In order to negotiate a packfile, we need to dereference refs to see
which commits we have in common with the remote. To do so, we first look
up the object's type -- if it's a tag, we peel until we hit a non-tag
object. If we hit a commit eventually, then we return that commit.
In case the object ID points to a commit directly, we can avoid the
initial lookup of the object type by opportunistically looking up the
commit via the commit-graph, if available, which gives us a slight speed
bump of about 2% in a huge repository with about 2.3M refs:
Benchmark #1: HEAD~: git-fetch
Time (mean ± σ): 31.634 s ± 0.258 s [User: 28.400 s, System: 5.090 s]
Range (min … max): 31.280 s … 31.896 s 5 runs
Benchmark #2: HEAD: git-fetch
Time (mean ± σ): 31.129 s ± 0.543 s [User: 27.976 s, System: 5.056 s]
Range (min … max): 30.172 s … 31.479 s 5 runs
Summary
'HEAD: git-fetch' ran
1.02 ± 0.02 times faster than 'HEAD~: git-fetch'
In case this fails, we fall back to the old code which peels the
objects to a commit.
---
fetch-pack.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/fetch-pack.c b/fetch-pack.c
index 1a6242cd71..c57faf278f 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -119,6 +119,11 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
{
enum object_type type;
struct object_info info = { .typep = &type };
+ struct commit *commit;
+
+ commit = lookup_commit_in_graph(the_repository, oid);
+ if (commit)
+ return commit;
while (1) {
if (oid_object_info_extended(the_repository, oid, &info,
@@ -139,7 +144,7 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
}
if (type == OBJ_COMMIT) {
- struct commit *commit = lookup_commit(the_repository, oid);
+ commit = lookup_commit(the_repository, oid);
if (!commit || repo_parse_commit(the_repository, commit))
return NULL;
return commit;
--
2.33.0
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