This patch series implements the corrected commit date offsets as generation number v2, along with other pre-requisites. Git uses topological levels in the commit-graph file for commit-graph traversal operations like git log --graph. Unfortunately, using topological levels can result in a worse performance than without them when compared with committer date as a heuristics. For example, git merge-base v4.8 v4.9 on the Linux repository walks 635,579 commits using topological levels and walks 167,468 using committer date. Thus, the need for generation number v2 was born. New generation number needed to provide good performance, increment updates, and backward compatibility. Due to an unfortunate problem, we also needed a way to distinguish between the old and new generation number without incrementing graph version. Various candidates were examined (https://github.com/derrickstolee/gen-test, https://github.com/abhishekkumar2718/git/pull/1). The proposed generation number v2, Corrected Commit Date with Mononotically Increasing Offsets performed much worse than committer date (506,577 vs. 167,468 commits walked for git merge-base v4.8 v4.9) and was dropped. Using Generation Data chunk (GDAT) relieves the requirement of backward compatibility as we would continue to store topological levels in Commit Data (CDAT) chunk. Thus, Corrected Commit Date was chosen as generation number v2. The Corrected Commit Date is defined as: For a commit C, let its corrected commit date be the maximum of the commit date of C and the corrected commit dates of its parents. Then corrected commit date offset is the difference between corrected commit date of C and commit date of C. We will introduce an additional commit-graph chunk, Generation Data chunk, and store corrected commit date offsets in GDAT chunk while storing topological levels in CDAT chunk. The old versions of Git would ignore GDAT chunk, using topological levels from CDAT chunk. In contrast, new versions of Git would use corrected commit dates, falling back to topological level if the generation data chunk is absent in the commit-graph file. Here's what left for the PR (which I intend to take on with the second version of pull request): 1. Add an option to skip writing generation data chunk (to test whether new Git works without GDAT as intended). 2. Handle writing to commit-graph for mismatched version (that is, merging all graphs into a new graph with a GDAT chunk). 3. Update technical documentation. I look forward to everyone's reviews! Thanks * Abhishek ---------------------------------------------------------------------------- The build fails for t9807-git-p4-submit.sh on osx-clang, which I feel is unrelated to my code changes. Still need to investigate further. Abhishek Kumar (6): commit-graph: fix regression when computing bloom filter revision: parse parent in indegree_walk_step() commit-graph: consolidate fill_commit_graph_info commit-graph: consolidate compare_commits_by_gen commit-graph: implement generation data chunk commit-graph: implement corrected commit date offset blame.c | 2 +- commit-graph.c | 181 +++++++++++++++++++++------------- commit-graph.h | 7 +- commit-reach.c | 47 +++------ commit-reach.h | 2 +- commit.c | 9 +- commit.h | 3 + revision.c | 17 ++-- t/helper/test-read-graph.c | 2 + t/t4216-log-bloom.sh | 4 +- t/t5000-tar-tree.sh | 4 +- t/t5318-commit-graph.sh | 21 ++-- t/t5324-split-commit-graph.sh | 12 +-- upload-pack.c | 2 +- 14 files changed, 178 insertions(+), 135 deletions(-) base-commit: 47ae905ffb98cc4d4fd90083da6bc8dab55d9ecc Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-676%2Fabhishekkumar2718%2Fcorrected_commit_date-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-676/abhishekkumar2718/corrected_commit_date-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/676 -- gitgitgadget
