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

From: Theodore Tso <tytso@mit.edu>
Date: May 9, 2007, 13:49 UTC
Message-ID: <20070509134958.GA21489@thunk.org>
In-Reply-To: <alpine.LFD.0.99.0705081439140.24220@xanadu.home>

On Tue, May 08, 2007 at 02:46:35PM -0400, Nicolas Pitre wrote:

Show 6 quoted lines

> The window size has absolutely no effect on the runtime pack access, 
> except maybe for the increased number of deltas.  It is really a pack 
> time cost.  The delta depth is the opposite: it has no effect on the 
> packing time, but it has the potential to slow down runtime access.  But 
> the delta base cache is apparently working really well to mitigate that 
> cost, as long as it is big enough of course.

Exactly, and the numbers bear out with the theory. Junio's already applied the change to up the default to 50, but for the record, here are the results using a kernel git repository, with times for doing a git-gc (with the --no-reuse-delta, although I like the suggestion to change it to just be "--hard", or maybe "--agressive"), and with a git pickaxe and a git-log with pathname restrictions. It essentially confirms that the delta base cache is doing the job just fine, up to even a depth of 100. However, there isn't much difference in pack size between a depth of 50 and 100.

Increasing the window size from 10 to 30 increases the pack run time by roughly 40%, and saves an extra 5% or so on the pack size. Increasing the window beyond 30 has ever-smaller diminishing returns, while the time to repack gets bigger and bigger.

The timing information is real/user/system/minor pagefaults, and as before, the results are done using a hot cache. Apologies in advance for the long lines in the benchmark results. These results were generated using a relatively recent, post-2.6.21 kernel git tree on a Thinkpad T60p laptop with a 2GHz T2500 Intel Core Duo processor.

Regards,

             git-gc                      pack size   git-log -S'object'    git-log include/scsi drivers/scsi
w=10,d=10   5:08.70/293.20/5.56/1027802  162316k  5:31.12/322.20/4.14/153298  0:01.80/1.72/0.03/19077
w=10,d=30   4:14.79/245.46/2.57/398754   149636k  5:41.56/331.21/4.86/517220  0:01.83/1.74/0.05/17880
w=10,d=50   4:31.89/257.16/3.50/576538   146608k  5:51.58/336.63/5.07/259643  0:01.88/1.80/0.04/17658
w=10,d=100  4:35.93/262.77/4.08/715195   144152k  5:58.23/341.26/6.22/624510  0:01.89/1.80/0.04/17571

w=30,d=10 7:23.06/424.08/6.35/1222640 153752k 5:32.85/323.96/2.13/213150 0:01.73/1.64/0.06/18429 w=30,d=30 6:15.76/364.27/3.53/407546 141200k 5:42.29/333.53/2.10/338301 0:01.81/1.71/0.04/17237 w=30,d=50 6:26.85/372.24/5.38/578343 139408k 5:42.21/336.65/1.30/260234 0:01.77/1.70/0.05/17108 w=30,d=100 6:34.31/381.72/5.40/744886 138040k 5:59.03/342.65/4.90/607681 0:01.93/1.79/0.06/17043

w=50,d=10 8:51.08/508.80/5.75/1050358 152168k 5:35.43/327.48/3.70/209655 0:01.70/1.64/0.04/18264 w=50,d=30 8:04.53/471.65/5.58/423755 139000k 5:42.06/335.67/1.55/335333 0:01.78/1.72/0.04/17046 w=50,d=50 8:14.87/479.93/7.09/617310 137244k 5:47.22/339.26/2.73/462412 0:01.79/1.72/0.04/16871 w=50,d=100 8:23.13/490.09/4.10/751742 136152k 5:51.10/343.89/2.29/503573 0:01.83/1.73/0.06/16920

w=100,d=10 12:00.34/702.07/6.27/1167403 150736k 5:34.42/328.88/1.09/207522 0:01.72/1.63/0.07/18024 w=100,d=30 11:56.34/702.36/2.93/436950 137240k 5:42.64/335.82/2.07/422743 0:01.77/1.67/0.05/16852 w=100,d=50 12:20.39/722.99/3.94/671655 135488k 5:47.20/339.38/2.44/468974 0:01.78/1.68/0.03/16621 w=100,d=100 12:37.69/740.63/4.12/733593 134492k 5:52.26/344.16/3.00/636120 0:01.85/1.80/0.03/16681

							- Ted