[PATCH v5 01/11] commit-graph: fix regression when computing Bloom filters

From: Abhishek Kumar <abhishekkumar8222@gmail.com>

Before computing Bloom fitlers, the commit-graph machinery uses commit_gen_cmp to sort commits by generation order for improved diff performance. 3d11275505 (commit-graph: examine commits by generation number, 2020-03-30) claims that this sort can reduce the time spent to compute Bloom filters by nearly half.

But since c49c82aa4c (commit: move members graph_pos, generation to a slab, 2020-06-17), this optimization is broken, since asking for a 'commit_graph_generation()' directly returns GENERATION_NUMBER_INFINITY while writing.

Not all hope is lost, though: 'commit_graph_generation()' falls back to comparing commits by their date when they have equal generation number, and so since c49c82aa4c is purely a date comparision function. This heuristic is good enough that we don't seem to loose appreciable performance while computing Bloom filters. Applying this patch (compared with v2.29.1) speeds up computing Bloom filters by around ~4 seconds.

So, avoid the useless 'commit_graph_generation()' while writing by instead accessing the slab directly. This returns the newly-computed generation numbers, and allows us to avoid the heuristic by directly comparing generation numbers.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
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 commit-graph.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)