350 lines
15 KiB
Text
350 lines
15 KiB
Text
Git Commit Graph Design Notes
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=============================
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Git walks the commit graph for many reasons, including:
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1. Listing and filtering commit history.
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2. Computing merge bases.
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These operations can become slow as the commit count grows. The merge
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base calculation shows up in many user-facing commands, such as 'merge-base'
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or 'status' and can take minutes to compute depending on history shape.
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There are two main costs here:
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1. Decompressing and parsing commits.
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2. Walking the entire graph to satisfy topological order constraints.
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The commit-graph file is a supplemental data structure that accelerates
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commit graph walks. If a user downgrades or disables the 'core.commitGraph'
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config setting, then the existing ODB is sufficient. The file is stored
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as "commit-graph" either in the .git/objects/info directory or in the info
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directory of an alternate.
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The commit-graph file stores the commit graph structure along with some
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extra metadata to speed up graph walks. By listing commit OIDs in lexi-
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cographic order, we can identify an integer position for each commit and
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refer to the parents of a commit using those integer positions. We use
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binary search to find initial commits and then use the integer positions
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for fast lookups during the walk.
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A consumer may load the following info for a commit from the graph:
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1. The commit OID.
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2. The list of parents, along with their integer position.
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3. The commit date.
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4. The root tree OID.
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5. The generation number (see definition below).
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Values 1-4 satisfy the requirements of parse_commit_gently().
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Define the "generation number" of a commit recursively as follows:
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* A commit with no parents (a root commit) has generation number one.
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* A commit with at least one parent has generation number one more than
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the largest generation number among its parents.
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Equivalently, the generation number of a commit A is one more than the
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length of a longest path from A to a root commit. The recursive definition
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is easier to use for computation and observing the following property:
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If A and B are commits with generation numbers N and M, respectively,
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and N <= M, then A cannot reach B. That is, we know without searching
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that B is not an ancestor of A because it is further from a root commit
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than A.
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Conversely, when checking if A is an ancestor of B, then we only need
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to walk commits until all commits on the walk boundary have generation
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number at most N. If we walk commits using a priority queue seeded by
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generation numbers, then we always expand the boundary commit with highest
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generation number and can easily detect the stopping condition.
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This property can be used to significantly reduce the time it takes to
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walk commits and determine topological relationships. Without generation
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numbers, the general heuristic is the following:
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If A and B are commits with commit time X and Y, respectively, and
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X < Y, then A _probably_ cannot reach B.
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This heuristic is currently used whenever the computation is allowed to
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violate topological relationships due to clock skew (such as "git log"
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with default order), but is not used when the topological order is
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required (such as merge base calculations, "git log --graph").
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In practice, we expect some commits to be created recently and not stored
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in the commit graph. We can treat these commits as having "infinite"
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generation number and walk until reaching commits with known generation
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number.
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We use the macro GENERATION_NUMBER_INFINITY = 0xFFFFFFFF to mark commits not
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in the commit-graph file. If a commit-graph file was written by a version
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of Git that did not compute generation numbers, then those commits will
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have generation number represented by the macro GENERATION_NUMBER_ZERO = 0.
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Since the commit-graph file is closed under reachability, we can guarantee
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the following weaker condition on all commits:
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If A and B are commits with generation numbers N amd M, respectively,
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and N < M, then A cannot reach B.
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Note how the strict inequality differs from the inequality when we have
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fully-computed generation numbers. Using strict inequality may result in
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walking a few extra commits, but the simplicity in dealing with commits
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with generation number *_INFINITY or *_ZERO is valuable.
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We use the macro GENERATION_NUMBER_MAX = 0x3FFFFFFF to for commits whose
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generation numbers are computed to be at least this value. We limit at
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this value since it is the largest value that can be stored in the
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commit-graph file using the 30 bits available to generation numbers. This
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presents another case where a commit can have generation number equal to
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that of a parent.
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Design Details
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--------------
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- The commit-graph file is stored in a file named 'commit-graph' in the
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.git/objects/info directory. This could be stored in the info directory
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of an alternate.
