On Sun, Jul 02, 2023 at 08:56:11AM -0400, Jeff King wrote:
> My biggest question would be whether this introduces any performance
> penalty for the more common cases (lightweight tags and single-level
> annotated tags). The answer is "no", I think; we are already paying the
> cost to parse every object to find out if it's a tag, and your new loop
> only does an extra parse if we see a tag-of-tag. Good.
Reading more carefully, I think this does actually change the
performance a bit, because we end up parsing the pointed-to commits, as
well. So here's before and after your patch running "git for-each-ref
--points-at=HEAD" on linux.git (785 refs, all but 3 are tags):
Benchmark 1: ./git.old for-each-ref --points-at=HEAD
Time (mean ± σ): 11.4 ms ± 0.2 ms [User: 6.5 ms, System: 4.9 ms]
Range (min … max): 11.0 ms … 12.3 ms 239 runs
Benchmark 2: ./git.new for-each-ref --points-at=HEAD
Time (mean ± σ): 20.6 ms ± 0.5 ms [User: 10.4 ms, System: 10.2 ms]
Range (min … max): 19.8 ms … 22.7 ms 133 runs
Summary
'./git.old for-each-ref --points-at=HEAD' ran
1.80 ± 0.06 times faster than './git.new for-each-ref --points-at=HEAD'
The absolute numbers are pretty small, but the percent change isn't
great. I'll send some patches in a minute that can be applied on top to
improve this case, as well as fix the other issues I pointed out in the
existing code.
-Peff
