Re: Why Git is so fast (was: Re: Eric Sink's blog - notes on git, dscms and a "whole product" approach)

From: Daniel Barkalow <barkalow@iabervon.org>
Date: May 1, 2009, 21:17 UTC
Message-ID: <alpine.LNX.2.00.0905011618370.2147@iabervon.org>
In-Reply-To: <20090501190854.GA13770@coredump.intra.peff.net>

On Fri, 1 May 2009, Jeff King wrote:

Show 27 quoted lines

> On Fri, May 01, 2009 at 02:43:49PM -0400, Linus Torvalds wrote:
> 
> > > Like all generalizations, this is only mostly true. Fast network servers
> > > with big caches can outperform disks for some loads.
> > [...]
> > In contrast, a workstation with local filesystems and enough memory to 
> > cache it well will just be a lot nicer.
> > [...]
> > > I have never used perforce, but I get the impression that it is more 
> > > optimized for such a situation.
> > 
> > I doubt it. I suspect git will outperform pretty much anything else in 
> > that kind of situation too.
> 
> Thanks for the analysis; what you said makes sense to me. However, there
> is at least one case of somebody complaining that git doesn't scale as
> well as perforce for their load:
> 
>   http://gandolf.homelinux.org/blog/index.php?id=50
> 
> Part of his issue is with git-p4 sucking, which it probably does. But
> part of it sounds like he has a gigantic workload (the description of
> which sounds silly to me, but I respect the fact that he is probably
> describing standard practice among some companies), and that workload is
> just a little too gigantic for the workstations to handle. I.e., by
> throwing resources at the central server they can avoid throwing as many
> at each workstation.

I think his problem is that he's trying to replace his p4 repository with a git repository, which is a bit like trying to download github, rather than a project from github. Perforce is good at dealing with the case where people check in a vast quantity of junk that you don't check out.

That is, you can back up your workstation into Perforce, and it won't affect anyone's performance if you use a path that's not in the range that anybody else checks out. And people actually do that. And Perforce doesn't make a distinction between different projects and different branches of the same project and different subdirectories of a branch of the same project, so it's impossible to tease apart except by company policy.

Git doesn't scale in that it can't do the extremely narrow checkouts you need if your repository root directory contains thousands of complete unrelated projects with each branch of each project getting a subdirectory. On the other hand, it does a great job when the data is already partitioned into useful repositories.

	-Daniel
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