Re: git-last-modified(1) slower than git-log(1)?

[Toon Claes <toon@iotcl.com>]

Gusted <gusted@codeberg.org> writes:

> Hi,
>
> I'm working at switching Forgejo's implementation of getting the last
> modified commits in a directory to git-last-modified(1). I'd expected
> equal or better performance than the current implementation, but have
> not yet been able to get this and I'm a bit puzzled as to why.
>
> The current implementation of Forgejo (inherited from Gitea) works
> roughly like this:
> 1. Run `git log --name-status -c --format=commit%x00%H %P%x00" --parents
> --no-renames -t -z $OID -- :(literal)some/path`, the output of this is
> quite complex and possible outputs more information than necessary.
> 2. The output of this is piped to some code to a parser and reconstructs
> what commit ID last modified each file in the directory.
> 3. Via `git cat-file --batch` get each unique commits information.
>
> With git-last-modified(1) (-z --show-trees --max-depth=0) this replaces
> step 1-2, but is slower. I've isolated the degraded performance to the
> fact that git-last-changed(1) takes more time to finish. So from my
> perspective it does not seem worth it to replace the current
> implementation with git-last-modified(1), and I would like to know if
> I'm missing something here or if git-last-modified(1) possibly could see
> a speedup?
>
> The repository I'm currently using to evaluate the performance is
> https://codeberg.org/ziglang/zig
>
> Reproduction steps:
> 1. `git clone https://codeberg.org/ziglang/zig $(mktemp -d)`
> 2. cd to tmp directory.
> 3. `git commit-graph write --changed-paths`. As git-last-modified(1)
> makes good use of the bloom filters.
> 4. `hyperfine 'git last-modified -z -t --max-depth=0
> 80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/' 'git log
> --name-status -c "--format=commit%x00%H %P%x00" --parents --no-renames
> -t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- ":(literal)doc/langref"'`
>
> With as output:
> Benchmark 1: git last-modified -z -t --max-depth=0
> 80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/
>  Time (mean ± σ): 66.5 ms ± 0.6 ms [User: 60.6 ms, System: 5.2 ms]
>  Range (min … max): 65.3 ms … 67.7 ms 44 runs
>
> Benchmark 2: git log --name-status -c "--format=commit%x00%H %P%x00"
> --parents --no-renames -t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 --
> ":(literal)doc/langref"
>  Time (mean ± σ): 26.2 ms ± 1.0 ms [User: 17.3 ms, System: 8.4 ms]
>  Range (min … max): 24.3 ms … 30.1 ms 110 runs
>
> Summary
>  git log --name-status -c "--format=commit%x00%H %P%x00" --parents
> --no-renames -t -z 80d06578ac66bce3aa0a21e9610cdb782b9a0593 --
> ":(literal)doc/langref" ran
>  2.54 ± 0.10 times faster than git last-modified -z -t --max-depth=0
> 80d06578ac66bce3aa0a21e9610cdb782b9a0593 -- doc/langref/

Hi Gusted,

Thanks for reaching out.

You're actually not the first to notice this, and I've been aware of
this.

The thing is, you're testing the difference on a single file. For us at
GitLab, it wasn't very useful to optimize that use-case, because usually
we want to see the last commit for a bunch of files at once.
So the use-case for git-last-modified(1) for us has been to replace
(pseudo code):

$ FILES=$(git ls-tree $COMMIT $PATH)
$ foreach $FILE in $FILES; do git log -1 $COMMIT -- $FILE; end

GitLab is batching files 25 at once, and in my benchmarking, it was
shown git-last-modified(1) is faster:

$ git last-modified $COMMIT -- <files

(I did this benchmarking in our Gitaly component to have a real-world
experience and you can visit the results at:
https://gitlab.com/gitlab-org/gitaly/-/merge_requests/7999#note_2850505479
)

So we left the door open for future improvement, although I never have
gotten to it. At some point I was trying to chase down when git-log(1)
was doing differently, but I never figured it out.

But this email challenged me already. And with some help of AI, I
managed to work on some improvements. You can expect a patch series
soon.

(Right before sending out this mail I noticed Peff sent out some changes
as well. I'll coordinate how to combine.)

-- 
Cheers,
Toon