Re: [PATCH v2 00/10] [GSoC] Implement Corrected Commit Date

> 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.
> 
> Thanks to Dr. Stolee, Dr. Narębski, and Taylor for their reviews on the
> first version.
> 
> I look forward to everyone's reviews!
> 
> Thanks
> 
>  * Abhishek
> 
> 
> ----------------------------------------------------------------------------
> 
> Changes in version 2:
> 
>  * Add tests for generation data chunk.
>  * Add an option GIT_TEST_COMMIT_GRAPH_NO_GDAT to control whether to write
>    generation data chunk.
>  * Compare commits with corrected commit dates if present in
>    paint_down_to_common().
>  * Update technical documentation.
>  * Handle mixed graph version commit chains.
>  * Improve commit messages for
>  * Revert unnecessary whitespace changes.
>  * Split uint_32 -> timestamp_t change into a new commit.