Re: [PATCH] Use configurable zlib compression level everywhere.

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jul 8, 2006, 02:10 UTC
Message-ID: <Pine.LNX.4.63.0607080406370.29667@wbgn013.biozentrum.uni-wuerzburg.de>
In-Reply-To: <Pine.LNX.4.63.0607071451430.1836@qynat.qvtvafvgr.pbz>

Hi,

On Fri, 7 Jul 2006, David Lang wrote:

Show 10 quoted lines

> On Mon, 3 Jul 2006, Junio C Hamano wrote:
> 
> > * sha1write_compressed() in csum-file.c is for producing packs
> >   and most of the things we compress there are deltas and less
> >   compressible, so even when core.compression is set to high we
> >   might be better off using faster compression.
> 
> why would deltas have poor compression? I'd expect them to have about the same
> as the files they are deltas of (or slightly better due to the fact that the
> deta metainfo is highly repetitive)

Deltas should have poor compression by definition, because compression tries to encode those parts of the file more efficiently, which do not bear much information (think entropy).

If you have deltas which really make sense, they are almost _pure_ information, i.e. they do not contain much redundancy, as compared to real files. So, the compression (which does not know anything about the characteristics of deltas in particular) cannot take much redundancy out of the delta. Therefore, the entropy is very high, and the compression rate is low.

Hope this makes sense to you, Dscho