commit-graph write: use pack order when finding commits

Slightly optimize the "commit-graph write" step by using
FOR_EACH_OBJECT_PACK_ORDER with for_each_object_in_pack(). See commit
[1] and [2] for the facility and a similar optimization for "cat-file".

On Linux it is around 5% slower to run:

echo 1 >/proc/sys/vm/drop_caches &&
cat .git/objects/pack/* >/dev/null &&
git cat-file --batch-all-objects --batch-check --unordered

Than the same thing with the "cat" omitted. This is as expected, since
we're iterating in pack order and the "cat" is extra work.

Before this change the opposite was true of "commit-graph write". We
were 6% faster if we first ran "cat" to efficiently populate the FS
cache for our sole big pack on linux.git, than if we had populated it
via for_each_object_in_pack(). Now we're 3% faster without the "cat"
instead.

My tests were done on an unloaded Linux 3.10 system with 10 runs for
each. Derrick Stolee did his own tests on Windows[3] showing a 2%
improvement with a high degree of accuracy.

1. 736eb88fdc ("for_each_packed_object: support iterating in
pack-order", 2018-08-10)

2. 0750bb5b51 ("cat-file: support "unordered" output for
--batch-all-objects", 2018-08-10)

3. https://public-inbox.org/git/f71fa868-25e8-a9c9-46a6-611b987f1a8f@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit-graph.c b/commit-graph.c
index 5c8fb4b134e96354cb25ea2d5bb060ce77ae3e2b..981faf0465a5c8605fb93b212fba5257c3ed7b17 100644 (file)
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -816,7 +816,8 @@ void write_commit_graph(const char *obj_dir,
                                die(_("error adding pack %s"), packname.buf);
                        if (open_pack_index(p))
                                die(_("error opening index for %s"), packname.buf);
-                       for_each_object_in_pack(p, add_packed_commits, &oids, 0);
+                       for_each_object_in_pack(p, add_packed_commits, &oids,
+                                               FOR_EACH_OBJECT_PACK_ORDER);
                        close_pack(p);
                        free(p);
                }
@@ -854,7 +855,8 @@ void write_commit_graph(const char *obj_dir,
                if (report_progress)
                        oids.progress = start_delayed_progress(
                                _("Finding commits for commit graph"), 0);
-               for_each_packed_object(add_packed_commits, &oids, 0);
+               for_each_packed_object(add_packed_commits, &oids,
+                                      FOR_EACH_OBJECT_PACK_ORDER);
                stop_progress(&oids.progress);
        }