Re: [PATCH 3/3] commit: use commit graph in `lookup_commit_reference_gently()`

> In the preceding commit we refactored `lookup_commit_reference_gently()`
> so that it doesn't parse non-commit objects anymore. This has led to a
> speedup when git-receive-pack(1) accepts a shallow push into a repo
> with lots of refs that point to blobs or trees.
> 
> But while this case is now faster, we still have the issue that
> accepting pushes with lots of "normal" refs that point to commits are
> still slow. This is mostly because we look up the commits via the object
> database, and that is rather costly.
> 
> Adapt the code to use `repo_parse_commit_gently()` instead of
> `parse_object()` to parse the resulting commit object. This function
> knows to use the commit-graph to fill in the object, which is way more
> cost efficient.

> This leads to another significant speedup when accepting shallow pushes.
> The following benchmark pushes a single objects from a shallow clone
> into a repository with 600,000 references that all point to commits:
> 
>   Benchmark 1: git-receive-pack (rev = HEAD~)
>     Time (mean ± σ):      9.179 s ±  0.031 s    [User: 8.858 s, System: 0.528 s]
>     Range (min … max):    9.154 s …  9.213 s    3 runs
> 
>   Benchmark 2: git-receive-pack (rev = HEAD)
>     Time (mean ± σ):      2.337 s ±  0.032 s    [User: 2.331 s, System: 0.234 s]
>     Range (min … max):    2.308 s …  2.371 s    3 runs
> 
>   Summary
>     git-receive-pack . </tmp/input (rev = HEAD) ran
>       3.93 ± 0.05 times faster than git-receive-pack (rev = HEAD~)
> 
> Also, this again leads to a significant reduction in memory allocations.
> Before this change:
> 
>   HEAP SUMMARY:
>       in use at exit: 17,524,978 bytes in 22,393 blocks
>     total heap usage: 33,313 allocs, 10,920 frees, 407,774,251 bytes allocated
> 
> And after this change:
> 
>   HEAP SUMMARY:
>       in use at exit: 11,534,036 bytes in 12,406 blocks
>     total heap usage: 13,284 allocs, 878 frees, 15,521,451 bytes allocated

> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  commit.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/commit.c b/commit.c
> index b7c4ec2eb5..014f74822c 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -45,7 +45,7 @@ struct commit *lookup_commit_reference_gently(struct repository *r,
>  {
>  	const struct object_id *maybe_peeled;
>  	struct object_id peeled_oid;
> -	struct object *object;
> +	struct commit *commit;
>  	enum object_type type;
>  
>  	switch (peel_object_ext(r, oid, &peeled_oid, 0, &type)) {
> @@ -67,11 +67,11 @@ struct commit *lookup_commit_reference_gently(struct repository *r,
>  		return NULL;
>  	}
>  
> -	object = parse_object(r, maybe_peeled);
> -	if (!object)
> +	commit = lookup_commit(r, maybe_peeled);
> +	if (!commit || repo_parse_commit_gently(r, commit, quiet) < 0)
>  		return NULL;

>  
> -	return object_as_type(object, OBJ_COMMIT, quiet);
> +	return commit;
>  }
>  
>  struct commit *lookup_commit_reference(struct repository *r, const struct object_id *oid)