[PATCH 0/4] optimizing upload-pack ref peeling

[Jeff King <peff@peff.net>]

On Thu, Oct 04, 2012 at 01:54:47AM +0200, Ævar Arnfjörð Bjarmason wrote:

> > Hmm. It seems like we should not need to open the tags at all. The main
> > reason is to produce the "peeled" advertisement just after it. But for a
> > packed ref with a modern version of git that supports the "peeled"
> > extension, we should already have that information.
> 
> B.t.w. do you plan to submit this as a non-hack, I'd like to have it
> in git.git, so if you're not going to I could pick it up and clean it
> up a bit. But I think it would be better coming from you.

Here's a cleaned-up version. I realized that peel_ref actually is quite
close to what we want; the key thing is that when you are calling it
from a for_each_ref loop it will not only use peeled information from
packed-refs (if available), but it will also short-circuit the ref
lookup for non-packed refs. So it really does what we want.

I put some timings into the final patch. The results are very satisfying
from a pure numbers standpoint. However, I'm not 100% sure this is all
that useful a benchmark. The repos where you get a noticeable benefit
are quite big, so I suspect the time to send the advertisements out over
the wire will dominate. Still, a CPU second saved is a CPU second
earned.

And the series actually ends up making the code cleaner and shares some
of the optimizations I put into upload-pack with other users of
peel_ref. So I think I'd be inclined to use it, even if the speedup
doesn't help that much in practice. We'll see what others think.

  [1/4]: peel_ref: use faster deref_tag_noverify
  [2/4]: peel_ref: do not return a null sha1
  [3/4]: peel_ref: check object type before loading
  [4/4]: upload-pack: use peel_ref for ref advertisements

-Peff