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connected: do not sort input revisions

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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>
This commit is contained in:
Patrick Steinhardt
2021-08-09 10:11:50 +02:00
committed by Junio C Hamano
parent 29ef1f27fe
commit f45022dc2f
4 changed files with 48 additions and 1 deletions
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31
t/t6000-rev-list-misc.sh
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@ -169,4 +169,35 @@ test_expect_success 'rev-list --count --objects' '
test_line_count = $count actual
'
test_expect_success 'rev-list --unsorted-input results in different sorting' '
git rev-list --unsorted-input HEAD HEAD~ >first &&
git rev-list --unsorted-input HEAD~ HEAD >second &&
! test_cmp first second &&
sort first >first.sorted &&
sort second >second.sorted &&
test_cmp first.sorted second.sorted
'
test_expect_success 'rev-list --unsorted-input incompatible with --no-walk' '
cat >expect <<-EOF &&
fatal: --no-walk is incompatible with --unsorted-input
EOF
test_must_fail git rev-list --unsorted-input --no-walk HEAD 2>error &&
test_cmp expect error &&
test_must_fail git rev-list --unsorted-input --no-walk=sorted HEAD 2>error &&
test_cmp expect error &&
test_must_fail git rev-list --unsorted-input --no-walk=unsorted HEAD 2>error &&
test_cmp expect error &&
cat >expect <<-EOF &&
fatal: --unsorted-input is incompatible with --no-walk
EOF
test_must_fail git rev-list --no-walk --unsorted-input HEAD 2>error &&
test_cmp expect error &&
test_must_fail git rev-list --no-walk=sorted --unsorted-input HEAD 2>error &&
test_cmp expect error &&
test_must_fail git rev-list --no-walk=unsorted --unsorted-input HEAD 2>error &&
test_cmp expect error
'
test_done