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- The core.commitGraph config setting must be on to consume graph files.
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- The file format includes parameters for the object ID hash function,
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so a future change of hash algorithm does not require a change in format.
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- Commit grafts and replace objects can change the shape of the commit
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history. The latter can also be enabled/disabled on the fly using
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`--no-replace-objects`. This leads to difficultly storing both possible
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interpretations of a commit id, especially when computing generation
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numbers. The commit-graph will not be read or written when
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replace-objects or grafts are present.
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- Shallow clones create grafts of commits by dropping their parents. This
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leads the commit-graph to think those commits have generation number 1.
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If and when those commits are made unshallow, those generation numbers
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become invalid. Since shallow clones are intended to restrict the commit
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history to a very small set of commits, the commit-graph feature is less
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helpful for these clones, anyway. The commit-graph will not be read or
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written when shallow commits are present.
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Commit Graphs Chains
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--------------------
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Typically, repos grow with near-constant velocity (commits per day). Over time,
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the number of commits added by a fetch operation is much smaller than the
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number of commits in the full history. By creating a "chain" of commit-graphs,
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we enable fast writes of new commit data without rewriting the entire commit
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history -- at least, most of the time.
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## File Layout
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A commit-graph chain uses multiple files, and we use a fixed naming convention
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to organize these files. Each commit-graph file has a name
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`$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-
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valued hash stored in the footer of that file (which is a hash of the file's
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contents before that hash). For a chain of commit-graph files, a plain-text
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file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the
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hashes for the files in order from "lowest" to "highest".
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For example, if the `commit-graph-chain` file contains the lines
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```
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{hash0}
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{hash1}
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{hash2}
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```
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then the commit-graph chain looks like the following diagram:
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+-----------------------+
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| graph-{hash2}.graph |
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+-----------------------+
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+-----------------------+
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| graph-{hash1}.graph |
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+-----------------------+
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+-----------------------+
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| graph-{hash0}.graph |
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+-----------------------+
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Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of
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commits in `graph-{hash1}.graph`, and X2 be the number of commits in
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`graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,
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then we interpret this as being the commit in position (X0 + X1 + i), and that
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will be used as its "graph position". The commits in `graph-{hash2}.graph` use these
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positions to refer to their parents, which may be in `graph-{hash1}.graph` or
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`graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking
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its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +
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X2).
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Each commit-graph file (except the base, `graph-{hash0}.graph`) contains data
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specifying the hashes of all files in the lower layers. In the above example,
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`graph-{hash1}.graph` contains `{hash0}` while `graph-{hash2}.graph` contains
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`{hash0}` and `{hash1}`.
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## Merging commit-graph files
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If we only added a new commit-graph file on every write, we would run into a
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linear search problem through many commit-graph files. Instead, we use a merge
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strategy to decide when the stack should collapse some number of levels.
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The diagram below shows such a collapse. As a set of new commits are added, it
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is determined by the merge strategy that the files should collapse to
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`graph-{hash1}`. Thus, the new commits, the commits in `graph-{hash2}` and
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the commits in `graph-{hash1}` should be combined into a new `graph-{hash3}`
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file.
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+---------------------+
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| (new commits) |
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+---------------------+
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+-----------------------+ +---------------------+
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| graph-{hash2} |->| |
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+-----------------------+ +---------------------+
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+-----------------------+ +---------------------+
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| graph-{hash1} |->| |
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+-----------------------+ +---------------------+
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| tmp_graphXXX
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+-----------------------+
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| graph-{hash0} |
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+-----------------------+
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During this process, the commits to write are combined, sorted and we write the
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contents to a temporary file, all while holding a `commit-graph-chain.lock`
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lock-file. When the file is flushed, we rename it to `graph-{hash3}`
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according to the computed `{hash3}`. Finally, we write the new chain data to
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`commit-graph-chain.lock`:
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```
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{hash3}
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{hash0}
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```
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We then close the lock-file.
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## Merge Strategy
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When writing a set of commits that do not exist in the commit-graph stack of
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height N, we default to creating a new file at level N + 1. We then decide to
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merge with the Nth level if one of two conditions hold:
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1. `--size-multiple=<X>` is specified or X = 2, and the number of commits in
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level N is less than X times the number of commits in level N + 1.
