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).
