Subject: Re: Why Git is so fast

From: Jakub Narebski <[email-masked]>
Date: Thu, 30 Apr 2009 21:33:59 +0200

On Thu, 30 Apr 2009, Nicolas Pitre wrote:
> On Thu, 30 Apr 2009, Jakub Narebski wrote:
> > Jakub Narebski <[email-masked]> writes:

> > es> Maybe Git is fast because every time they faced one of these "buy
> > es> vs. build" choices, they decided to just write it themselves.
> > 
> > I don't think so.  Rather the opposite is true.  Git uses libcurl for
> > HTTP transport.  Git uses zlib for compression.  Git uses SHA-1 from
> > OpenSSL or from Mozilla.  Git uses (modified, internal) LibXDiff for
> > (binary) deltaifying, for diffs and for merges.
> 
> Well, I think he's right on this point as well.  [...]
> The fact that libxdiff was made internal is indeed to have a better 
> impedance matching with the core code, otherwise it could have remained 
> fully external just like zlib.  And the binary delta code is not 
> libxdiff anymore but a much smaller, straight forward, and optimized to 
> death version to achieve speed over versatility (no need to be versatile 
> when strictly dealing with Git's needs only).

Hrmmmm... I have thought that LibXDiff was internalized mainly for ease
of modification, as my impression is that LibXDiff is single developer
effort, while Git from beginning have many contributors (and submodules
didn't exist then).  If I remember correctly the rcsmerge/diff3 algorithm
was added first in internalized git's xdiff... was it added to LibXDiff
proper, anyway?

BTW. I wonder what other F/OSS version control systems: Bazaar,
Mercurial, Darcs, Monotone use for binary deltas, for diff engine,
and for textual three-way merge engine.  Hmmm... perhaps I'll ask
on #revctrl

-- 
Jakub Narebski
Poland