Re: [PATCH v1] diffcore-rename: speed up register_rename_src

From: Jeff King <peff@peff.net>
Date: Apr 20, 2017, 15:50 UTC
Message-ID: <20170420155003.cugkol6rbv25lpdi@sigill.intra.peff.net>
In-Reply-To: <alpine.DEB.2.20.1704201231350.3480@virtualbox>

On Thu, Apr 20, 2017 at 12:40:52PM +0200, Johannes Schindelin wrote:

Show 16 quoted lines

> > > Teach register_rename_src() to see if new file pair can simply be
> > > appended to the rename_src[] array before performing the binary search
> > > to find the proper insertion point.
> > 
> > I guess your perf results show some minor improvement. But I suspect
> > this is because your synthetic repo does not resemble the real world
> > very much.
> 
> Please note that the synthetic test repo was added *after* coming up with
> the patch, *after* performance benchmarking on a certain really big
> repository (it is not hard to guess what use case we are optimizing,
> right?).
> 
> In that light, I would like to register the fact that Jeff's performance
> work is trying to improve a very real world, that of more than 2,000
> developers in our company [*1*].

Sure; I didn't think it came out of thin air. What are the benchmarks on this real-world repository, then?

Specifically, it looks like this optimization isn't really about the number of files in the repository so much as the number of additions/deletions in a particular diff (which is what become rename sources and destinations).

Is it common to add or delete 4 million tiny files and then run "git status"?

Note that I think the optimization probably _is_ worth doing in the general case. These "is it sorted" tradeoffs can backfire if we sometimes get unsorted input, but I don't think that would ever be the case here. My main complaint is not that it's not worth doing, but that I'm not excited about sprinkling these checks ad-hoc throughout the code base.

-Peff