connected: do not sort input revisions

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.

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'

[1]: https://gitlab.com/gitlab-org/gitlab.git. Note that not all refs
     are visible to clients.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/revision.c b/revision.c
index 86bbcd10d2..47541407d2 100644
--- a/revision.c
+++ b/revision.c
@@ -2256,6 +2256,10 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--author-date-order")) {
 		revs->sort_order = REV_SORT_BY_AUTHOR_DATE;
 		revs->topo_order = 1;
+	} else if (!strcmp(arg, "--unsorted-input")) {
+		if (revs->no_walk)
+			die(_("--unsorted-input is incompatible with --no-walk"));
+		revs->unsorted_input = 1;
 	} else if (!strcmp(arg, "--early-output")) {
 		revs->early_output = 100;
 		revs->topo_order = 1;
@@ -2651,8 +2655,13 @@ static int handle_revision_pseudo_opt(const char *submodule,
 	} else if (!strcmp(arg, "--not")) {
 		*flags ^= UNINTERESTING | BOTTOM;
 	} else if (!strcmp(arg, "--no-walk")) {
+		if (!revs->no_walk && revs->unsorted_input)
+			die(_("--no-walk is incompatible with --unsorted-input"));
 		revs->no_walk = 1;
 	} else if (skip_prefix(arg, "--no-walk=", &optarg)) {
+		if (!revs->no_walk && revs->unsorted_input)
+			die(_("--no-walk is incompatible with --unsorted-input"));
+
 		/*
 		 * Detached form ("--no-walk X" as opposed to "--no-walk=X")
 		 * not allowed, since the argument is optional.