Re: [PATCH] git exproll: steps to tackle gc aggression

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 8, 2013, 19:37 UTC
Message-ID: <7vwqnw7z47.fsf@alter.siamese.dyndns.org>
In-Reply-To: <CALkWK0kqE8azzxp_GkzhPNT41nD8NzeLqXSe1xi0jbVo=7Xz3A@mail.gmail.com>

Ramkumar Ramachandra <artagnon@gmail.com> writes:

> I asked you a very simple question: what happens when I do "git push"?

You asked "does push send unreachable cruft?" and I said No. Does that answer your question?

> 3. After a few days of working, the gc heuristics figure out that I
> have too much garbage and too many packs; a cleanup is required. The
> gc --auto which doesn't tolerate fragmentation: it tries to put
> everything into one large pack.

Today's "gc --auto" skims the history shallowly and packs loose objects into a single _new_ pack. If you start from a single pack and enough loose objects and run "gc --auto", you will have two packs, one intact from the original, one new.

> We're talking about tackling the gc aggression problem in 3.

> when I say "git push" (or when gc decides to repack), won't I need
> to explode the young pack into loose objects, do a reachability
> analysis, and repack anyway?

You do not explode anything. "push" will read objects from anywhere, either from loose or packed, and send a newly created pack over the wire.

And I see from "(or when ...)" part that you do not know what you are talking about. "push" will not stream existing pack to the other end without computation, and it never will. It will reuse existing individual pieces when it can, and having data in a pack (even without deltifying, or as a suboptimal delta) is much better for push performance than having the same data in a loose object if only for this reason, as you do not have to recompress and reencode it in a different way (loose objects and undelitifed object in pack are encoded differently).

> ... are you willing to explain it to me?

Hope that the above clarifies.

I've treated this thread as a design discussion, not an education session, but the above ended up having more education material than anything that would advance the design.