Subject: [PATCH v2 0/9] address packed-refs speed regressions

From: Jeff King <[email-masked]>
Date: Thu, 16 Apr 2015 04:47:34 -0400

On Sat, Apr 04, 2015 at 09:06:11PM -0400, Jeff King wrote:

> As I've mentioned before, I have some repositories with rather large
> numbers of refs. The worst one has ~13 million refs, for a 1.6GB
> packed-refs file. So I was saddened by this:
> 
>   $ time git.v2.0.0 rev-parse refs/heads/foo >/dev/null 2>&1
>   real    0m6.840s
>   user    0m6.404s
>   sys     0m0.440s
> 
>   $ time git.v2.4.0-rc1 rev-parse refs/heads/foo >/dev/null 2>&1
>   real    0m19.432s
>   user    0m18.996s
>   sys     0m0.456s

Here's a re-roll incorporating feedback from the list. Thanks everybody
for your comments. Last time the final number was ~8.5s, which was
disappointingly slower than v2.0.0. In this iteration, my final numbers
are:

  real    0m5.703s
  user    0m5.276s
  sys     0m0.432s

which is quite pleasing.

The big changes that resulted in this additional speedup are:

  1. Use getdelim() when it is available. This is much faster than even
     a getc_unlocked() loop.

  2. The slowdown from d0f810f was from adding in refname_is_safe calls.
     But what I didn't notice before is that we run them in _addition_
     to check_refname_format, rather than instead of it. So in the
     common case of a sanely-formatted refname, we can skip the call,
     rather than writing a lot of code to micro-optimize it.

It was also mentioned in a nearby thread that the config code could
benefit from some of the same micro-optimizations. It can't make use of
getdelim(), as it really does want to do character-by-character parsing.
But it can still use getc_unlocked() and the strbuf_avail() trick, which
speeds up config reading by 47%. Those patches are included here.

  [1/9]: strbuf_getwholeline: use getc macro
  [2/9]: git-compat-util: add fallbacks for unlocked stdio
  [3/9]: strbuf_getwholeline: use getc_unlocked
  [4/9]: config: use getc_unlocked when reading from file
  [5/9]: strbuf_addch: avoid calling strbuf_grow
  [6/9]: strbuf_getwholeline: avoid calling strbuf_grow
  [7/9]: strbuf_getwholeline: use getdelim if it is available
  [8/9]: read_packed_refs: avoid double-checking sane refs
  [9/9]: t1430: add another refs-escape test

-Peff