When fetch.writeCommitGraph was introduced in 50f26bd035 (fetch: add
fetch.writeCommitGraph config setting, 2019-09-02), the stated goal
was to stay updated with the latest commits after fetching new
objects. The implementation used write_commit_graph_reachable()
because it was the only API available, but two things have changed
since then:
1. write_commit_graph() was added, and it accepts an explicit set of
commits as seeds, enabling more targeted commit-graph updates.
2. The ref-scanning callback add_ref_to_set() became more expensive
in 630cd5194e (commit-graph.c: peel refs in 'add_ref_to_set',
2020-07-22) when it started to validate the refs against the odb
for correctness. On a repository with many refs, this makes the
full reachable scan unnecessarily costly for a targeted fetch.
Optimize the commit-graph write by using only the newly updated refs
as seeds instead of scanning all refs after every fetch. To keep
this change small, skip the optimization for multi-remote fetches
(since that would require propagating the set of refs across process
boundaries).
This relies on the commit-graph write being additive, keeping the
commits that are already in the graph. fetch already operates in
this mode (COMMIT_GRAPH_WRITE_SPLIT) and now that becomes
required for correctness. Without that mode, the write would
replace the commit-graph and lose other commits.
After fetch_one() returns, call prepare_commit_graph() (which is
made non-static by this commit) to determine the graph-write mode:
- If no commit-graph exists yet, fall back to the full reachable
scan so the first graph creation covers all refs.
- If a commit-graph exists and the fetch updated at least one ref,
write incrementally using only the new refs as seeds.
- If a commit-graph exists but the fetch is a no-op, skip the
commit-graph write entirely.
- For the multi-remote path (fetch --all), where child processes
do the actual fetching, fall back to the full reachable scan.
Full commit-graph coverage of all refs remains the responsibility
of "git maintenance", "git gc" and "git commit-graph write".
Regular Git operations may trigger "git maintenance run --auto",
which periodically rebuilds the commit-graph from all reachable
refs.
The effect was measured on a synthetic setup: git.git with 200K
extra packed refs (~206K total), a local file:// remote, an existing
split commit-graph and a warm page cache. The times below are the
median of 9 runs of the trace2 region fetch/write-commit-graph:
scenario before after
no-op fetch 380 ms (skipped)
1 ref updated 357 ms 9.3 ms
10 refs updated 359 ms 8.9 ms
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Kristofer Karlsson
cef2d8c4cb