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commit-graph: introduce commit_graph_data_slab

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The struct commit is used in many contexts. However, members
`generation` and `graph_pos` are only used for commit-graph related
operations and otherwise waste memory.

This wastage would have been more pronounced as we transition to
generation number v2, which uses 64-bit generation number instead of
current 32-bits.

As they are often accessed together, let's introduce struct
commit_graph_data and move them to a commit_graph_data slab.

While the overall test suite runs just as fast as master,
(series: 26m48s, master: 27m34s, faster by 2.87%), certain commands
like `git merge-base --is-ancestor` were slowed by 40% as discovered
by Szeder Gábor [1]. After minimizing commit-slab access, the slow down
persists but is closer to 20%.

Derrick Stolee believes the slow down is attributable to the underlying
algorithm rather than the slowness of commit-slab access [2] and we will
follow-up in a later series.

[1]: https://lore.kernel.org/git/20200607195347.GA8232@szeder.dev/
[2]: https://lore.kernel.org/git/13db757a-9412-7f1e-805c-8a028c4ab2b1@gmail.com/

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Abhishek Kumar
2020-06-17 14:44:09 +05:30
committed by Junio C Hamano
parent 6da43d937c
commit 4844812b9e
2 changed files with 78 additions and 10 deletions
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10
commit-graph.h
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@ -135,4 +135,14 @@ void free_commit_graph(struct commit_graph *);
*/
void disable_commit_graph(struct repository *r);
struct commit_graph_data {
uint32_t graph_pos;
uint32_t generation;
};
/*
* Commits should be parsed before accessing generation, graph positions.
*/
uint32_t commit_graph_generation(const struct commit *);
uint32_t commit_graph_position(const struct commit *);
#endif