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Diffstat (limited to 'Documentation/technical/commit-graph.txt')
| -rw-r--r-- | Documentation/technical/commit-graph.txt | 77 |
1 files changed, 43 insertions, 34 deletions
diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt index 0550c6d0dc..7805b0968c 100644 --- a/Documentation/technical/commit-graph.txt +++ b/Documentation/technical/commit-graph.txt @@ -15,13 +15,13 @@ There are two main costs here: 1. Decompressing and parsing commits. 2. Walking the entire graph to satisfy topological order constraints. -The commit graph file is a supplemental data structure that accelerates +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 as "commit-graph" either in the .git/objects/info directory or in the info directory of an alternate. -The commit graph file stores the commit graph structure along with some +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 @@ -77,10 +77,33 @@ in the commit graph. We can treat these commits as having "infinite" generation number and walk until reaching commits with known generation number. +We use the macro GENERATION_NUMBER_INFINITY = 0xFFFFFFFF to mark commits not +in the commit-graph file. If a commit-graph file was written by a version +of Git that did not compute generation numbers, then those commits will +have generation number represented by the macro GENERATION_NUMBER_ZERO = 0. + +Since the commit-graph file is closed under reachability, we can guarantee +the following weaker condition on all commits: + + If A and B are commits with generation numbers N amd M, respectively, + and N < M, then A cannot reach B. + +Note how the strict inequality differs from the inequality when we have +fully-computed generation numbers. Using strict inequality may result in +walking a few extra commits, but the simplicity in dealing with commits +with generation number *_INFINITY or *_ZERO is valuable. + +We use the macro GENERATION_NUMBER_MAX = 0x3FFFFFFF to for commits whose +generation numbers are computed to be at least this value. We limit at +this value since it is the largest value that can be stored in the +commit-graph file using the 30 bits available to generation numbers. This +presents another case where a commit can have generation number equal to +that of a parent. + Design Details -------------- -- The commit graph file is stored in a file named 'commit-graph' in the +- The commit-graph file is stored in a file named 'commit-graph' in the .git/objects/info directory. This could be stored in the info directory of an alternate. @@ -89,48 +112,34 @@ Design Details - The file format includes parameters for the object ID hash function, so a future change of hash algorithm does not require a change in format. +- Commit grafts and replace objects can change the shape of the commit + history. The latter can also be enabled/disabled on the fly using + `--no-replace-objects`. This leads to difficultly storing both possible + interpretations of a commit id, especially when computing generation + numbers. The commit-graph will not be read or written when + replace-objects or grafts are present. + +- Shallow clones create grafts of commits by dropping their parents. This + leads the commit-graph to think those commits have generation number 1. + If and when those commits are made unshallow, those generation numbers + become invalid. Since shallow clones are intended to restrict the commit + history to a very small set of commits, the commit-graph feature is less + helpful for these clones, anyway. The commit-graph will not be read or + written when shallow commits are present. + Future Work ----------- -- The commit graph feature currently does not honor commit grafts. This can - be remedied by duplicating or refactoring the current graft logic. - -- The 'commit-graph' subcommand does not have a "verify" mode that is - necessary for integration with fsck. - -- 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. - - 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' + - 'tag --merged' -- 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' subcommand 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 could compute a graph on 'clone', 'fetch', or 'repack' - commands. - -- A server could provide a commit graph file as part of the network protocol +- A server could provide a commit-graph file as part of the network protocol to avoid extra calculations by clients. This feature is only of benefit if the user is willing to trust the file, because verifying the file is correct is as hard as computing it from scratch. |