402 lines
18 KiB
Plaintext
402 lines
18 KiB
Plaintext
Git Commit-Graph Design Notes
|
|
=============================
|
|
|
|
Git walks the commit graph for many reasons, including:
|
|
|
|
1. Listing and filtering commit history.
|
|
2. Computing merge bases.
|
|
|
|
These operations can become slow as the commit count grows. The merge
|
|
base calculation shows up in many user-facing commands, such as 'merge-base'
|
|
or 'status' and can take minutes to compute depending on history shape.
|
|
|
|
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
|
|
commit graph walks. If a user downgrades or disables the 'core.commitGraph'
|
|
config setting, then the existing object database 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
|
|
extra metadata to speed up graph walks. By listing commit OIDs in
|
|
lexicographic order, we can identify an integer position for each commit
|
|
and refer to the parents of a commit using those integer positions. We
|
|
use binary search to find initial commits and then use the integer
|
|
positions for fast lookups during the walk.
|
|
|
|
A consumer may load the following info for a commit from the graph:
|
|
|
|
1. The commit OID.
|
|
2. The list of parents, along with their integer position.
|
|
3. The commit date.
|
|
4. The root tree OID.
|
|
5. The generation number (see definition below).
|
|
|
|
Values 1-4 satisfy the requirements of parse_commit_gently().
|
|
|
|
There are two definitions of generation number:
|
|
1. Corrected committer dates (generation number v2)
|
|
2. Topological levels (generation number v1)
|
|
|
|
Define "corrected committer date" of a commit recursively as follows:
|
|
|
|
* A commit with no parents (a root commit) has corrected committer date
|
|
equal to its committer date.
|
|
|
|
* A commit with at least one parent has corrected committer date equal to
|
|
the maximum of its committer date and one more than the largest corrected
|
|
committer date among its parents.
|
|
|
|
* As a special case, a root commit with timestamp zero has corrected commit
|
|
date of 1, to be able to distinguish it from GENERATION_NUMBER_ZERO
|
|
(that is, an uncomputed corrected commit date).
|
|
|
|
Define the "topological level" of a commit recursively as follows:
|
|
|
|
* A commit with no parents (a root commit) has topological level of one.
|
|
|
|
* A commit with at least one parent has topological level one more than
|
|
the largest topological level among its parents.
|
|
|
|
Equivalently, the topological level of a commit A is one more than the
|
|
length of a longest path from A to a root commit. The recursive definition
|
|
is easier to use for computation and observing the following property:
|
|
|
|
If A and B are commits with generation numbers N and M, respectively,
|
|
and N <= M, then A cannot reach B. That is, we know without searching
|
|
that B is not an ancestor of A because it is further from a root commit
|
|
than A.
|
|
|
|
Conversely, when checking if A is an ancestor of B, then we only need
|
|
to walk commits until all commits on the walk boundary have generation
|
|
number at most N. If we walk commits using a priority queue seeded by
|
|
generation numbers, then we always expand the boundary commit with highest
|
|
generation number and can easily detect the stopping condition.
|
|
|
|
The property applies to both versions of generation number, that is both
|
|
corrected committer dates and topological levels.
|
|
|
|
This property can be used to significantly reduce the time it takes to
|
|
walk commits and determine topological relationships. Without generation
|
|
numbers, the general heuristic is the following:
|
|
|
|
If A and B are commits with commit time X and Y, respectively, and
|
|
X < Y, then A _probably_ cannot reach B.
|
|
|
|
In absence of corrected commit dates (for example, old versions of Git or
|
|
mixed generation graph chains),
|
|
this heuristic is currently used whenever the computation is allowed to
|
|
violate topological relationships due to clock skew (such as "git log"
|
|
with default order), but is not used when the topological order is
|
|
required (such as merge base calculations, "git log --graph").
|
|
|
|
In practice, we expect some commits to be created recently and not stored
|
|
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 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 and 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_V1_MAX = 0x3FFFFFFF for commits whose
|
|
topological levels (generation number v1) 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 topological levels. 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
|
|
.git/objects/info directory. This could be stored in the info directory
|
|
of an alternate.
|
|
|
|
- The core.commitGraph config setting must be on to consume graph files.
|
|
|
|
- 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.
|
|
|
|
Commit-Graphs Chains
|
|
--------------------
|
|
|
|
Typically, repos grow with near-constant velocity (commits per day). Over time,
|
|
the number of commits added by a fetch operation is much smaller than the
|
|
number of commits in the full history. By creating a "chain" of commit-graphs,
|
|
we enable fast writes of new commit data without rewriting the entire commit
|
|
history -- at least, most of the time.
|
|
|
|
## File Layout
|
|
|
|
A commit-graph chain uses multiple files, and we use a fixed naming convention
|
|
to organize these files. Each commit-graph file has a name
|
|
`$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-
|
|
valued hash stored in the footer of that file (which is a hash of the file's
|
|
contents before that hash). For a chain of commit-graph files, a plain-text
|
|
file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the
|
|
hashes for the files in order from "lowest" to "highest".
