Subject: Re: [PATCH v3 10/11] commit-reach: use corrected commit dates in paint_down_to_common()

From: [email-masked] (Jakub Narębski)
Date: Sat, 22 Aug 2020 21:09:21 +0200

Hello Abhishek,

"Abhishek Kumar via GitGitGadget" <[email-masked]> writes:

> From: Abhishek Kumar <[email-masked]>
>
> With corrected commit dates implemented, we no longer have to rely on
> commit date as a heuristic in paint_down_to_common().

All right, but it would be nice to have some benchmark data: what were
performance when using topological levels, what was performance when
using commit date heuristics (before this patch), what is performace now
when using corrected commit date.

>
> t6024-recursive-merge setups a unique repository where all commits have
> the same committer date without well-defined merge-base. As this has
> already caused problems (as noted in 859fdc0 (commit-graph: define
> GIT_TEST_COMMIT_GRAPH, 2018-08-29)), we disable commit graph within the
> test script.

OK?

>
> Signed-off-by: Abhishek Kumar <[email-masked]>
> ---
>  commit-graph.c             | 14 ++++++++++++++
>  commit-graph.h             |  6 ++++++
>  commit-reach.c             |  2 +-
>  t/t6024-recursive-merge.sh |  4 +++-
>  4 files changed, 24 insertions(+), 2 deletions(-)
>

I have reorderd files for easier review.

> diff --git a/commit-graph.h b/commit-graph.h
> index 3cf89d895d..e22ec1e626 100644
> --- a/commit-graph.h
> +++ b/commit-graph.h
> @@ -91,6 +91,12 @@ struct commit_graph *parse_commit_graph(void *graph_map, size_t graph_size);
>   */
>  int generation_numbers_enabled(struct repository *r);
>
> +/*
> + * Return 1 if and only if the repository has a commit-graph
> + * file and generation data chunk has been written for the file.
> + */
> +int corrected_commit_dates_enabled(struct repository *r);
> +
>  enum commit_graph_write_flags {
>  	COMMIT_GRAPH_WRITE_APPEND     = (1 << 0),
>  	COMMIT_GRAPH_WRITE_PROGRESS   = (1 << 1),

All right.

> diff --git a/commit-graph.c b/commit-graph.c
> index c1292f8e08..6411068411 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -703,6 +703,20 @@ int generation_numbers_enabled(struct repository *r)
>  	return !!first_generation;
>  }
>
> +int corrected_commit_dates_enabled(struct repository *r)
> +{
> +	struct commit_graph *g;
> +	if (!prepare_commit_graph(r))
> +		return 0;
> +
> +	g = r->objects->commit_graph;
> +
> +	if (!g->num_commits)
> +		return 0;
> +
> +	return !!g->chunk_generation_data;
> +}

The previous commit introduced validate_mixed_generation_chain(), which
walked whole split commit-graph chain, and set `read_generation_data`
field in `struct commit_graph` for all layers in the chain.

This function examines only the top layer, so it follows the assumption
that Git would behave in such way that oly topmost layers in the chai
can be GDAT-less.

Why the difference?  Couldn't validate_mixed_generation_chain() simply
call corrected_commit_dates_enabled()?

> +
>  static void close_commit_graph_one(struct commit_graph *g)
>  {
>  	if (!g)
> diff --git a/commit-reach.c b/commit-reach.c
> index 470bc80139..3a1b925274 100644
> --- a/commit-reach.c
> +++ b/commit-reach.c
> @@ -39,7 +39,7 @@ static struct commit_list *paint_down_to_common(struct repository *r,
>  	int i;
>  	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
>
> -	if (!min_generation)

This check was added in 091f4cf (commit: don't use generation numbers if
not needed, 2018-08-30) by Derrick Stolee, and its commit message
includes benchmark results for running 'git merge-base v4.8 v4.9' in
Linux kernel repository:

      v2.18.0: 0.122s    167,468 walked
  v2.19.0-rc1: 0.547s    635,579 walked
         HEAD: 0.127s

> +	if (!min_generation && !corrected_commit_dates_enabled(r))
>  		queue.compare = compare_commits_by_commit_date;

It would be nice to have similar benchmark for this change... unless of
course there is no change in performance, but I think then it needs to
be stated explicitly.  I think.

>
>  	one->object.flags |= PARENT1;
> diff --git a/t/t6024-recursive-merge.sh b/t/t6024-recursive-merge.sh
> index 332cfc53fd..d3def66e7d 100755
> --- a/t/t6024-recursive-merge.sh
> +++ b/t/t6024-recursive-merge.sh
> @@ -15,6 +15,8 @@ GIT_COMMITTER_DATE="2006-12-12 23:28:00 +0100"
>  export GIT_COMMITTER_DATE
>
>  test_expect_success 'setup tests' '
> +	GIT_TEST_COMMIT_GRAPH=0 &&
> +	export GIT_TEST_COMMIT_GRAPH &&
>  	echo 1 >a1 &&
>  	git add a1 &&
>  	GIT_AUTHOR_DATE="2006-12-12 23:00:00" git commit -m 1 a1 &&
> @@ -66,7 +68,7 @@ test_expect_success 'setup tests' '
>  '
>
>  test_expect_success 'combined merge conflicts' '
> -	test_must_fail env GIT_TEST_COMMIT_GRAPH=0 git merge -m final G
> +	test_must_fail git merge -m final G
>  '
>
>  test_expect_success 'result contains a conflict' '

OK, so instead of disabling commit-graph for this test, now we disable
it for the whole script.

Maybe this change should be in a separate patch?

Best,
-- 
Jakub Narębski