> But we won't pack those trees (the ones at the tips of the refs) in the
> same pack as the tree containing their merge. If we later on tried to
> repack, would we evaluate the tip trees as possible delta candidates
> against the merged tree? Or would we look at the merged tree, realize it
> isn't delta'd with anything, and then not attempt to find any
> candidates?

When we repack (either all-into-one, or in a geometric roll-up), we
should consider those trees as candidates. The only deltas we don't
consider are:

  - if something is already a delta in a pack, then we will usually
    reuse that delta verbatim (so you might get fooled by a mediocre
    delta and not go to the trouble to search again. But I don't think
    that applies here; there is no other tree in your new pack to make
    such a mediocre delta from, and anyway you are skipping deltas
    entirely)

  - if two objects are in the same pack but there's no delta
    relationship, the try_delta() heuristics will skip them immediately
    (under the assumption that we tried during the last repack and
    didn't find anything good).

So yes, if you had a big old pack with the original trees, and then a
new pack with the merge result, we should try to delta the new merge
result tree against the others, just as we would if it were loose.

> > I was going to suggest the same thing. ;) Unfortunately it's a bit
> > tricky to do as we have no room in the file format for an optional flag.
> > You'd have to add a ".mediocre-delta" file or something.
> 
> Yeah, I figured that we'd add a new ".baddeltas" file or something. (As
> an aside, we probably should have an optional flags section in the .pack
> format, since we seem to have a lot of optional pack extensions: .rev,
> .bitmap, .keep, .promisor, etc.)

Yes, though since packv2 is the on-the-wire format, it's very hard to
change now. It might be easier to put an annotation into the .idx file.

-Peff