db8791aaf2
Remove the fallback that switched paint_down_to_common() from
generation ordering to commit-date ordering when the commit-graph
lacks corrected commit dates (v1 graph with topo levels only).
The fallback was added in 091f4cf3 (commit: don't use generation
numbers if not needed, 2018-08-30) to avoid a performance
regression on the Linux kernel repo where v1 topo levels caused
"git merge-base v4.8 v4.9" to walk 636k commits instead of 167k.
A side branch with a low topo level stayed in the queue behind a
long chain, preventing early STALE propagation.
Side-exhaustion (added in the previous commits) solves this
differently by terminating the walk as soon as one paint side
empties from the queue, preventing the deep walk regardless of
queue ordering. Benchmarks of "git merge-base --all v4.8 v4.9"
on the Linux kernel repo show that side-exhaustion reduces the
step count far below what the date-ordering fallback achieved:
steps time
no graph, baseline: 167,413 3.25 s
v1 graph, baseline: 167,413 0.25 s
v2 graph, baseline: 167,441 0.29 s
v1 graph, this series: 5,725 0.02 s
v2 graph, this series: 3,887 0.01 s
With generation ordering always active, the existing min_generation
check in paint_queue_get() correctly identifies when the walk has
reached the finite generation region. The date ordering fallback
broke this invariant: a commit could have a finite topo level
while the queue was date-ordered, causing the early exit to fire
before all merge bases were found.
Also remove corrected_commit_dates_enabled() from commit-graph.c
which has no remaining callers.
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
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