Re: [PATCH 4/6] fetch-pack: optimize loading of refs via commit graph

[Derrick Stolee <stolee@gmail.com>]

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