Add Documentation/technical/commit-graph.txt with details of the planned commit graph feature, including future plans. Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx> --- Documentation/technical/commit-graph.txt | 189 +++++++++++++++++++++++++++++++ 1 file changed, 189 insertions(+) create mode 100644 Documentation/technical/commit-graph.txt diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt new file mode 100644 index 0000000000..fc86b06041 --- /dev/null +++ b/Documentation/technical/commit-graph.txt @@ -0,0 +1,189 @@ +Git Commit Graph Design Notes +============================= + +Git walks the commit graph for many reasons, including: + +1. Listing and filtering commit history. +2. Computing merge bases. + +These operations can become slow as the commit count grows. The merge +base calculation shows up in many user-facing commands, such as 'merge-base' +or 'status' and can take minutes to compute depending on history shape. + +There are two main costs here: + +1. Decompressing and parsing commits. +2. Walking the entire graph to avoid topological order mistakes. + +The commit graph file is a supplemental data structure that accelerates +commit graph walks. If a user downgrades or disables the 'core.commitGraph' +config setting, then the existing ODB is sufficient. The file is stored +next to packfiles either in the .git/objects/pack directory or in the pack +directory of an alternate. + +The commit graph file stores the commit graph structure along with some +extra metadata to speed up graph walks. By listing commit OIDs in lexi- +cographic order, we can identify an integer position for each commit and +refer to the parents of a commit using those integer positions. We use +binary search to find initial commits and then use the integer positions +for fast lookups during the walk. + +A consumer may load the following info for a commit from the graph: + +1. The commit OID. +2. The list of parents, along with their integer position. +3. The commit date. +4. The root tree OID. +5. The generation number (see definition below). + +Values 1-4 satisfy the requirements of parse_commit_gently(). + +Define the "generation number" of a commit recursively as follows: + + * A commit with no parents (a root commit) has generation number one. + + * A commit with at least one parent has generation number one more than + the largest generation number among its parents. + +Equivalently, the generation number of a commit A is one more than the +length of a longest path from A to a root commit. The recursive definition +is easier to use for computation and observing the following property: + + If A and B are commits with generation numbers N and M, respectively, + and N <= M, then A cannot reach B. That is, we know without searching + that B is not an ancestor of A because it is further from a root commit + than A. + + Conversely, when checking if A is an ancestor of B, then we only need + to walk commits until all commits on the walk boundary have generation + number at most N. If we walk commits using a priority queue seeded by + generation numbers, then we always expand the boundary commit with highest + generation number and can easily detect the stopping condition. + +This property can be used to significantly reduce the time it takes to +walk commits and determine topological relationships. Without generation +numbers, the general heuristic is the following: + + If A and B are commits with commit time X and Y, respectively, and + X < Y, then A _probably_ cannot reach B. + +This heuristic is currently used whenever the computation can make +mistakes with topological orders (such as "git log" with default order), +but is not used when the topological order is required (such as merge +base calculations, "git log --graph"). + +In practice, we expect some commits to be created recently and not stored +in the commit graph. We can treat these commits as having "infinite" +generation number and walk until reaching commits with known generation +number. + +Design Details +-------------- + +- A graph file is stored in a file named 'graph-<hash>.graph' in the pack + directory. This could be stored in an alternate. + +- The most-recent graph file hash is stored in a 'graph-head' file for + immediate access and storing backup graphs. This could be stored in an + alternate, and refers to a 'graph-<hash>.graph' file in the same pack + directory. + +- The core.commitGraph config setting must be on to consume graph files. + +- The file format includes parameters for the object id length and hash + algorithm, so a future change of hash algorithm does not require a change + in format. + +Current Limitations +------------------- + +- Only one graph file is used at one time. This allows the integer position + to seek into the single graph file. It is possible to extend the model + for multiple graph files, but that is currently not part of the design. + +- .graph files are managed only by the 'commit-graph' builtin. These are not + updated automatically during clone, fetch, repack, or creating new commits. + +- There is no '--verify' option for the 'commit-graph' builtin to verify the + contents of the graph file agree with the contents in the ODB. + +- When rewriting the graph, we do not check for a commit still existing + in the ODB, so garbage collection may remove commits. + +- Generation numbers are not computed in the current version. The file + format supports storing them, along with a mechanism to upgrade from + a file without generation numbers to one that uses them. + +Future Work +----------- + +- The file format includes room for precomputed generation numbers. These + are not currently computed, so all generation numbers will be marked as + 0 (or "uncomputed"). A later patch will include this calculation. + +- The commit graph is currently incompatible with commit grafts. This can be + remedied by duplicating or refactoring the current graft logic. + +- After computing and storing generation numbers, we must make graph + walks aware of generation numbers to gain the performance benefits they + enable. This will mostly be accomplished by swapping a commit-date-ordered + priority queue with one ordered by generation number. The following + operations are important candidates: + + - paint_down_to_common() + - 'log --topo-order' + +- The graph currently only adds commits to a previously existing graph. + When writing a new graph, we could check that the ODB still contains + the commits and choose to remove the commits that are deleted from the + ODB. For performance reasons, this check should remain optional. + +- Currently, parse_commit_gently() requires filling in the root tree + object for a commit. This passes through lookup_tree() and consequently + lookup_object(). Also, it calls lookup_commit() when loading the parents. + These method calls check the ODB for object existence, even if the + consumer does not need the content. For example, we do not need the + tree contents when computing merge bases. Now that commit parsing is + removed from the computation time, these lookup operations are the + slowest operations keeping graph walks from being fast. Consider + loading these objects without verifying their existence in the ODB and + only loading them fully when consumers need them. Consider a method + such as "ensure_tree_loaded(commit)" that fully loads a tree before + using commit->tree. + +- The current design uses the 'commit-graph' builtin to generate the graph. + When this feature stabilizes enough to recommend to most users, we should + add automatic graph writes to common operations that create many commits. + For example, one coulde compute a graph on 'clone', 'fetch', or 'repack' + commands. + +- A server could provide a commit graph file as part of the network protocol + to avoid extra calculations by clients. + +Related Links +------------- +[0] https://bugs.chromium.org/p/git/issues/detail?id=8 + Chromium work item for: Serialized Commit Graph + +[1] https://public-inbox.org/git/20110713070517.GC18566@xxxxxxxxxxxxxxxxxxxxx/ + An abandoned patch that introduced generation numbers. + +[2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@xxxxxxxxxxxxxxxxxxxxx/ + Discussion about generation numbers on commits and how they interact + with fsck. + +[3] https://public-inbox.org/git/20170907094718.b6kuzp2uhvkmwcso@xxxxxxxxxxxxxxxxxxxxx/t/#m7a2ea7b355aeda962e6b86404bcbadc648abfbba + More discussion about generation numbers and not storing them inside + commit objects. A valuable quote: + + "I think we should be moving more in the direction of keeping + repo-local caches for optimizations. Reachability bitmaps have been + a big performance win. I think we should be doing the same with our + properties of commits. Not just generation numbers, but making it + cheap to access the graph structure without zlib-inflating whole + commit objects (i.e., packv4 or something like the "metapacks" I + proposed a few years ago)." + +[4] https://public-inbox.org/git/20180108154822.54829-1-git@xxxxxxxxxxxxxxxxx/T/#u + A patch to remove the ahead-behind calculation from 'status'. + -- 2.15.1.45.g9b7079f
