Re: Git commit generation numbers

From: Jeff King <peff@peff.net>
Date: Jul 14, 2011, 20:31 UTC
Message-ID: <20110714203141.GA28548@sigill.intra.peff.net>
In-Reply-To: <69e0ad24-32b7-4e14-9492-6d0c3d653adf@email.android.com>

On Thu, Jul 14, 2011 at 01:19:51PM -0700, Linus Torvalds wrote:

Show 21 quoted lines

> >Out of curiosity, what don't you like about the generation cache?
> 
> The thing I hate about it is very fundamental: I think it's a hack
> around a basic git design mistake. And it's a mistake we have known
> about for a long time.
> 
> Now, I don't think it's a *fatal* mistake, but I do find it very
> broken to basically say "we made a mistake in the original commit
> design, and instead of fixing it we create a separate workaround for
> it".
> 
> THAT I find distasteful. My reaction is that if we're going to add
> generation numbers, then were should just do it the way we should have
> done them originally, rather than as some separate hack.
> 
> See? That's why I wouldn't have any problem with adding a separate
> cache on top of it, if it's really required, but I would hope that it
> isn't really needed.
> 
> So a cache in itself is not necessarily wrong. But leaving the
> original design mistake in place IS.

Thanks, that makes some sense to me.

However, I'm not 100% convinced leaving generation numbers out was a mistake. The git philosophy seems always to have been to keep the minimal required information in the DAG. And I think that has served us well, because we're not saddled with cruft that seemed like a good idea early on, but isn't.

Generation numbers are _completely_ redundant with the actual structure of history represented by the parent pointers. Having them in there is not about giving git more information that it doesn't have, but about being a cheap place to stuff a value that is a little expensive to calculate.

And so that seems a bit hack-ish to me.

I liken it somewhat to the "don't store renames" debate. We don't want to crystallize forever in the history whatever crappy rename-detection algorithm is done at the time of commit. We put the minimum amount of information in the DAG, and it's the runtime's responsibility to get the answer.

I think the decision is a little more gray with generation numbers, because it's not about "you got this information with a wrong and crappy algorithm" like it might be with rename detection, but rather "we're sticking this redundant number in the commit object, and we assume that it will always be useful enough to future algorithms to merit being here".

> And fixing it really ended up being a very tiny patch, no?

Well, yes. But it also doesn't yield a 100-fold speedup in "git tag --contains" for existing repositories. So it's not quite a full solution.

-Peff