Re: [PATCH 1/6] fetch: speed up lookup of want refs via commit-graph

On 8/20/2021 6:08 AM, Patrick Steinhardt wrote:

When updating our local refs based on the refs fetched from the remote,
we need to iterate through all requested refs and load their respective
commits such that we can determine whether they need to be appended to
FETCH_HEAD or not. In cases where we're fetching from a remote with
exceedingly many refs, resolving these refs can be quite expensive given
that we repeatedly need to unpack object headers for each of the
referenced objects.

Speed this up by opportunistcally trying to resolve object IDs via the
commit graph: more likely than not, they're going to be a commit anyway,
and this lets us avoid having to unpack object headers completely in
case the object is a commit that is part of the commit-graph. This
significantly speeds up mirror-fetches in a real-world repository with
2.3M refs:

    Benchmark #1: HEAD~: git-fetch
      Time (mean ± σ):     56.942 s ±  0.449 s    [User: 53.360 s, System: 5.356 s]
      Range (min … max):   56.372 s … 57.533 s    5 runs

    Benchmark #2: HEAD: git-fetch
      Time (mean ± σ):     33.657 s ±  0.167 s    [User: 30.302 s, System: 5.181 s]
      Range (min … max):   33.454 s … 33.844 s    5 runs

    Summary
      'HEAD: git-fetch' ran
        1.69 ± 0.02 times faster than 'HEAD~: git-fetch'

These numbers are impressive, and it makes sense that performing a
binary search on the OID lookup chunk of the commit-graph is faster
than doing a binary search on the OIDs across the pack-index(es).

I do worry about the case where annotated tags greatly outnumber
branches, so this binary search is extra overhead and the performance
may degrade. Would it be worth checking the ref to see if it lies
within "refs/heads/" (or even _not_ in "refs/tags/") before doing
this commit-graph check?

-			commit = lookup_commit_reference_gently(the_repository,
-								&rm->old_oid,
-								1);
-			if (!commit)
-				rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
+			commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
+			if (!commit) {
+				commit = lookup_commit_reference_gently(the_repository,
+									&rm->old_oid,
+									1);
+				if (!commit)
+					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;

nit: I wouldn't nest this last "if (!commit)".

Code will work as advertised.

Thanks,
-Stolee