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

[ZheNing Hu <adlternative@gmail.com>]

Jeff King <peff@peff.net> 于2021年9月4日周六 下午8:40写道：
>
> Somebody complained to me off-list recently that for-each-ref was slow
> when doing a trivial listing (like refname + objectname) of a large
> number of refs, even though you could get mostly the same output by
> just dumping the packed-refs file.
>
> So I was nerd-sniped into making this horrible series, which does speed
> up some cases. It was a combination of "how fast can we easily get it",
> plus I was curious _which_ parts of for-each-ref were actually slow.
>
> 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?

>   - directly outputting items rather than building up a ref_array yields
>     about a 1.5x speedup
>
>   - avoiding the whole atom-formatting system yields a 2.5x speedup on
>     top of that
>
> I had originally written it against a slightly older version of Git,
> before show_ref_array_item() was inlined. In that version, there was a
> middle ground where we still created a ref_array_item for each ref, and
> then fed it to the "quick" formatter (so we could see the cost of
> having to malloc/free a bunch of ref_array_item structs). The numbers
> there wre:
>
>   - streaming out items gave a 1.5x speedup
>
>   - using the "quick" formatter gave a 1.8x speedup
>
>   - avoiding the extra malloc/free for each item gave a 1.4x
>
> which makes sense; 1.8 * 1.4 is ~2.5, so it's the same speedup just
> broken down further.
>
> I'm not sure I'm really advocating that we should do something like
> this, though it is tempting because it does make some common queries
> much faster. But I wanted to share it here to give some insight on where
> the time may be going in ref-filter / for-each-ref.
>

Well, this acceleration looks gratifying. But so far I haven't seen
its effect on
git cat-file --batch | --batch-check.


>   [1/2]: ref-filter: hacky "streaming" mode
>   [2/2]: ref-filter: implement "quick" formats
>
>  builtin/for-each-ref.c |   7 +++
>  ref-filter.c           | 113 +++++++++++++++++++++++++++++++++++++----
>  ref-filter.h           |  21 ++++++++
>  3 files changed, 132 insertions(+), 9 deletions(-)
>
> -Peff