fetch-pack: avoid quadratic list insertion in mark_complete

We insert the commit pointed to by each ref one-by-one into
the "complete" commit_list using insert_by_date. Because
each insertion is O(n), we end up with O(n^2) behavior.

This typically doesn't matter, because the number of refs is
reasonably small. And even if there are a lot of refs, they
often point to a smaller set of objects (in which case the
optimization in commit ea5f220 keeps our "n" small).

However, in pathological repositories (hundreds of thousands
of refs, each pointing to a unique commit), this quadratic
behavior can make a difference. Since we do not care about
the list order until we have finished building it, we can
simply keep it unsorted during the insertion phase, then
sort it afterwards.

On a repository like the one described above, this dropped
the time to do a no-op fetch from 2.0s to 1.7s. On normal
repositories, it probably does not matter at all, but it
does not hurt to protect ourselves from pathological cases.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/fetch-pack.c b/fetch-pack.c
index abe5ffb..4df8abd 100644 (file)
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -505,7 +505,7 @@ static int mark_complete(const char *refname, const unsigned char *sha1, int fla
                struct commit *commit = (struct commit *)o;
                if (!(commit->object.flags & COMPLETE)) {
                        commit->object.flags |= COMPLETE;
-                       commit_list_insert_by_date(commit, &complete);
+                       commit_list_insert(commit, &complete);
                }
        }
        return 0;
@@ -622,6 +622,7 @@ static int everything_local(struct fetch_pack_args *args,
        if (!args->depth) {
                for_each_ref(mark_complete, NULL);
                for_each_alternate_ref(mark_alternate_complete, NULL);
+               commit_list_sort_by_date(&complete);
                if (cutoff)
                        mark_recent_complete_commits(args, cutoff);
        }