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

[Derrick Stolee <stolee@gmail.com>]

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>

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.

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'

(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.


>  			else if (opts->merge)
>  				errs |= checkout_merged(pos, &state,
> -							&nr_unmerged);
> +							&nr_unmerged,
> +							&ce_mem_pool);

I see the need to populate these merged entries in the pool for now,
before...

>  			pos = skip_same_name(ce, pos) - 1;
>  		}
>  	}
> +	if (pc_workers > 1)
> +		errs |= run_parallel_checkout(&state, pc_workers, pc_threshold,
> +					      NULL, NULL);
> +	mem_pool_discard(&ce_mem_pool, should_validate_cache_entries());
...now running the parallel checkout and discarding the entries.

Thanks,
-Stolee