Documentation/technical/commit-graph.txt

   1 Git Commit Graph Design Notes
   2 =============================
   3
   4 Git walks the commit graph for many reasons, including:
   5
   6 1. Listing and filtering commit history.
   7 2. Computing merge bases.
   8
   9 These operations can become slow as the commit count grows. The merge
  10 base calculation shows up in many user-facing commands, such as 'merge-base'
  11 or 'status' and can take minutes to compute depending on history shape.
  12
  13 There are two main costs here:
  14
  15 1. Decompressing and parsing commits.
  16 2. Walking the entire graph to satisfy topological order constraints.
  17
  18 The commit graph file is a supplemental data structure that accelerates
  19 commit graph walks. If a user downgrades or disables the 'core.commitGraph'
  20 config setting, then the existing ODB is sufficient. The file is stored
  21 as "commit-graph" either in the .git/objects/info directory or in the info
  22 directory of an alternate.
  23
  24 The commit graph file stores the commit graph structure along with some
  25 extra metadata to speed up graph walks. By listing commit OIDs in lexi-
  26 cographic order, we can identify an integer position for each commit and
  27 refer to the parents of a commit using those integer positions. We use
  28 binary search to find initial commits and then use the integer positions
  29 for fast lookups during the walk.
  30
  31 A consumer may load the following info for a commit from the graph:
  32
  33 1. The commit OID.
  34 2. The list of parents, along with their integer position.
  35 3. The commit date.
  36 4. The root tree OID.
  37 5. The generation number (see definition below).
  38
  39 Values 1-4 satisfy the requirements of parse_commit_gently().
  40
  41 Define 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
  48 Equivalently, the generation number of a commit A is one more than the
  49 length of a longest path from A to a root commit. The recursive definition
  50 is 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
  63 This property can be used to significantly reduce the time it takes to
  64 walk commits and determine topological relationships. Without generation
  65 numbers, 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
  70 This heuristic is currently used whenever the computation is allowed to
  71 violate topological relationships due to clock skew (such as "git log"
  72 with default order), but is not used when the topological order is
  73 required (such as merge base calculations, "git log --graph").
  74
  75 In practice, we expect some commits to be created recently and not stored
  76 in the commit graph. We can treat these commits as having "infinite"
  77 generation number and walk until reaching commits with known generation
  78 number.
  79
  80 Design Details
  81 --------------
  82
  83 - The commit graph file is stored in a file named 'commit-graph' in the
  84   .git/objects/info directory. This could be stored in the info directory
  85   of an alternate.
  86
  87 - The core.commitGraph config setting must be on to consume graph files.
  88
  89 - The file format includes parameters for the object ID hash function,
  90   so a future change of hash algorithm does not require a change in format.
  91
  92 Future Work
  93 -----------
  94
  95 - The commit graph feature currently does not honor commit grafts. This can
  96   be remedied by duplicating or refactoring the current graft logic.
  97
  98 - The 'commit-graph' subcommand does not have a "verify" mode that is
  99   necessary for integration with fsck.
 100
 101 - The file format includes room for precomputed generation numbers. These
 102   are not currently computed, so all generation numbers will be marked as
 103   0 (or "uncomputed"). A later patch will include this calculation.
 104
 105 - After computing and storing generation numbers, we must make graph
 106   walks aware of generation numbers to gain the performance benefits they
 107   enable. This will mostly be accomplished by swapping a commit-date-ordered
 108   priority queue with one ordered by generation number. The following
 109   operations are important candidates:
 110
 111     - paint_down_to_common()
 112     - 'log --topo-order'
 113
 114 - Currently, parse_commit_gently() requires filling in the root tree
 115   object for a commit. This passes through lookup_tree() and consequently
 116   lookup_object(). Also, it calls lookup_commit() when loading the parents.
 117   These method calls check the ODB for object existence, even if the
 118   consumer does not need the content. For example, we do not need the
 119   tree contents when computing merge bases. Now that commit parsing is
 120   removed from the computation time, these lookup operations are the
 121   slowest operations keeping graph walks from being fast. Consider
 122   loading these objects without verifying their existence in the ODB and
 123   only loading them fully when consumers need them. Consider a method
 124   such as "ensure_tree_loaded(commit)" that fully loads a tree before
 125   using commit->tree.
 126
 127 - The current design uses the 'commit-graph' subcommand to generate the graph.
 128   When this feature stabilizes enough to recommend to most users, we should
 129   add automatic graph writes to common operations that create many commits.
 130   For example, one could compute a graph on 'clone', 'fetch', or 'repack'
 131   commands.
 132
 133 - A server could provide a commit graph file as part of the network protocol
 134   to avoid extra calculations by clients. This feature is only of benefit if
 135   the user is willing to trust the file, because verifying the file is correct
 136   is as hard as computing it from scratch.
 137
 138 Related Links
 139 -------------
 140 [0] https://bugs.chromium.org/p/git/issues/detail?id=8
 141     Chromium work item for: Serialized Commit Graph
 142
 143 [1] https://public-inbox.org/git/20110713070517.GC18566@sigill.intra.peff.net/
 144     An abandoned patch that introduced generation numbers.
 145
 146 [2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@sigill.intra.peff.net/
 147     Discussion about generation numbers on commits and how they interact
 148     with fsck.
 149
 150 [3] https://public-inbox.org/git/20170908034739.4op3w4f2ma5s65ku@sigill.intra.peff.net/
 151     More discussion about generation numbers and not storing them inside
 152     commit objects. A valuable quote:
 153
 154     "I think we should be moving more in the direction of keeping
 155      repo-local caches for optimizations. Reachability bitmaps have been
 156      a big performance win. I think we should be doing the same with our
 157      properties of commits. Not just generation numbers, but making it
 158      cheap to access the graph structure without zlib-inflating whole
 159      commit objects (i.e., packv4 or something like the "metapacks" I
 160      proposed a few years ago)."
 161
 162 [4] https://public-inbox.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u
 163     A patch to remove the ahead-behind calculation from 'status'.