As promised [1], this patch contains a way to serialize the commit graph.
The current implementation defines a new file format to store the graph
structure (parent relationships) and basic commit metadata (commit date,
root tree OID) in order to prevent parsing raw commits while performing
basic graph walks. For example, we do not need to parse the full commit
when performing these walks:
* 'git log --topo-order -1000' walks all reachable commits to avoid
incorrect topological orders, but only needs the commit message for
the top 1000 commits.
* 'git merge-base <A> <B>' may walk many commits to find the correct
boundary between the commits reachable from A and those reachable
from B. No commit messages are needed.
* 'git branch -vv' checks ahead/behind status for all local branches
compared to their upstream remote branches. This is essentially as
hard as computing merge bases for each.
The current patch speeds up these calculations by injecting a check in
parse_commit_gently() to check if there is a graph file and using that
to provide the required metadata to the struct commit.
The file format has room to store generation numbers, which will be
provided as a patch after this framework is merged. Generation numbers
are referenced by the design document but not implemented in order to
make the current patch focus on the graph construction process. Once
that is stable, it will be easier to add generation numbers and make
graph walks aware of generation numbers one-by-one.
Here are some performance results for a copy of the Linux repository
where 'master' has 704,766 reachable commits and is behind 'origin/master'
by 19,610 commits.
| Command | Before | After | Rel % |
|----------------------------------|--------|--------|-------|
| log --oneline --topo-order -1000 | 5.9s | 0.7s | -88% |
| branch -vv | 0.42s | 0.27s | -35% |
| rev-list --all | 6.4s | 1.0s | -84% |
| rev-list --all --objects | 32.6s | 27.6s | -15% |
To test this yourself, run the following on your repo:
git config core.graph true
git show-ref -s | git graph --write --update-head
The second command writes a commit graph file containing every commit
reachable from your refs. Now, all git commands that walk commits will
check your graph first before consulting the ODB. You can run your own
performance comparisions by toggling the 'core.graph' setting.
[1]
https://public-inbox.org/git/d154319e-bb9e-b300-7c37-27b1dcd2a...@jeffhostetler.com/
Re: What's cooking in git.git (Jan 2018, #03; Tue, 23)
[2] https://github.com/derrickstolee/git/pull/2
A GitHub pull request containing the latest version of this patch.
P.S. I'm sending this patch from my gmail address to avoid Outlook
munging the URLs included in the design document.
Derrick Stolee (14):
graph: add packed graph design document
packed-graph: add core.graph setting
packed-graph: create git-graph builtin
packed-graph: add format document
packed-graph: implement construct_graph()
packed-graph: implement git-graph --write
packed-graph: implement git-graph --read
graph: implement git-graph --update-head
packed-graph: implement git-graph --clear
packed-graph: teach git-graph --delete-expired
commit: integrate packed graph with commit parsing
packed-graph: read only from specific pack-indexes
packed-graph: close under reachability
packed-graph: teach git-graph to read commits
Documentation/config.txt | 3 +
Documentation/git-graph.txt | 102 ++++
Documentation/technical/graph-format.txt | 88 ++++
Documentation/technical/packed-graph.txt | 185 +++++++
Makefile | 2 +
alloc.c | 1 +
builtin.h | 1 +
builtin/graph.c | 231 +++++++++
cache.h | 1 +
command-list.txt | 1 +
commit.c | 20 +-
commit.h | 2 +
config.c | 5 +
environment.c | 1 +
git.c | 1 +
log-tree.c | 3 +-
packed-graph.c | 840 +++++++++++++++++++++++++++++++
packed-graph.h | 65 +++
packfile.c | 4 +-
packfile.h | 2 +
t/t5319-graph.sh | 271 ++++++++++
21 files changed, 1822 insertions(+), 7 deletions(-)
create mode 100644 Documentation/git-graph.txt
create mode 100644 Documentation/technical/graph-format.txt
create mode 100644 Documentation/technical/packed-graph.txt
create mode 100644 builtin/graph.c
create mode 100644 packed-graph.c
create mode 100644 packed-graph.h
create mode 100755 t/t5319-graph.sh
--
2.16.0
