Re: Huge performance bottleneck reading packs

[Jeff King <peff@peff.net>]

On Wed, Oct 12, 2016 at 07:01:43PM -0400, Jeff King wrote:

> On Thu, Oct 13, 2016 at 12:30:52AM +0200, Vegard Nossum wrote:
> 
> > However, the commit found by 'git blame' above appears just fine to me,
> > I haven't been able to spot a bug in it.
> > 
> > A closer inspection reveals the problem to really be that this is an
> > extremely hot path with more than -- holy cow -- 4,106,756,451
> > iterations on the 'packed_git' list for a single 'git fetch' on my
> > repository. I'm guessing the patch above just made the inner loop
> > ever so slightly slower.
> > 
> > My .git/objects/pack/ has ~2088 files (1042 idx files, 1042 pack files,
> > and 4 tmp_pack_* files).
> 
> Yeah. I agree that the commit you found makes the check a little more
> expensive, but I think the root of the problem is calling
> prepare_packed_git_one many times. This _should_ happen once for each
> pack at program startup, and possibly again if we need to re-scan the
> pack directory to account for racing with a simultaneous repack.
> 
> The latter is generally triggered when we fail to look up an object we
> expect to exist. So I'd suspect 45e8a74 (has_sha1_file: re-check pack
> directory before giving up, 2013-08-30) is playing a part. We dealt with
> that to some degree in 0eeb077 (index-pack: avoid excessive re-reading
> of pack directory, 2015-06-09), but it would not surprise me if there is
> another spot that needs similar treatment.
> 
> Does the patch below help?

Also, is it possible to make the repository in question available? I
might be able to reproduce based on your description, but it would save
time if I could directly run gdb on your example.

Specifically, I'm interested in the call graph that leads to calling
reprepare_packed_git().

-Peff