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

From: Jeff King <peff@peff.net>
Date: Oct 11, 2021, 16:32 UTC
Message-ID: <YWRnMbAOQ1t2DVHb@coredump.intra.peff.net>
In-Reply-To: <87wnmmkzaa.fsf@evledraar.gmail.com>

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

Show 12 quoted lines

> 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