Re: "git reflog expire" blindly trusting timestamps in reflogs

[Jeff King <peff@peff.net>]

On Sat, Oct 09, 2021 at 04:57:20PM +0200, Ævar Arnfjörð Bjarmason wrote:

> On Sat, Oct 09 2021, René Scharfe wrote:
> 
> > Turn off automatic background maintenance for perf tests by default to
> > avoid interference with performance measurements.
> 
> Turning off background GC during the perf tests seems like a good idea
> in general, so I think this patch should go in. Even with the WIP
> (haven't picked it up again in a while) test mode I menitoned in[1] it
> still wouldn't make any sense to run detached background GC in t/perf.
> 
> Because first of all we take the repo as-is (hardlinks and all), so
> changing it is a bug in itself.

Lots of perf tests modify the repository. This is generally OK as Git
tends to write and rename tempfiles (breaking the hardlink) rather than
modifying anything in place. We also only do the hardlink thing for the
objects/ directory, so the scope is limited.

It does make me a little nervous, but to my knowledge we've never had a
perf test which hurt the original donor repo in this way. And there are
more subtle ways to screw things up with various filesystem references;
see 36e834abc1 (t/perf: avoid copying worktree files from test repo,
2021-02-26).

That's all just an aside; I agree that we should avoid background gc
just because it confuses performance tests.

-Peff