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2. `--max-commits=<C>` is specified with non-zero C and the number of commits
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in level N + 1 is more than C commits.
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This decision cascades down the levels: when we merge a level we create a new
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set of commits that then compares to the next level.
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The first condition bounds the number of levels to be logarithmic in the total
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number of commits. The second condition bounds the total number of commits in
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a `graph-{hashN}` file and not in the `commit-graph` file, preventing
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significant performance issues when the stack merges and another process only
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partially reads the previous stack.
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The merge strategy values (2 for the size multiple, 64,000 for the maximum
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number of commits) could be extracted into config settings for full
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flexibility.
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## Deleting graph-{hash} files
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After a new tip file is written, some `graph-{hash}` files may no longer
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be part of a chain. It is important to remove these files from disk, eventually.
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The main reason to delay removal is that another process could read the
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`commit-graph-chain` file before it is rewritten, but then look for the
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`graph-{hash}` files after they are deleted.
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To allow holding old split commit-graphs for a while after they are unreferenced,
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we update the modified times of the files when they become unreferenced. Then,
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we scan the `$OBJDIR/info/commit-graphs/` directory for `graph-{hash}`
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files whose modified times are older than a given expiry window. This window
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defaults to zero, but can be changed using command-line arguments or a config
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setting.
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## Chains across multiple object directories
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In a repo with alternates, we look for the `commit-graph-chain` file starting
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in the local object directory and then in each alternate. The first file that
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exists defines our chain. As we look for the `graph-{hash}` files for
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each `{hash}` in the chain file, we follow the same pattern for the host
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directories.
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This allows commit-graphs to be split across multiple forks in a fork network.
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The typical case is a large "base" repo with many smaller forks.
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As the base repo advances, it will likely update and merge its commit-graph
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chain more frequently than the forks. If a fork updates their commit-graph after
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the base repo, then it should "reparent" the commit-graph chain onto the new
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chain in the base repo. When reading each `graph-{hash}` file, we track
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the object directory containing it. During a write of a new commit-graph file,
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we check for any changes in the source object directory and read the
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`commit-graph-chain` file for that source and create a new file based on those
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files. During this "reparent" operation, we necessarily need to collapse all
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levels in the fork, as all of the files are invalid against the new base file.
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It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
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files in this scenario. It falls to the user to define the proper settings for
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their custom environment:
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1. When merging levels in the base repo, the unreferenced files may still be
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referenced by chains from fork repos.
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2. The expiry time should be set to a length of time such that every fork has
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time to recompute their commit-graph chain to "reparent" onto the new base
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file(s).
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3. If the commit-graph chain is updated in the base, the fork will not have
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access to the new chain until its chain is updated to reference those files.
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(This may change in the future [5].)
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Related Links
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-------------
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[0] https://bugs.chromium.org/p/git/issues/detail?id=8
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Chromium work item for: Serialized Commit Graph
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[1] https://public-inbox.org/git/20110713070517.GC18566@sigill.intra.peff.net/
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An abandoned patch that introduced generation numbers.
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[2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@sigill.intra.peff.net/
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Discussion about generation numbers on commits and how they interact
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with fsck.
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[3] https://public-inbox.org/git/20170908034739.4op3w4f2ma5s65ku@sigill.intra.peff.net/
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More discussion about generation numbers and not storing them inside
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commit objects. A valuable quote:
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"I think we should be moving more in the direction of keeping
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repo-local caches for optimizations. Reachability bitmaps have been
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a big performance win. I think we should be doing the same with our
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properties of commits. Not just generation numbers, but making it
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cheap to access the graph structure without zlib-inflating whole
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commit objects (i.e., packv4 or something like the "metapacks" I
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proposed a few years ago)."
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[4] https://public-inbox.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u
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A patch to remove the ahead-behind calculation from 'status'.
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[5] https://public-inbox.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/
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A discussion of a "two-dimensional graph position" that can allow reading
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multiple commit-graph chains at the same time.
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