On Tue, Jan 30, 2018 at 1:39 PM, Derrick Stolee <stolee@xxxxxxxxx> wrote: > Add Documentation/technical/commit-graph.txt with details of the planned > commit graph feature, including future plans. > > Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx> > --- > Documentation/technical/commit-graph.txt | 189 +++++++++++++++++++++++++++++++ > 1 file changed, 189 insertions(+) > create mode 100644 Documentation/technical/commit-graph.txt > > diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt > new file mode 100644 > index 0000000000..cbf88f7264 > --- /dev/null > +++ b/Documentation/technical/commit-graph.txt > @@ -0,0 +1,189 @@ > +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 'git show --remerge-diff' and can take minutes to compute depending on > +history shape. Sorry for appearing more authoritative than I am here. The --remerge-diff flag is just floating around the mailing list, and was never merged. (It is such a cool feature though, but it would actually confuse users looking for it, not finding it) > +There are two main costs here: > + > +1. Decompressing and parsing commits. > +2. Walking the entire graph to avoid topological order mistakes. > + > +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 ODB is sufficient. The file is stored > +next to packfiles either in the .git/objects/pack directory or in the pack > +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 lexi- > +cographic 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(). > + > +Define the "generation number" of a commit recursively as follows: > + > + * A commit with no parents (a root commit) has generation number one. > + > + * A commit with at least one parent has generation number one more than > + the largest generation number among its parents. > + > +Equivalently, the generation number 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. > + > +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. > + > +This heuristic is currently used whenever the computation can make > +mistakes with topological orders (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. > + > +Design Details > +-------------- > + > +- A graph file is stored in a file named 'graph-<hash>.graph' in the pack > + directory. This could be stored in an alternate. > + > +- The most-recent graph file hash is stored in a 'graph-head' file for > + immediate access and storing backup graphs. This could be stored in an > + alternate, and refers to a 'graph-<hash>.graph' file in the same pack > + directory. > + > +- The core.commitgraph config setting must be on to consume graph files. > + > +- The file format includes parameters for the object id length and hash > + algorithm, so a future change of hash algorithm does not require a change > + in format. > + > +Current Limitations > +------------------- > + > +- Only one graph file is used at one time. This allows the integer position > + to seek into the single graph file. It is possible to extend the model > + for multiple graph files, but that is currently not part of the design. > + > +- .graph files are managed only by the 'commit-graph' builtin. These are not > + updated automatically during clone, fetch, repack, or creating new commits. > + > +- There is no '--verify' option for the 'commit-graph' builtin to verify the > + contents of the graph file agree with the contents in the ODB. > + > +- When rewriting the graph, we do not check for a commit still existing > + in the ODB, so garbage collection may remove commits. > + > +- Generation numbers are not computed in the current version. The file > + format supports storing them, along with a mechanism to upgrade from > + a file without generation numbers to one that uses them. > + > +Future Work > +----------- > + > +- 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. > + > +- The commit graph is currently incompatible with commit grafts. This can be > + remedied by duplicating or refactoring the current graft logic. > + > +- 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' > + > +- The graph currently only adds commits to a previously existing graph. > + When writing a new graph, we could check that the ODB still contains > + the commits and choose to remove the commits that are deleted from the > + ODB. For performance reasons, this check should remain optional. > + > +- 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' builtin 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 coulde compute a graph on 'clone', 'fetch', or 'repack' > + commands. > + > +- A server could provide a commit graph file as part of the network protocol > + to avoid extra calculations by clients. > + > +Related Links > +------------- > +[0] https://bugs.chromium.org/p/git/issues/detail?id=8 > + Chromium work item for: Serialized Commit Graph > + > +[1] https://public-inbox.org/git/20110713070517.GC18566@xxxxxxxxxxxxxxxxxxxxx/ > + An abandoned patch that introduced generation numbers. > + > +[2] https://public-inbox.org/git/20170908033403.q7e6dj7benasrjes@xxxxxxxxxxxxxxxxxxxxx/ > + Discussion about generation numbers on commits and how they interact > + with fsck. > + > +[3] https://public-inbox.org/git/20170907094718.b6kuzp2uhvkmwcso@xxxxxxxxxxxxxxxxxxxxx/t/#m7a2ea7b355aeda962e6b86404bcbadc648abfbba > + 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://public-inbox.org/git/20180108154822.54829-1-git@xxxxxxxxxxxxxxxxx/T/#u > + A patch to remove the ahead-behind calculation from 'status'. > -- > 2.16.0.15.g9c3cf44.dirty >
