On Mon, Aug 02 2021, Patrick Steinhardt wrote:

[[PGP Signed Part:Undecided]]
In order to compute whether objects reachable from a set of tips are all
connected, we do a revision walk with these tips as positive references
and `--not --all`. `--not --all` will cause the revision walk to load
all preexisting references as uninteresting, which can be very expensive
in repositories with many references.

Benchmarking the git-rev-list(1) command highlights that by far the most
expensive single phase is initial sorting of the input revisions: after
all references have been loaded, we first sort commits by author date.
In a real-world repository with about 2.2 million references, it makes
up about 40% of the total runtime of git-rev-list(1).

Ultimately, the connectivity check shouldn't really bother about the
order of input revisions at all. We only care whether we can actually
walk all objects until we hit the cut-off point. So sorting the input is
a complete waste of time.

Really good results:

Introduce a new "--unsorted-input" flag to git-rev-list(1) which will
cause it to not sort the commits and adjust the connectivity check to
always pass the flag. This results in the following speedups, executed
in a clone of gitlab-org/gitlab [1]:

    Benchmark #1: git rev-list  --objects --quiet --not --all --not $(cat newrev)
      Time (mean ± σ):      7.639 s ±  0.065 s    [User: 7.304 s, System: 0.335 s]
      Range (min … max):    7.543 s …  7.742 s    10 runs

    Benchmark #2: git rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.995 s ±  0.044 s    [User: 4.657 s, System: 0.337 s]
      Range (min … max):    4.909 s …  5.048 s    10 runs

    Summary
      'git rev-list --unsorted-input --objects --quiet --not --all --not $(cat newrev)' ran
        1.53 ± 0.02 times faster than 'git rev-list  --objects --quiet --not --all --not $newrev'

Just bikeshedding for a potential re-roll, perhaps --unordered-input, so
that it matches/rhymes with the existing "git cat-file --unordered",
which serves the same conceptual purpose (except this one's input, that
one's output).