Re: [PATCH] packfile: fix perf regression with many packs

On Wed, Aug 12, 2026 at 12:51:30PM -0700, Junio C Hamano wrote:

> "Johannes Schindelin via GitGitGadget" <[email protected]>
> writes:
> 
> > In one reported use case (https://github.com/microsoft/git/issues/970),
> > N equals 37,815 and caused a slow-down of a simple `git rev-parse
> > --short HEAD` (which is regularly executed as part of `GIT_PS1`) from
> > 0.4s to 4.5s. In another, heavily exercised CI scenario, clone times
> > increased from under 2 minutes to over half an hour.
> 
> Face with Rolling Eyes (1f644) 🙄
> 
> As we grow older, more and more extreme use cases that we initially
> thought were simply crazy become reality.

Sort of. The quadratic adding became a problem long ago, hence
ec48540fe8 (packfile.c: speed up loading lots of packfiles, 2019-11-27).

So this was something we already dealt with that regressed. We can even
see the regression in our perf suite:

  $ GIT_SKIP_TESTS='p5303.[1-9] p5303.1[0-9]' ./run 589127caa730^ 589127caa730 p5303-many-packs.sh
  Test                         589127caa730^     589127caa730
  ----------------------------------------------------------------------
  5303.21: load 10,000 packs   0.13(0.11+0.02)   0.45(0.42+0.02) +246.2%

Unfortunately I don't think anybody pays close attention to the perf
suite (partially because it's clunky and expensive to run, but also
because it often requires human judgement to decide when something is a
real change and not just a blip).

None of that has any bearing on the fix, which seems reasonable to me,
but...

> > --- a/t/perf/p5303-many-packs.sh
> > +++ b/t/perf/p5303-many-packs.sh
> > @@ -141,4 +141,8 @@ test_perf "load 10,000 packs" '
> >  	git rev-parse --verify "HEAD^{commit}"
> >  '
> >  
> > +test_perf "abbreviate with 10,000 packs" '
> > +	git rev-parse --short HEAD
> > +'

...I wonder what value this is adding. It shows the same slowdown as the
existing test you can see in the context (and whose results I showed
above).

-Peff