|
|
|
|
For example, if the `commit-graph-chain` file contains the lines
|
|
|
|
```
|
|
{hash0}
|
|
{hash1}
|
|
{hash2}
|
|
```
|
|
|
|
then the commit-graph chain looks like the following diagram:
|
|
|
|
+-----------------------+
|
|
| graph-{hash2}.graph |
|
|
+-----------------------+
|
|
|
|
|
+-----------------------+
|
|
| |
|
|
| graph-{hash1}.graph |
|
|
| |
|
|
+-----------------------+
|
|
|
|
|
+-----------------------+
|
|
| |
|
|
| |
|
|
| |
|
|
| graph-{hash0}.graph |
|
|
| |
|
|
| |
|
|
| |
|
|
+-----------------------+
|
|
|
|
Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of
|
|
commits in `graph-{hash1}.graph`, and X2 be the number of commits in
|
|
`graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,
|
|
then we interpret this as being the commit in position (X0 + X1 + i), and that
|
|
will be used as its "graph position". The commits in `graph-{hash2}.graph` use these
|
|
positions to refer to their parents, which may be in `graph-{hash1}.graph` or
|
|
`graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking
|
|
its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +
|
|
X2).
|
|
|
|
Each commit-graph file (except the base, `graph-{hash0}.graph`) contains data
|
|
specifying the hashes of all files in the lower layers. In the above example,
|
|
`graph-{hash1}.graph` contains `{hash0}` while `graph-{hash2}.graph` contains
|
|
`{hash0}` and `{hash1}`.
|
|
|
|
## Merging commit-graph files
|
|
|
|
If we only added a new commit-graph file on every write, we would run into a
|
|
linear search problem through many commit-graph files. Instead, we use a merge
|
|
strategy to decide when the stack should collapse some number of levels.
|
|
|
|
The diagram below shows such a collapse. As a set of new commits are added, it
|
|
is determined by the merge strategy that the files should collapse to
|
|
`graph-{hash1}`. Thus, the new commits, the commits in `graph-{hash2}` and
|
|
the commits in `graph-{hash1}` should be combined into a new `graph-{hash3}`
|
|
file.
|
|
|
|
+---------------------+
|
|
| |
|
|
| (new commits) |
|
|
| |
|
|
+---------------------+
|
|
| |
|
|
+-----------------------+ +---------------------+
|
|
| graph-{hash2} |->| |
|
|
+-----------------------+ +---------------------+
|
|
| | |
|
|
+-----------------------+ +---------------------+
|
|
| | | |
|
|
| graph-{hash1} |->| |
|
|
| | | |
|
|
+-----------------------+ +---------------------+
|
|
| tmp_graphXXX
|
|
+-----------------------+
|
|
| |
|
|
| |
|
|
| |
|
|
| graph-{hash0} |
|
|
| |
|
|
| |
|
|
| |
|
|
+-----------------------+
|
|
|
|
During this process, the commits to write are combined, sorted and we write the
|
|
contents to a temporary file, all while holding a `commit-graph-chain.lock`
|
|
lock-file. When the file is flushed, we rename it to `graph-{hash3}`
|
|
according to the computed `{hash3}`. Finally, we write the new chain data to
|
|
`commit-graph-chain.lock`:
|
|
|
|
```
|
|
{hash3}
|
|
{hash0}
|
|
```
|
|
|
|
We then close the lock-file.
|
|
|
|
## Merge Strategy
|
|
|
|
When writing a set of commits that do not exist in the commit-graph stack of
|
|
height N, we default to creating a new file at level N + 1. We then decide to
|
|
merge with the Nth level if one of two conditions hold:
|
|
|
|
1. `--size-multiple=<X>` is specified or X = 2, and the number of commits in
|
|
level N is less than X times the number of commits in level N + 1.
|
|
|
|
2. `--max-commits=<C>` is specified with non-zero C and the number of commits
|
|
in level N + 1 is more than C commits.
|
|
|
|
This decision cascades down the levels: when we merge a level we create a new
|
|
set of commits that then compares to the next level.
|
|
|
|
The first condition bounds the number of levels to be logarithmic in the total
|
|
number of commits. The second condition bounds the total number of commits in
|
|
a `graph-{hashN}` file and not in the `commit-graph` file, preventing
|
|
significant performance issues when the stack merges and another process only
|
|
partially reads the previous stack.
|
|
|
|
The merge strategy values (2 for the size multiple, 64,000 for the maximum
|
|
number of commits) could be extracted into config settings for full
|
|
flexibility.
|
|
|
|
## Handling Mixed Generation Number Chains
|
|
|
|
With the introduction of generation number v2 and generation data chunk, the
|
|
following scenario is possible:
|
|
|
|
1. "New" Git writes a commit-graph with the corrected commit dates.
|
|
2. "Old" Git writes a split commit-graph on top without corrected commit dates.
