Re: [hacky PATCH 0/2] speeding up trivial for-each-ref invocations

From: ZheNing Hu <adlternative@gmail.com>
Date: Sep 6, 2021, 13:30 UTC
Message-ID: <CAOLTT8SYx4f8x-0zjXLJVOpW4-Mj34mjsfwr6tWo0PbZAV9B0w@mail.gmail.com>
In-Reply-To: <YTS8yiYwR+eeZf4+@coredump.intra.peff.net>

Jeff King <peff@peff.net> 于2021年9月5日周日 下午8:49写道：

Show 19 quoted lines

>
> On Sun, Sep 05, 2021 at 04:19:53PM +0800, ZheNing Hu wrote:
>
> > > In this version there are 2 patches, tested against 'git for-each-ref
> > > --format="%(objectname) %(refname)"' on a fully packed repo with 500k
> > > refs:
> > >
> >
> > Regarding this 500k refs, is there any way I can reproduce it?
>
> Try this in a clone of linux.git (or any other repo):
>
>   git rev-list HEAD |
>   head -500000 |
>   perl -lne 'print "create refs/foo/$. $_"' |
>   git update-ref --stdin
>
>   git pack-refs --all --prune
>

Sorry, It seems that the above command is difficult to complete on my machine (it took more than ten minutes). It may be stuck on git update-ref. So I tried to reproduce it in a repo which containing 76K refs:

Benchmark #1: jk-for-each-ref-speedup~2: git for-each-ref
--format='%(refname) %(objectname)'
 Time (mean ± σ):     108.0 ms ±   1.9 ms    [User: 55.2 ms, System: 52.1 ms]
 Range (min … max):   105.7 ms … 112.4 ms    26 runs

Benchmark #2: jk-for-each-ref-speedup~1: git for-each-ref
--format='%(refname) %(objectname)'
 Time (mean ± σ):      88.2 ms ±   1.7 ms    [User: 44.8 ms, System: 43.1 ms]
 Range (min … max):    85.8 ms …  93.2 ms    32 runs

Benchmark #3:jk-for-each-ref-speedup: git for-each-ref
--format='%(refname) %(objectname)'
  Time (mean ± σ):      69.0 ms ±   2.0 ms    [User: 22.7 ms, System: 46.1 ms]
 Range (min … max):    66.2 ms …  74.1 ms    41 runs

For %(refname) and %(objectname), this performance optimization is indeed amazing.

Show 5 quoted lines

> Though I actually think for these tests that it is not important that
> each ref point to a unique commit (we are not opening up the objects at
> all, and just treating the oids as strings).
>
> -Peff

Thanks. -- ZheNing Hu