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Diffstat (limited to 'Documentation/technical/commit-graph.txt')
| -rw-r--r-- | Documentation/technical/commit-graph.txt | 69 |
1 files changed, 39 insertions, 30 deletions
diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt index 0550c6d0dc..001395e950 100644 --- a/Documentation/technical/commit-graph.txt +++ b/Documentation/technical/commit-graph.txt @@ -77,6 +77,29 @@ 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 -------------- @@ -89,46 +112,32 @@ 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' - -- 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. + - 'tag --merged' - 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 |