|
|
|
|
A naive approach of using the newest available generation number from
|
|
each layer would lead to violated expectations: the lower layer would
|
|
use corrected commit dates which are much larger than the topological
|
|
levels of the higher layer. For this reason, Git inspects the topmost
|
|
layer to see if the layer is missing corrected commit dates. In such a case
|
|
Git only uses topological level for generation numbers.
|
|
|
|
When writing a new layer in split commit-graph, we write corrected commit
|
|
dates if the topmost layer has corrected commit dates written. This
|
|
guarantees that if a layer has corrected commit dates, all lower layers
|
|
must have corrected commit dates as well.
|
|
|
|
When merging layers, we do not consider whether the merged layers had corrected
|
|
commit dates. Instead, the new layer will have corrected commit dates if the
|
|
layer below the new layer has corrected commit dates.
|
|
|
|
While writing or merging layers, if the new layer is the only layer, it will
|
|
have corrected commit dates when written by compatible versions of Git. Thus,
|
|
rewriting split commit-graph as a single file (`--split=replace`) creates a
|
|
single layer with corrected commit dates.
|
|
|
|
## Deleting graph-{hash} files
|
|
|
|
After a new tip file is written, some `graph-{hash}` files may no longer
|
|
be part of a chain. It is important to remove these files from disk, eventually.
|
|
The main reason to delay removal is that another process could read the
|
|
`commit-graph-chain` file before it is rewritten, but then look for the
|
|
`graph-{hash}` files after they are deleted.
|
|
|
|
To allow holding old split commit-graphs for a while after they are unreferenced,
|
|
we update the modified times of the files when they become unreferenced. Then,
|
|
we scan the `$OBJDIR/info/commit-graphs/` directory for `graph-{hash}`
|
|
files whose modified times are older than a given expiry window. This window
|
|
defaults to zero, but can be changed using command-line arguments or a config
|
|
setting.
|
|
|
|
## Chains across multiple object directories
|
|
|
|
In a repo with alternates, we look for the `commit-graph-chain` file starting
|
|
in the local object directory and then in each alternate. The first file that
|
|
exists defines our chain. As we look for the `graph-{hash}` files for
|
|
each `{hash}` in the chain file, we follow the same pattern for the host
|
|
directories.
|
|
|
|
This allows commit-graphs to be split across multiple forks in a fork network.
|
|
The typical case is a large "base" repo with many smaller forks.
|
|
|
|
As the base repo advances, it will likely update and merge its commit-graph
|
|
chain more frequently than the forks. If a fork updates their commit-graph after
|
|
the base repo, then it should "reparent" the commit-graph chain onto the new
|
|
chain in the base repo. When reading each `graph-{hash}` file, we track
|
|
the object directory containing it. During a write of a new commit-graph file,
|
|
we check for any changes in the source object directory and read the
|
|
`commit-graph-chain` file for that source and create a new file based on those
|
|
files. During this "reparent" operation, we necessarily need to collapse all
|
|
levels in the fork, as all of the files are invalid against the new base file.
|
|
|
|
It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
|
|
files in this scenario. It falls to the user to define the proper settings for
|
|
their custom environment:
|
|
|
|
1. When merging levels in the base repo, the unreferenced files may still be
|
|
referenced by chains from fork repos.
|
|
|
|
2. The expiry time should be set to a length of time such that every fork has
|
|
time to recompute their commit-graph chain to "reparent" onto the new base
|
|
file(s).
|
|
|
|
3. If the commit-graph chain is updated in the base, the fork will not have
|
|
access to the new chain until its chain is updated to reference those files.
|
|
(This may change in the future [5].)
|
|
|
|
Related Links
|
|
-------------
|
|
[0] https://bugs.chromium.org/p/git/issues/detail?id=8
|
|
Chromium work item for: Serialized Commit Graph
|
|
|
|
[1] https://lore.kernel.org/git/20110713070517.GC18566@sigill.intra.peff.net/
|
|
An abandoned patch that introduced generation numbers.
|
|
|
|
[2] https://lore.kernel.org/git/20170908033403.q7e6dj7benasrjes@sigill.intra.peff.net/
|
|
Discussion about generation numbers on commits and how they interact
|
|
with fsck.
|
|
|
|
[3] https://lore.kernel.org/git/20170908034739.4op3w4f2ma5s65ku@sigill.intra.peff.net/
|
|
More discussion about generation numbers and not storing them inside
|
|
commit objects. A valuable quote:
|
|
|
|
"I think we should be moving more in the direction of keeping
|
|
repo-local caches for optimizations. Reachability bitmaps have been
|
|
a big performance win. I think we should be doing the same with our
|
|
properties of commits. Not just generation numbers, but making it
|
|
cheap to access the graph structure without zlib-inflating whole
|
|
commit objects (i.e., packv4 or something like the "metapacks" I
|
|
proposed a few years ago)."
|
|
|
|
[4] https://lore.kernel.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u
|
|
A patch to remove the ahead-behind calculation from 'status'.
|
|
|
|
[5] https://lore.kernel.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/
|
|
A discussion of a "two-dimensional graph position" that can allow reading
|
|
multiple commit-graph chains at the same time.
|