[PATCH 0/3] low-hanging performance fruit with promisor packs

On Mon, Apr 12, 2021 at 07:47:41PM -0400, Jeff King wrote:

I think we can do even better than that, after looking into the "do we really need to parse the objects?" comment I left (spoiler: the answer is no, we do not need to, at least for that caller).

Here are some cleaned-up patches that I think improve the situation quite a bit. This is just the low-hanging fruit from this part of the discussion; I'm sure there's more to do to make using partial clones pleasant. In particular:

  - this does nothing for the "oops, we turned all of the promisor
    objects loose and then deleted them" problem. Hopefully Rafael
    will produce a nice patch for that

  - In is_promisor_object(), we still call parse_object(), because it
    really does look at the contents. But doing so for blobs is wasteful
    (it's a lot of bytes we push through sha1, and we don't even look at
    them). It might be worth using oid_object_info() to avoid this. This
    introduces a little overhead, but I think would be a net win (it
    would be really nice if we could amortize the object lookup work;
    i.e., if there was a way to call oid_object_info_extended() and say
    "open the object and look at the type; only return the contents if
    it's a non-blob").

  - I still think it's probably worth having a mode where we store the
    set of pointed-to objects we don't have, rather than parsing on the
    fly. This could easily be made optional (it's a good thing if you
    _have_ a lot of objects that point to a small or moderate number of
    missing objects, like a blob:limit or blob:none filter; it's a bad
    thing if you have very few objects but they point to a very large
    number).

I didn't explore any of those here, and I don't plan to look into them anytime soon. I'm just documenting my findings for later.

Anyway, here are the patches.

  [1/3]: is_promisor_object(): free tree buffer after parsing
  [2/3]: lookup_unknown_object(): take a repository argument
  [3/3]: revision: avoid parsing with --exclude-promisor-objects

 builtin/fsck.c                   |  2 +-
 builtin/pack-objects.c           |  2 +-
 http-push.c                      |  2 +-
 object.c                         |  7 +++----
 object.h                         |  2 +-
 packfile.c                       |  1 +
 refs.c                           |  2 +-
 revision.c                       |  2 +-
 t/helper/test-example-decorate.c |  6 +++---
 t/perf/p5600-partial-clone.sh    | 12 ++++++++++++
 upload-pack.c                    |  2 +-
 walker.c                         |  2 +-
 12 files changed, 27 insertions(+), 15 deletions(-)

-Peff