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.
> @@ -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
