Re: [PATCH] Add pack.depth option to git-pack-objects and change default depth to 50

[Theodore Tso <tytso@mit.edu>]

On Tue, May 08, 2007 at 11:38:46AM -0400, Nicolas Pitre wrote:
> On Tue, 8 May 2007, Theodore Ts'o wrote:
> 
> > Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> 
> I'd prefer if tests were performed on the performance impact before 
> changing the default depth.  If done separately from this patch then the 
> commit log could contain those results as well.

The following results are on a recent git repository, using time to
record the real, user, and sys times on the two commands: "git-gc
--no-reuse-delta --window=X --depth=Y" and "git log --pretty=oneline
-S'object' > /dev/null".  All of these tests were done with a hot
cache, so disk speed didn't enter into the calculations.

                git-gc                     git log -S'object'   pack size
w=10,d=10       27.1s/25.2s/0.3s           20.8s/20.4s/0.1s     15292k
w=10,d=30	23.8s/22.3s/0.2s           21.2s/20.9s/0.1s     12996k
w=10,d=50       24.8s/22.4s/0.4s           21.8s/21.2s/0.1s     12340k
w=100,d=100     24.1s/22.8s/0.3s           22.4s/21.8s/0.2s     11772k

w=30,d=10       45.0s/43.1s/0.4s           20.8s/20.5s/0.1s     14388k
w=30,d=30       35.8s/34.1s/0.3s           21.6s/21.1s/0.1s     11800k
w=30,d=50       34.6s/33.0s/0.3s           22.1s/21.4s/0.1s     11376k
w=30,d=100      34.0s/32.2s/0.3s           22.2s/21.6s/0.1s     11012k

w=50,d=10       56.1s/54.3s/0.4s           21.3s/20.5s/0.1s     14224k
w=50,d=30       47.2s/45.4s/0.4s           21.6s/21.0s/0.1s     11496k      
w=50,d=50       44.5s/43.0s/0.3s           21.7s/21.2s/0.1s     11108k
w=50,d=100      44.3s/42.7s/0.4s           22.4s/21.7s/0.1s     10824k

So a couple of things immediately become evident.  First of all, as
Junio predicted, changing --depth makes no difference to the git-gc or
git log times.  The latter is thanks to the delta chaching.  Secondly,
changing --depth does make a signficiant difference to the pack size.

Finally, --window does help somewhat in reducing the pack size, but it
_significantly_ increases the time to calculate the pack.

My conclusion given this quick benchmark is that it seems to me that
changing the defaults of --depth to 50, and keeping --window at 10, is
a reasonable thing to do.

Regards,

						- Ted