path: root/t/perf/
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authorKevin Willford <>2016-07-29 16:19:20 (GMT)
committerJunio C Hamano <>2016-08-11 21:39:16 (GMT)
commitb3dfeebb92630c54db1e4f03dbcff0e05208c4c1 (patch)
treec994a03fe0911554245be1e40f4f68c2764ae922 /t/perf/
parent3e8e32c32ef8e49bcfd715837d51aca30925fdfe (diff)
rebase: avoid computing unnecessary patch IDs
The `rebase` family of Git commands avoid applying patches that were already integrated upstream. They do that by using the revision walking option that computes the patch IDs of the two sides of the rebase (local-only patches vs upstream-only ones) and skipping those local patches whose patch ID matches one of the upstream ones. In many cases, this causes unnecessary churn, as already the set of paths touched by a given commit would suffice to determine that an upstream patch has no local equivalent. This hurts performance in particular when there are a lot of upstream patches, and/or large ones. Therefore, let's introduce the concept of a "diff-header-only" patch ID, compare those first, and only evaluate the "full" patch ID lazily. Please note that in contrast to the "full" patch IDs, those "diff-header-only" patch IDs are prone to collide with one another, as adjacent commits frequently touch the very same files. Hence we now have to be careful to allow multiple hash entries with the same hash. We accomplish that by using the hashmap_add() function that does not even test for hash collisions. This also allows us to evaluate the full patch ID lazily, i.e. only when we found commits with matching diff-header-only patch IDs. We add a performance test that demonstrates ~1-6% improvement. In practice this will depend on various factors such as how many upstream changes and how big those changes are along with whether file system caches are cold or warm. As Git's test suite has no way of catching performance regressions, we also add a regression test that verifies that the full patch ID computation is skipped when the diff-header-only computation suffices. Signed-off-by: Kevin Willford <> Signed-off-by: Junio C Hamano <>
Diffstat (limited to 't/perf/')
1 files changed, 36 insertions, 0 deletions
diff --git a/t/perf/ b/t/perf/
new file mode 100644
index 0000000..b3e7d52
--- /dev/null
+++ b/t/perf/
@@ -0,0 +1,36 @@
+test_description='Tests rebase performance'
+. ./
+test_expect_success 'setup' '
+ git checkout -f -b base &&
+ git checkout -b to-rebase &&
+ git checkout -b upstream &&
+ for i in $(seq 100)
+ do
+ # simulate huge diffs
+ echo change$i >unrelated-file$i &&
+ seq 1000 >>unrelated-file$i &&
+ git add unrelated-file$i &&
+ test_tick &&
+ git commit -m commit$i unrelated-file$i &&
+ echo change$i >unrelated-file$i &&
+ seq 1000 | tac >>unrelated-file$i &&
+ git add unrelated-file$i &&
+ test_tick &&
+ git commit -m commit$i-reverse unrelated-file$i ||
+ break
+ done &&
+ git checkout to-rebase &&
+ test_commit our-patch interesting-file
+test_perf 'rebase on top of a lot of unrelated changes' '
+ git rebase --onto upstream HEAD^ &&
+ git rebase --onto base HEAD^