Re: [PATCH 2/7] builtin/checkout.c: complete parallel checkout support

[Matheus Tavares Bernardino <matheus.bernardino@usp.br>]

On Fri, Apr 23, 2021 at 1:19 PM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 4/22/2021 11:17 AM, Matheus Tavares wrote:
> > There is one code path in builtin/checkout.c which still doesn't benefit
> > from parallel checkout because it calls checkout_entry() directly,> instead of unpack_trees(). Let's add parallel checkout support for this
> > missing spot as well.
>
> I couldn't tell immediately from the patch what would trigger this
> code path. I had to trace the method calls to discover that it is
> for the case of a pathspec-limited checkout:
>
>         git checkout <ref> -- <pathspec>

Oops, I should have mentioned that in the commit message. Thanks for
pointing it out.

> I confirmed that this does work with this change, but it might be
> nice to have a test that verifies that parallelism is triggering for
> this case.
>
> Looking ahead to patches 4-6, which add tests, I do not see one for this
> code path. Yes, patch 7 will implicitly test it through optional
> settings, but it would be nice to verify that the code is actually using
> parallel workers. The test_checkout_workers helper in patch 4 should be
> helpful for this effort.
>
> Please point out the test that covers this case, in case I'm just not
> seeing it.

Hmm, there are some tests at t2081 and t2082 that check the
pathspec-limited case with parallel workers. For example the collision
tests run `test_checkout_workers 2 git checkout .`. We also test
direct pathnames in t2082, using `test_checkout_workers 2 git checkout
A B`.

> The good news is that I can see a difference. By alternating checkouts
> of the Git repository's "t" directory between v2.20 and v2.31.1, I can
> see these results for varying numbers of workers:
>
> Benchmark #1: 16 workers
>   Time (mean ± σ):     108.6 ms ±   5.2 ms    [User: 146.1 ms, System: 146.1 ms]
>   Range (min … max):    95.5 ms … 124.9 ms    100 runs
>
> Benchmark #2: 8 workers
>   Time (mean ± σ):     104.8 ms ±   4.8 ms    [User: 128.3 ms, System: 131.7 ms]
>   Range (min … max):    94.2 ms … 119.0 ms    100 runs
>
> Benchmark #3: 4 workers
>   Time (mean ± σ):     112.3 ms ±   6.2 ms    [User: 114.6 ms, System: 112.1 ms]
>   Range (min … max):   100.0 ms … 127.4 ms    100 runs
>
> Benchmark #4: 2 workers
>   Time (mean ± σ):     124.2 ms ±   4.2 ms    [User: 106.5 ms, System: 102.0 ms]
>   Range (min … max):   114.8 ms … 136.3 ms    100 runs
>
> Benchmark #5: sequential
>   Time (mean ± σ):     154.6 ms ±   6.7 ms    [User: 83.5 ms, System: 79.4 ms]
>   Range (min … max):   142.1 ms … 176.0 ms    100 runs
>
> Summary
>   '8 workers' ran
>     1.04 ± 0.07 times faster than '16 workers'
>     1.07 ± 0.08 times faster than '4 workers'
>     1.19 ± 0.07 times faster than '2 workers'
>     1.48 ± 0.09 times faster than 'sequential'

Nice! Thanks for the benchmark!

> (Note: these time measurements are for the round-trip of two checkout
> commands.)
> > @@ -359,16 +360,22 @@ static int checkout_worktree(const struct checkout_opts *opts,
> >       int nr_checkouts = 0, nr_unmerged = 0;
> >       int errs = 0;
> >       int pos;
> > +     int pc_workers, pc_threshold;
> > +     struct mem_pool ce_mem_pool;
> >
> >       state.force = 1;
> >       state.refresh_cache = 1;
> >       state.istate = &the_index;
> >
> > +     mem_pool_init(&ce_mem_pool, 0);
> > +     get_parallel_checkout_configs(&pc_workers, &pc_threshold);
> >       init_checkout_metadata(&state.meta, info->refname,
> >                              info->commit ? &info->commit->object.oid : &info->oid,
> >                              NULL);
> >
> >       enable_delayed_checkout(&state);
> > +     if (pc_workers > 1)
> > +             init_parallel_checkout();
>
> I'm late to looking at your parallel checkout work, but I find this
> to be a really nice API to get things initialized.

Thanks :)