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. While the overall test suite runs slightly faster than master (series: 27m10s, master: 27ms34s, faster by 2.35%), certain commands like `git merge-base --is-ancestory` are slowed by nearly 40% as discovered by SDEZER Gabor [1]. 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 on that in a later series. I did not mention maximum RSS in the commit messages as they were nearly identical (series: 68104kb, master: 68040kb, fewer by <0.1%). This leads me to conclude that either the test using maximum memory involves commit graph or did not involve the struct commit at all. The move to commit-slab reduces memory footprint for the cases where struct commit is used but members generation and graph position are not. Average RSS would have been a good and more representative measure, but unfortunately time(1) could not measure it on my system. With this, I feel the patch will require minor fixes, if any. I am moving ahead with working the next step of "Implement Generation Number v2" that is proper handling of commit-graph format change. Based on the discussions, I feel we should compute both generation number v1 and the date offset value with storing date offsets in a new chunk as the cost is mostly from walking the commits. Abhishek Kumar (4): alloc: introduce parsed_commits_count commit-graph: introduce commit_graph_data_slab commit: move members graph_pos, generation to a slab commit-graph: minimize commit_graph_data_slab access alloc.c | 6 +- blame.c | 2 +- bloom.c | 7 +- commit-graph.c | 122 ++++++++++++++++++++++++-------- commit-graph.h | 10 +++ commit-reach.c | 69 +++++++++++------- commit.c | 8 ++- contrib/coccinelle/commit.cocci | 18 +++++ revision.c | 20 +++--- 9 files changed, 188 insertions(+), 74 deletions(-) -- 2.27.0 Changes in v3: - Introduce alloc commit to fix the failing diff-submodule test. - Elaborate on performance and slow down noticed in the commit message. Changes in v2: - Introduce struct commit_graph_data. - Merge `graph_pos`, `generation` slabs into a single, `commit_graph_data` slab. - Use graph position for an intermediate check for generation, saving the cost of initializing generation numbers. - Add an follow-up patch caching results of slab access in local variables. - Move coccinelle transformation to commit.coccinelle instead of creating new scripts. - Elaborate on removing default values from init_commit_node(). - Revert moving macro constants (e.g. COMMIT_NOT_FROM_GRAPH, GENERATION_NUMBER_ZERO) from commit.h to commit-graph.h CI Build: https://travis-ci.com/github/abhishekkumar2718/git
