From 6cc017431c1c48f80d1c6512fdcc9866cf4b7f55 Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Fri, 20 Jul 2018 16:33:30 +0000 Subject: commit-reach: use can_all_from_reach The is_descendant_of method previously used in_merge_bases() to check if the commit can reach any of the commits in the provided list. This had two performance problems: 1. The performance is quadratic in worst-case. 2. A single in_merge_bases() call requires walking beyond the target commit in order to find the full set of boundary commits that may be merge-bases. The can_all_from_reach method avoids this quadratic behavior and can limit the search beyond the target commits using generation numbers. It requires a small prototype adjustment to stop using commit-date as a cutoff, as that optimization is no longer appropriate here. Since in_merge_bases() uses paint_down_to_common(), is_descendant_of() naturally found cutoffs to avoid walking the entire commit graph. Since we want to always return the correct result, we cannot use the min_commit_date cutoff in can_all_from_reach. We then rely on generation numbers to provide the cutoff. Since not all repos will have a commit-graph file, nor will we always have generation numbers computed for a commit-graph file, create a new method, generation_numbers_enabled(), that checks for a commit-graph file and sees if the first commit in the file has a non-zero generation number. In the case that we do not have generation numbers, use the old logic for is_descendant_of(). Performance was meausured on a copy of the Linux repository using the 'test-tool reach is_descendant_of' command using this input: A:v4.9 X:v4.10 X:v4.11 X:v4.12 X:v4.13 X:v4.14 X:v4.15 X:v4.16 X:v4.17 X.v3.0 Note that this input is tailored to demonstrate the quadratic nature of the previous method, as it will compute merge-bases for v4.9 versus all of the later versions before checking against v4.1. Before: 0.26 s After: 0.21 s Since we previously used the is_descendant_of method in the ref_newer method, we also measured performance there using 'test-tool reach ref_newer' with this input: A:v4.9 B:v3.19 Before: 0.10 s After: 0.08 s By adding a new commit with parent v3.19, we test the non-reachable case of ref_newer: Before: 0.09 s After: 0.08 s Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- commit-graph.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'commit-graph.h') diff --git a/commit-graph.h b/commit-graph.h index 76e098934a..0de8f88316 100644 --- a/commit-graph.h +++ b/commit-graph.h @@ -51,6 +51,12 @@ struct commit_graph { struct commit_graph *load_commit_graph_one(const char *graph_file); +/* + * Return 1 if and only if the repository has a commit-graph + * file and generation numbers are computed in that file. + */ +int generation_numbers_enabled(struct repository *r); + void write_commit_graph_reachable(const char *obj_dir, int append); void write_commit_graph(const char *obj_dir, struct string_list *pack_indexes, -- cgit v1.2.3