Re: gc --aggressive

From: Jeff King <peff@peff.net>
Date: Apr 28, 2012, 12:25 UTC
Message-ID: <20120428122533.GA12098@sigill.intra.peff.net>
In-Reply-To: <vpqbomqqdxo.fsf@bauges.imag.fr>

On Tue, Apr 17, 2012 at 10:52:03PM +0200, Matthieu Moy wrote:

Show 16 quoted lines

> Jay Soffian <jaysoffian@gmail.com> writes:
> 
> > + 3. `git gc --aggressive`; this is often much slower than (2) because git
> > +    throws out all of the existing deltas and recomputes them from
> > +    scratch. It uses a higher window parameter meaning it will spend
> > +    more time computing, and it may end up with a smaller pack. However,
> > +    unless the repository is known to have initially been poorly packed,
> > +    this option is not needed and will just cause git to perform
> > +    extra work.
> 
> I like your patch.
> 
> Maybe you should elaborate on "unless the repository is known to have
> initially been poorly packed". My understanding is that --aggressive was
> implemented to be called after an import from another VCS that would
> have computed very poor deltas, but I'm not sure about the details.

Coincidentally, I came across a case last week that shows --aggressive providing a large improvement. And it's a public repo, so I was able to grab a snapshot of the pre-packed state to experiment on and share.

The current packfile is ~246M. It was produced over time by pushes into the repository, which were then eventually grouped into a single pack by "git gc" (I'm not sure of the exact history, but this may even have been a set of "gc --auto" calls over time).

Here's a list of commands and the pack sizes they yield on the repo:

  1. `git repack -ad`: 246M
  2. `git repack -ad -f`: 376M
  3. `git repack -ad --window=250`: 246M
  4. `git repack -ad -f --window=250`: 145M

The most interesting thing is (4): repacking with a larger window size yields a 100M (40%) space improvement. The other commands show that it is not that the current pack is simply bad; command (2) repacks from scratch and actually ends up with a worse pack. So the increased window size really is important.

I haven't been able to figure out what it is about this dataset that makes the bigger window so much better. Certainly doing the same commands on git.git does not yield as impressive a speedup.

If anybody is interested in the repository as a packing test case, you can download it from:

  https://gist.github.com/raw/2518074/d9c0244bf0ced690fee1edb2c88c522ecce102e4/phpmyadmin-network.tar

-Peff