From: Karthik Nayak Date: Thu, 11 Jun 2026 08:16:52 GMT Subject: Re: [PATCH v2 2/2] ref-filter: memoize --contains with generations Message-ID: In-Reply-To: Tamir Duberstein writes: > On Wed, Jun 10, 2026 at 4:47 AM Karthik Nayak wrote: >> >> Tamir Duberstein writes: >> >> > git branch and git for-each-ref call repo_is_descendant_of() for >> > each candidate selected by --contains or --no-contains. Each call >> > starts a new graph walk, so refs with shared history repeatedly >> > traverse the same commits. >> > >> > ffc4b8012d (tag: speed up --contains calculation, 2011-06-11) >> > introduced a depth-first walk for git tag that caches positive and >> > negative answers across candidates. ee2bd06b0f (ref-filter: implement >> > '--contains' option, 2015-07-07) preserved both implementations when >> > ref-filter learned --contains. >> > >> > The memoized walk is not always faster. Without generation numbers, >> > a negative check can walk to the root even when the breadth-first >> > merge-base walk finds a nearby divergence. With generation numbers, >> > the depth-first walk can stop below the oldest target while still >> > reusing answers across candidates. >> > >> > Keep the existing memoized selection for git tag. Select it for other >> > ref-filter callers when generation numbers are enabled, and retain >> > the breadth-first walk otherwise. >> > >> > When generation numbers are unavailable, repo_is_descendant_of() can >> > return -1 if ancestry cannot be read. The ref-filter Boolean interface >> > treated that error as a match. Check it and exit instead. The memoized >> > path already dies on the same parse failure, so both selected paths now >> > fail rather than return a result. >> > >> > Add p1500 cases for up to 8,192 packed refs along one first-parent >> > history and for sibling refs near the tip with generation numbers >> > forced off. >> > >> > On a checkout with 62,174 remote-tracking refs and generation numbers >> > enabled, I ran: >> > >> > hyperfine --warmup 0 --runs 3 \ >> > --command-name parent \ >> > '"$parent" branch -r --contains c78ae85f3ce7e >/dev/null' \ >> > --command-name this-commit \ >> > '"$this" branch -r --contains c78ae85f3ce7e >/dev/null' >> > >> > The results were: >> > >> > parent this commit >> > elapsed 104.365 s 467.7 ms >> > user 93.702 s 220.2 ms >> > system 0.723 s 182.7 ms >> > >> > The wall-time standard deviations were 11.356 seconds and 133.8 >> > milliseconds, respectively. Separate runs without redirection produced >> > the same output with SHA-256 >> > 2466f6e2b72aa16b1a2126eddb81c8a1b2764ee251204ac034c191a925aa896f. >> > >> > Both revisions were built with the default -O2 flags using Apple >> > clang 21.0.0 on macOS 26.5. The machine was a MacBook Pro (Mac16,6) >> > with a 16-core Apple M4 Max (12 performance and four efficiency >> > cores) and 128 GB RAM. >> > >> > Link: https://lore.kernel.org/git/1445163904-24611-1-git-send-email-Karthik.188@gmail.com/ >> > Link: https://lore.kernel.org/r/20230324191009.GA536967@coredump.intra.peff.net >> > Link: https://lore.kernel.org/git/20260527070510.3510836-1-krka@spotify.com/ >> > Link: https://lore.kernel.org/r/20260608223430.GA340696@coredump.intra.peff.net >> > Suggested-by: Jeff King >> > Signed-off-by: Tamir Duberstein >> > --- >> > commit-reach.c | 13 +++++++++-- >> > commit-reach.h | 7 ++++++ >> > t/perf/p1500-graph-walks.sh | 49 +++++++++++++++++++++++++++++++++++++++++- >> > t/t6301-for-each-ref-errors.sh | 22 +++++++++++++++++++ >> > 4 files changed, 88 insertions(+), 3 deletions(-) >> > >> > diff --git a/commit-reach.c b/commit-reach.c >> > index 65b618959b..83a48004ef 100644 >> > --- a/commit-reach.c >> > +++ b/commit-reach.c >> > @@ -821,9 +821,18 @@ static enum contains_result contains_tag_algo(struct commit *candidate, >> > int commit_contains(struct ref_filter *filter, struct commit *commit, >> > struct commit_list *list, struct contains_cache *cache) >> > { >> > - if (filter->with_commit_tag_algo) >> > + int result; >> > + >> > + if (!list) >> > + return 1; >> > + if (filter->with_commit_tag_algo || >> > + generation_numbers_enabled(the_repository)) >> >> What's stopping us from dropping `filter->with_commit_tag_algo` >> completely and then doing? >> >> if (generation_numbers_enabled(the_repository)) >> return contains_algo(commit, list, cache) == CONTAINS_YES; >> return repo_is_descendant_of(the_repository, commit, list); > > Jeff raised this distinction during the v1 review: > > https://lore.kernel.org/r/20260608223430.GA340696@coredump.intra.peff.net/ > > `with_commit_tag_algo` preserves the existing behavior of `git tag` when > generation numbers are unavailable. `git tag --contains` has used the > memoized walk since ffc4b8012d (tag: speed up --contains calculation, > 2011-06-11). Dropping the flag would send it back through repeated > `repo_is_descendant_of()` walks in repositories without usable generation > numbers. > I did read that, my question is on top of that. Do we also want to use the non-memoized walk for 'git tag' when there are no generation numbers available or does that not work? If not, we should mention that too in the commit message. > The condition in v2 implements the rule discussed there: retain the > existing memoized path for `git tag`, and use it for other ref-filter > callers when generation numbers make the depth-first walk reliably > advantageous. > > This is probably my fault for breaking the threading between this and > v1. Sorry about that.