1Git Commit Graph Design Notes 2============================= 3 4Git walks the commit graph for many reasons, including: 5 61. Listing and filtering commit history. 72. Computing merge bases. 8 9These operations can become slow as the commit count grows. The merge 10base calculation shows up in many user-facing commands, such as 'merge-base' 11or 'status' and can take minutes to compute depending on history shape. 12 13There are two main costs here: 14 151. Decompressing and parsing commits. 162. Walking the entire graph to satisfy topological order constraints. 17 18The commit-graph file is a supplemental data structure that accelerates 19commit graph walks. If a user downgrades or disables the 'core.commitGraph' 20config setting, then the existing ODB is sufficient. The file is stored 21as "commit-graph" either in the .git/objects/info directory or in the info 22directory of an alternate. 23 24The commit-graph file stores the commit graph structure along with some 25extra metadata to speed up graph walks. By listing commit OIDs in lexi- 26cographic order, we can identify an integer position for each commit and 27refer to the parents of a commit using those integer positions. We use 28binary search to find initial commits and then use the integer positions 29for fast lookups during the walk. 30 31A consumer may load the following info for a commit from the graph: 32 331. The commit OID. 342. The list of parents, along with their integer position. 353. The commit date. 364. The root tree OID. 375. The generation number (see definition below). 38 39Values 1-4 satisfy the requirements of parse_commit_gently(). 40 41Define the "generation number" of a commit recursively as follows: 42 43 * A commit with no parents (a root commit) has generation number one. 44 45 * A commit with at least one parent has generation number one more than 46 the largest generation number among its parents. 47 48Equivalently, the generation number of a commit A is one more than the 49length of a longest path from A to a root commit. The recursive definition 50is easier to use for computation and observing the following property: 51 52 If A and B are commits with generation numbers N and M, respectively, 53 and N <= M, then A cannot reach B. That is, we know without searching 54 that B is not an ancestor of A because it is further from a root commit 55 than A. 56 57 Conversely, when checking if A is an ancestor of B, then we only need 58 to walk commits until all commits on the walk boundary have generation 59 number at most N. If we walk commits using a priority queue seeded by 60 generation numbers, then we always expand the boundary commit with highest 61 generation number and can easily detect the stopping condition. 62 63This property can be used to significantly reduce the time it takes to 64walk commits and determine topological relationships. Without generation 65numbers, the general heuristic is the following: 66 67 If A and B are commits with commit time X and Y, respectively, and 68 X < Y, then A _probably_ cannot reach B. 69 70This heuristic is currently used whenever the computation is allowed to 71violate topological relationships due to clock skew (such as "git log" 72with default order), but is not used when the topological order is 73required (such as merge base calculations, "git log --graph"). 74 75In practice, we expect some commits to be created recently and not stored 76in the commit graph. We can treat these commits as having "infinite" 77generation number and walk until reaching commits with known generation 78number. 79 80We use the macro GENERATION_NUMBER_INFINITY = 0xFFFFFFFF to mark commits not 81in the commit-graph file. If a commit-graph file was written by a version 82of Git that did not compute generation numbers, then those commits will 83have generation number represented by the macro GENERATION_NUMBER_ZERO = 0. 84 85Since the commit-graph file is closed under reachability, we can guarantee 86the following weaker condition on all commits: 87 88 If A and B are commits with generation numbers N amd M, respectively, 89 and N < M, then A cannot reach B. 90 91Note how the strict inequality differs from the inequality when we have 92fully-computed generation numbers. Using strict inequality may result in 93walking a few extra commits, but the simplicity in dealing with commits 94with generation number *_INFINITY or *_ZERO is valuable. 95 96We use the macro GENERATION_NUMBER_MAX = 0x3FFFFFFF to for commits whose 97generation numbers are computed to be at least this value. We limit at 98this value since it is the largest value that can be stored in the 99commit-graph file using the 30 bits available to generation numbers. This 100presents another case where a commit can have generation number equal to 101that of a parent. 102 103Design Details 104-------------- 105 106- The commit-graph file is stored in a file named 'commit-graph' in the 107 .git/objects/info directory. This could be stored in the info directory 108 of an alternate. 109 110- The core.commitGraph config setting must be on to consume graph files. 111 112- The file format includes parameters for the object ID hash function, 113 so a future change of hash algorithm does not require a change in format. 114 115- Commit grafts and replace objects can change the shape of the commit 116 history. The latter can also be enabled/disabled on the fly using 117 `--no-replace-objects`. This leads to difficultly storing both possible 118 interpretations of a commit id, especially when computing generation 119 numbers. The commit-graph will not be read or written when 120 replace-objects or grafts are present. 121 122- Shallow clones create grafts of commits by dropping their parents. This 123 leads the commit-graph to think those commits have generation number 1. 124 If and when those commits are made unshallow, those generation numbers 125 become invalid. Since shallow clones are intended to restrict the commit 126 history to a very small set of commits, the commit-graph feature is less 127 helpful for these clones, anyway. The commit-graph will not be read or 128 written when shallow commits are present. 129 130Related Links 131------------- 132[0] https://bugs.chromium.org/p/git/issues/detail?id=8 133 Chromium work item for: Serialized Commit Graph 134 135[1] https://public-inbox.org/git/20110713070517.GC18566@sigill.intra.peff.net/ 136 An abandoned patch that introduced generation numbers. 137 138[2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@sigill.intra.peff.net/ 139 Discussion about generation numbers on commits and how they interact 140 with fsck. 141 142[3] https://public-inbox.org/git/20170908034739.4op3w4f2ma5s65ku@sigill.intra.peff.net/ 143 More discussion about generation numbers and not storing them inside 144 commit objects. A valuable quote: 145 146 "I think we should be moving more in the direction of keeping 147 repo-local caches for optimizations. Reachability bitmaps have been 148 a big performance win. I think we should be doing the same with our 149 properties of commits. Not just generation numbers, but making it 150 cheap to access the graph structure without zlib-inflating whole 151 commit objects (i.e., packv4 or something like the "metapacks" I 152 proposed a few years ago)." 153 154[4] https://public-inbox.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u 155 A patch to remove the ahead-behind calculation from 'status'.