Re: [PATCH v2 2/4] pack-objects: support reachability bitmaps with `--path-walk`

[Taylor Blau <me@ttaylorr.com>]

On Fri, Jun 19, 2026 at 10:40:51AM -0400, Derrick Stolee wrote:
> > [...]
> > , which gives us:
> >
> >     Test                                            HEAD^             HEAD
> >     ----------------------------------------------------------------------------------------
> >     5311.3: size of bitmapped pack                           278.8M            278.8M -0.0%
> >     5311.38: size of bitmapped pack (--path-walk)            278.7M            278.7M +0.0%
> >
> > (eliding other tests). I considered whether there are other interesting
> > tests, but I think "repack" is the right layer to run perf tests, since
> > you're always writing a closed pack. We could try different subsets of
> > the repository's objects (which would also have to be closed), but I
> > don't think this is that interesting.
>
> This sort of thing does help to show that we're getting different
> behavior when repacking with and without --path-walk. And this test
> is showing the slightest change for git.git, but is likely more
> impactful for the other repos I've used to demonstrate the benefits.
>
> So this is the kind of data I'm hoping to see, but also with data
> from other repos whose data shapes benefit from --path-walk more
> than git.git and repos where name-hash v1 is sufficient to give a
> similar result.

I'm glad this is the sort of data you're looking for. I'm happy to run
this on other repositories.

> I'd also like to see if the repack _time_ changes with this, but
> these direct size comparisons are the biggest indicator I'd like to
> see.

Unfortunately a timing comparison is kind of a pain here. We'd have to
use test_perf, which will perform the same repack multiple times. We
could do that, though it's wasteful, and changes like bf4a60874af
(p5326: generate pack bitmaps before writing the MIDX bitmap,
2021-09-17) move us in the opposite direction.

I'm not opposed to changing this to test_perf if you feel strongly about
it.

Thanks,
Taylor