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p6012: add perf test for merge-base with deep side branch
Add a perf test that benchmarks merge-base on a synthetic 500k-commit repository with a deep side branch. Includes variants with and without a commit-graph to cover the case where both tips are outside the commit-graph (INFINITY region). Signed-off-by: Kristofer Karlsson <krka@spotify.com>
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‎t/perf/p6012-merge-base-deep-side-branch.sh‎

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#!/bin/sh
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test_description='Performance of merge-base with a deep side branch
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Synthetic repository that triggers slow merge-base when a merge commit
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introduces a side branch rooted far back in history. The pathological
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case forces paint_down_to_common() to chase STALE flags through many
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intermediate commits after the merge-base is already found.
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Topology (N = $NUM_COMMITS commits on main before the merge, F is
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the BRANCH_POINT-th of them, S = $SIDE_COMMITS commits on the side):
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# F -- s1 -- ... -- sS (side, dates set old)
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# / \
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# 1 -- 2 -- ... -- N-1 -- N -- M (main)
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# |
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# P (pr branch)
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merge-base(M, P) = N. Without early termination, the algorithm
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walks from N back through roughly N - BRANCH_POINT commits to F
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to exhaust STALE processing.
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'
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. ./perf-lib.sh
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NUM_COMMITS=500000
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BRANCH_POINT=10000
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SIDE_COMMITS=5
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EXTRA_COMMITS=10
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test_expect_success 'setup synthetic repo' '
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git init --bare repo.git &&
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awk -v N=$NUM_COMMITS -v BP=$BRANCH_POINT -v S=$SIDE_COMMITS \
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-v E=$EXTRA_COMMITS '\''
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BEGIN {
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# Shared blob
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print "blob"
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print "mark :1"
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print "data 8"
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print "content"
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print ""
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# Main branch commits: mark :2 is the 1st main
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# commit, mark :(N+1) is the Nth.
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for (i = 2; i <= N + 1; i++) {
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print "commit refs/heads/main"
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print "mark :" i
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print "committer C <c@c> " (1000000 + i) " +0000"
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print "data 2"
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print "x"
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if (i > 2)
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print "from :" (i - 1)
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print "M 100644 :1 file"
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print ""
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}
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# Side branch forks from the BP-th main commit
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# (mark :(BP+1)), with old dates.
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side_start = BP + 1
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side_mark_base = N + 2
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for (j = 0; j < S; j++) {
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mark = side_mark_base + j
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if (j == 0)
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from_mark = side_start
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else
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from_mark = mark - 1
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print "commit refs/heads/side"
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print "mark :" mark
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print "committer C <c@c> " (500000 + j) " +0000"
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print "data 2"
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print "x"
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print "from :" from_mark
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print ""
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}
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# Merge side into main
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main_tip = N + 1
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side_tip = side_mark_base + S - 1
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merge_mark = side_tip + 1
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print "commit refs/heads/main"
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print "mark :" merge_mark
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print "committer C <c@c> " (1000000 + N + 2) " +0000"
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print "data 2"
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print "x"
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print "from :" main_tip
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print "merge :" side_tip
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print ""
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# PR branch forked from main tip (just before merge)
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pr_mark = merge_mark + 1
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print "commit refs/heads/pr"
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print "mark :" pr_mark
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print "committer C <c@c> " (1000000 + N + 3) " +0000"
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print "data 2"
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print "x"
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print "from :" main_tip
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print ""
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# Save refs before extra commits for the
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# commit-graph boundary.
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print "reset refs/markers/graph-boundary-main"
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print "from :" merge_mark
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print ""
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print "reset refs/markers/graph-boundary-pr"
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print "from :" pr_mark
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print ""
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# Extra commits on main (beyond commit-graph).
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next_mark = pr_mark + 1
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from_mark = merge_mark
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for (k = 0; k < E; k++) {
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print "commit refs/heads/main"
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print "mark :" next_mark
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print "committer C <c@c> " (2000000 + k) " +0000"
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print "data 2"
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print "x"
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print "from :" from_mark
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print ""
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from_mark = next_mark
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next_mark++
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}
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# Extra commits on pr (beyond commit-graph).
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from_mark = pr_mark
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for (k = 0; k < E; k++) {
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print "commit refs/heads/pr"
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print "mark :" next_mark
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print "committer C <c@c> " (2000000 + k) " +0000"
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print "data 2"
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print "x"
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print "from :" from_mark
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print ""
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from_mark = next_mark
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next_mark++
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}
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}
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'\'' </dev/null |
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git -C repo.git fast-import --quiet &&
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# Write commit-graph covering everything up to the boundary
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# markers, leaving the extra commits in the INFINITY region.
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git -C repo.git rev-list refs/markers/graph-boundary-main \
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refs/markers/graph-boundary-pr |
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git -C repo.git commit-graph write --stdin-commits &&
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git -C repo.git rev-parse main~1~$EXTRA_COMMITS >expect
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'
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test_expect_success 'merge-base result is correct' '
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git -C repo.git merge-base --all main pr >actual &&
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test_cmp expect actual
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'
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test_perf 'merge-base: both tips in INFINITY region' '
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git -C repo.git merge-base --all main pr
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'
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# Rewrite commit-graph to cover all commits, so both tips are in
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# the finite region.
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test_expect_success 'setup: full commit-graph' '
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git -C repo.git commit-graph write --reachable
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'
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test_perf 'merge-base: both tips in finite region' '
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git -C repo.git merge-base --all main pr
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'
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test_perf 'merge-base: no side branch (baseline)' '
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git -C repo.git merge-base --all main~1~$EXTRA_COMMITS pr
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'
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# Remove commit-graph entirely: all 500k commits are INFINITY.
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test_expect_success 'setup: remove commit-graph' '
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rm -f repo.git/objects/info/commit-graph &&
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rm -rf repo.git/objects/info/commit-graphs
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'
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test_perf 'merge-base: all INFINITY (no commit-graph)' '
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git -C repo.git merge-base --all main pr
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'
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test_expect_success 'merge-base result correct without commit-graph' '
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git -C repo.git merge-base --all main pr >actual &&
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test_cmp expect actual
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'
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test_done

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