[PATCH] fetch-pack: speed up loading of refs via commit graph

From: Patrick Steinhardt <ps@pks.im>
Date: Aug 4, 2021, 13:56 UTC
Message-ID: <08519b8ab6f395cffbcd5e530bfba6aaf64241a2.1628085347.git.ps@pks.im>

When doing reference negotiation, git-fetch-pack(1) is loading all refs from disk in order to determine which commits it has in common with the remote repository. This can be quite expensive in repositories with many references though: in a real-world repository with around 2.2 million refs, fetching a single commit by its ID takes around 44 seconds.

Dominating the loading time is decompression and parsing of the objects which are referenced by commits. Given the fact that we only care about commits (or tags which can be peeled to one) in this context, there is thus an easy performance win by switching the parsing logic to make use of the commit graph in case we have one available. Like this, we avoid hitting the object database to parse these commits but instead only load them from the commit-graph. This results in a significant performance boost when executing git-fetch in said repository with 2.2 million refs:

    Benchmark #1: HEAD~: git fetch $remote $commit
      Time (mean ± σ):     44.168 s ±  0.341 s    [User: 42.985 s, System: 1.106 s]
      Range (min … max):   43.565 s … 44.577 s    10 runs

    Benchmark #2: HEAD: git fetch $remote $commit
      Time (mean ± σ):     19.498 s ±  0.724 s    [User: 18.751 s, System: 0.690 s]
      Range (min … max):   18.629 s … 20.454 s    10 runs

    Summary
      'HEAD: git fetch $remote $commit' ran
        2.27 ± 0.09 times faster than 'HEAD~: git fetch $remote $commit'

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 fetch-pack.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fetch-pack.c b/fetch-pack.c
index b0c7be717c..0bf7ed7e47 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -137,8 +137,14 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
 			break;
 		}
 	}
-	if (type == OBJ_COMMIT)
-		return (struct commit *) parse_object(the_repository, oid);
+
+	if (type == OBJ_COMMIT) {
+		struct commit *commit = lookup_commit(the_repository, oid);
+		if (!commit || repo_parse_commit(the_repository, commit))
+			return NULL;
+		return commit;
+	}
+
 	return NULL;
 }

-- 
2.32.0