Re: [PATCH 4/6] fetch-pack: optimize loading of refs via commit graph
From: Derrick Stolee <hidden>
Date: 2021-08-20 14:37:37
On 8/20/2021 6:08 AM, Patrick Steinhardt wrote:
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'This 2% gain is nice, especially because you are measuring the end-to-end scenario. If you use GIT_TRACE2_PERF=1 on a few runs, then you could likely isolate some of the regions from mark_complete_and_common_ref() and demonstrate a larger improvement in that focused area.
quoted hunk ↗ jump to hunk
@@ -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;
Obviously a correct thing to do.
if (type == OBJ_COMMIT) {
- struct commit *commit = lookup_commit(the_repository, oid);
+ commit = lookup_commit(the_repository, oid);Re-using the local simplifies this. Good. Thanks, -Stolee