Re: [PATCH 2/2] commit-graph: propagate topo_levels slab to all chain layers

[Junio C Hamano <gitster@pobox.com>]

"Kristofer Karlsson via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> From: Kristofer Karlsson <krka@spotify.com>
>
> Fix a regression introduced in 199d452758 (commit-graph: fix
> "filling in" topological levels, 2025-04-07) where the loop

I guess the same comment from [1/2] applies.  We might be chasing
ghosts here.  Is that elusive commit a total hallucination?

> On a repository with 2.78M commits and a multi-layer split
> commit-graph, this caused a single incremental commit-graph
> write to spend ~3.7 seconds in the generation DFS instead of
> microseconds.

Nice.

> Signed-off-by: Kristofer Karlsson <krka@spotify.com>
> ---
>  commit-graph.c                | 2 +-
>  t/t5324-split-commit-graph.sh | 6 +-----
>  2 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 4e39a048c4..c2a711cceb 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -2610,7 +2610,7 @@ int write_commit_graph(struct odb_source *source,
>  
>  	g = prepare_commit_graph(ctx.r);
>  	for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
> -		g->topo_levels = &topo_levels;
> +		chain->topo_levels = &topo_levels;
>  
>  	if (flags & COMMIT_GRAPH_WRITE_BLOOM_FILTERS)
>  		ctx.changed_paths = 1;
> diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
> index f9c57760f4..9e5ab7dbd0 100755
> --- a/t/t5324-split-commit-graph.sh
> +++ b/t/t5324-split-commit-graph.sh
> @@ -738,11 +738,7 @@ test_expect_success 'incremental write reads topo levels from all layers' '
>  		GIT_TRACE2_EVENT="$(pwd)/trace.txt" \
>  			git commit-graph write --reachable --split=no-merge &&
>  
> -		# BUG: topo levels from lower graph layers are not
> -		# propagated, so the DFS re-walks from base-3 down to
> -		# the root (7 steps) instead of reading topo levels
> -		# from the existing graph (1 step).
> -		test_trace2_data commit-graph generation-dfs-steps 7 <trace.txt
> +		test_trace2_data commit-graph generation-dfs-steps 1 <trace.txt
>  	)
>  '