Thanks for all of the feedback. I've learned a lot working on this patch. As discussed [0], this version changes several fundamental structures and operations, including:
* Graph files are stored in .git/objects/info * The "graph-head" file is now called "graph-latest" to avoid confusion with HEAD. We use "--set-latest" argument instead of "--update-head". * The graph format no longer stores the hash width, but expects the hash version to imply a length. This frees up one byte for future use while preserving 4-byte alignment. * The graph format is more explicit about optional chunks and no longer dies when seeing an unknown chunk. The large edges chunk is now optional. * There is no longer a "clear" subcommand to git-commit-graph. * Fixed the bug related to "--stdin-commits" and check that the command only includes commits reachable from the input OIDs. * The struct packed_oid_list type is now an array of struct object_id instead of an array of pointers. In my testing, I saw no performance difference between these two options, so I switched to the simpler pattern. * My patch is based on jt/binsearch-with-fanout (b4e00f730), with the newer method prototype since v3. Thanks, -Stolee [0] https://public-inbox.org/git/1517348383-112294-1-git-send-email-dsto...@microsoft.com/T/#m22bfdb7cf7b3d6e5f380b8bf0eec957e2cfd2dd7 -- >8 -- As promised [1], this patch contains a way to serialize the commit graph. The current implementation defines a new file format to store the graph structure (parent relationships) and basic commit metadata (commit date, root tree OID) in order to prevent parsing raw commits while performing basic graph walks. For example, we do not need to parse the full commit when performing these walks: * 'git log --topo-order -1000' walks all reachable commits to avoid incorrect topological orders, but only needs the commit message for the top 1000 commits. * 'git merge-base <A> <B>' may walk many commits to find the correct boundary between the commits reachable from A and those reachable from B. No commit messages are needed. * 'git branch -vv' checks ahead/behind status for all local branches compared to their upstream remote branches. This is essentially as hard as computing merge bases for each. The current patch speeds up these calculations by injecting a check in parse_commit_gently() to check if there is a graph file and using that to provide the required metadata to the struct commit. The file format has room to store generation numbers, which will be provided as a patch after this framework is merged. Generation numbers are referenced by the design document but not implemented in order to make the current patch focus on the graph construction process. Once that is stable, it will be easier to add generation numbers and make graph walks aware of generation numbers one-by-one. Here are some performance results for a copy of the Linux repository where 'master' has 704,766 reachable commits and is behind 'origin/master' by 19,610 commits. | Command | Before | After | Rel % | |----------------------------------|--------|--------|-------| | log --oneline --topo-order -1000 | 5.9s | 0.7s | -88% | | branch -vv | 0.42s | 0.27s | -35% | | rev-list --all | 6.4s | 1.0s | -84% | | rev-list --all --objects | 32.6s | 27.6s | -15% | To test this yourself, run the following on your repo: git config core.commitGraph true git show-ref -s | git commit-graph write --set-latest --stdin-commits The second command writes a commit graph file containing every commit reachable from your refs. Now, all git commands that walk commits will check your graph first before consulting the ODB. You can run your own performance comparisions by toggling the 'core.commitgraph' setting. [1] https://public-inbox.org/git/d154319e-bb9e-b300-7c37-27b1dcd2a...@jeffhostetler.com/ Re: What's cooking in git.git (Jan 2018, #03; Tue, 23) [2] https://github.com/derrickstolee/git/pull/2 A GitHub pull request containing the latest version of this patch. Derrick Stolee (13): commit-graph: add format document graph: add commit graph design document commit-graph: create git-commit-graph builtin commit-graph: implement write_commit_graph() commit-graph: implement 'git-commit-graph write' commit-graph: implement git commit-graph read commit-graph: implement --set-latest commit-graph: implement --delete-expired commit-graph: add core.commitGraph setting commit-graph: close under reachability commit: integrate commit graph with commit parsing commit-graph: read only from specific pack-indexes commit-graph: build graph from starting commits .gitignore | 1 + Documentation/config.txt | 3 + Documentation/git-commit-graph.txt | 105 ++++ Documentation/technical/commit-graph-format.txt | 90 +++ Documentation/technical/commit-graph.txt | 185 ++++++ Makefile | 2 + alloc.c | 1 + builtin.h | 1 + builtin/commit-graph.c | 261 +++++++++ cache.h | 1 + command-list.txt | 1 + commit-graph.c | 724 ++++++++++++++++++++++++ commit-graph.h | 49 ++ commit.c | 3 + commit.h | 3 + config.c | 5 + environment.c | 1 + git.c | 1 + log-tree.c | 3 +- packfile.c | 4 +- packfile.h | 2 + t/t5318-commit-graph.sh | 244 ++++++++ 22 files changed, 1686 insertions(+), 4 deletions(-) create mode 100644 Documentation/git-commit-graph.txt create mode 100644 Documentation/technical/commit-graph-format.txt create mode 100644 Documentation/technical/commit-graph.txt create mode 100644 builtin/commit-graph.c create mode 100644 commit-graph.c create mode 100644 commit-graph.h create mode 100755 t/t5318-commit-graph.sh -- 2.15.1.44.g453ed2b
