Subject: Re: [RFC] On watchman support

From: Jeff King <[email-masked]>
Date: Tue, 18 Nov 2014 20:46:00 -0500

On Tue, Nov 18, 2014 at 01:26:56PM -0800, Junio C Hamano wrote:

> It is not check_refname_format() that is the real problem. It's the
> fact that we do O(# of refs) work whenever we have to access the
> packed-refs file. check_refname_format() is part of that, surely,
> but so is reading the file, creating all of the refname structs in
> memory, etc. (credit to peff@).

Yeah, I'd agree very much with that. I am not sure if I am cc'd here
because of my general complaining about packed-refs, or if I have said
something clever on the subject.

I did implement at one point a packed-refs reader that does a binary
search on the mmap'd packed-refs file, and can return a single value or
even locate the first entry matching a prefix (like "refs/tags/") and
iterate until we're out of the prefix. Unfortunately this runs very
contrary to the caching design of the refs.c code. It is focused on
caching _loose_ refs, where we may read an outer directory (like
"refs/"), and would like to avoid descending into an inner directory
(likes "refs/foo/") unless we are interested in what is in it. But
caching partial reads of packed-refs like this is "inside out"; we might
read all of "refs/tags/*", but have no clue what else is in "refs/". So
integrating it into refs.c would take pretty major surgery.

-Peff