Accelerate "git merge-base --is-ancestor"

It was recently [1] reported (and not-so-recently [2] reported) that "git merge-base --is-ancestor" can be pretty slow. In fact, it is regularly slower than "git branch --contains" or "git tag --contains", which are answering a "harder" query.

[1] https://lore.kernel.org/git/[email protected]/

[2] https://lore.kernel.org/git/[email protected]/

The root cause is that the in_merge_base() implementation is skipping the fast can_all_from_reach() implementation and using paint_down_to_common() instead. Note that these are equivalent: a commit A is in the set of merge-bases between A and B if and only if B can reach A.

This fixes the issue, and makes the performance degradation reported by Szeder a non-issue.

Thanks, -Stolee

Derrick Stolee (2):
commit-reach: create repo_is_descendant_of()
commit-reach: use fast logic in repo_in_merge_base

 commit-reach.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

base-commit: b3d7a52fac39193503a0b6728771d1bf6a161464
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-664%2Fderrickstolee%2Fmerge-base-is-ancestor-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-664/derrickstolee/merge-base-is-ancestor-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/664

gitgitgadget

From: Derrick Stolee <[email protected]>

The next change will make repo_in_merge_bases() depend on the logic in is_descendant_of(), but we need to make the method independent of the_repository first.

---
 commit-reach.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

index 4ca7e706a18..13722430aa5 100644
@@ -283,7 +283,9 @@ struct commit_list *repo_get_merge_bases(struct repository *r,
 /*
  * Is "commit" a descendant of one of the elements on the "with_commit" list?
  */
-int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
+static int repo_is_descendant_of(struct repository *r,
+				 struct commit *commit,
+				 struct commit_list *with_commit)
 {
 	if (!with_commit)
 		return 1;

@@ -301,13 +303,18 @@ int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
 
 			other = with_commit->item;
 			with_commit = with_commit->next;
-			if (in_merge_bases(other, commit))
+			if (repo_in_merge_bases(r, other, commit))
 				return 1;
 		}
 		return 0;
 	}
 }
 
+int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
+{
+	return repo_is_descendant_of(the_repository, commit, with_commit);
+}
+
 /*
  * Is "commit" an ancestor of one of the "references"?
  */

gitgitgadget

Thanks,
Taylor

I have good news for you. [1]

[1] https://lore.kernel.org/git/[email protected]/

Thanks,
-Stolee

From: Derrick Stolee <[email protected]>

The repo_is_descendant_of() method is aware of the existence of the commit-graph file. It checks for generation_numbers_enabled() before deciding on using can_all_from_reach() or repo_in_merge_bases() depending on the situation. The reason here is that can_all_from_reach() uses a depth-first search that is limited by the minimum generation number of the target commits, and that algorithm can be very slow when generation numbers are not present. The alternative uses paint_down_to_common() which will walk the entire merge-base boundary, which is typically slower.

This method is used by commands like "git tag --contains" and "git branch --contains" for very fast results when a commit-graph file exists. Unfortunately, it is _not_ used in commands like "git merge-base --is-ancestor" which is doing an even simpler request.

This issue was raised recently [1] with respect to a change to how generation numbers are stored, but was also reported much earlier [2] before commit-reach.c existed to simplify these reachability queries.

[1] https://lore.kernel.org/git/[email protected]/
[2] https://lore.kernel.org/git/[email protected]/

The root cause is that builtin/merge-base.c has a method handle_is_ancestor() that calls in_merge_bases(), an older version of repo_in_merge_bases(). It would be better if we have every caller to in_merge_bases() use the logic in can_all_from_reach() when possible.

This is where things get a little tricky: repo_is_descendant_of() calls repo_in_merge_bases() in the non-generation numbers enabled case! If we simply update repo_in_merge_bases() to call repo_is_descendant_of() instead of repo_in_merge_bases_many(), then we will get a recursive call loop. Thankfully, this is caught by the test suite in the default mode (i.e. GIT_TEST_COMMIT_GRAPH=0).

The trick, then, is to make the non-generation number case for repo_is_descendant_of() call repo_in_merge_bases_many() directly, skipping the non-_many version. This allows us to take advantage of this faster code path, when possible.

The easiest way to measure the performance impact is to test the following command on the Linux kernel repository:

	git merge-base --is-ancestor <A> <B>

  | A    | B    | Time Before | Time After |
  |------|------|-------------|------------|
  | v3.0 | v5.7 | 0.459s      | 0.028s     |
  | v4.0 | v5.7 | 0.267s      | 0.021s     |
  | v5.0 | v5.7 | 0.074s      | 0.013s     |

Note that each of these samples return success. The old code performed the same operation when <A> and <B> are swapped. However, can_all_from_reach() will return immediately if the generation numbers show that <A> has larger generation number than <B>. Thus, the time for the swapped case is universally 0.004s in each case.

---
 commit-reach.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

index 13722430aa5..43e303d5f25 100644
@@ -303,7 +303,7 @@ static int repo_is_descendant_of(struct repository *r,
 
 			other = with_commit->item;
 			with_commit = with_commit->next;
-			if (repo_in_merge_bases(r, other, commit))
+			if (repo_in_merge_bases_many(r, other, 1, &commit))
 				return 1;
 		}
 		return 0;

@@ -355,7 +355,15 @@ int repo_in_merge_bases(struct repository *r,
 			struct commit *commit,
 			struct commit *reference)
 {
-	return repo_in_merge_bases_many(r, commit, 1, &reference);
+	int res;
+	struct commit_list *list = NULL;
+	struct commit_list **next = &list;
+
+	next = commit_list_append(commit, next);
+	res = repo_is_descendant_of(r, reference, list);
+	free_commit_list(list);
+
+	return res;
 }
 
 struct commit_list *reduce_heads(struct commit_list *heads)

gitgitgadget

OK, so in short, this codepath was not taking advantage of the generation numbers and that was the difference?

Correct. We now use generation numbers more often, when available. All callers of in_merge_bases() will become faster, which includes callers in these builtins:

- branch
- fetch
- log
- merge-base
- receive-pack

and these libgit files:

- fast-import.c
- http-push.c
- merge-recursive.c (several!)
- pack-bitmap-write.c

Thanks,
-Stolee

Wow! Thanks for investigating through the issue and following up. The performance numbers speak for themselves.

By applying this series on the commit-slab patch series, both are now just as fast (master: 0.048s, commit-slab: 0.050s).

Regards
Abhishek