From: Kristofer Karlsson via GitGitGadget Date: Tue, 06 Oct 2026 09:46:30 GMT Subject: [PATCH v2 0/2] fetch: write commit-graph using updated refs only Message-ID: In-Reply-To: When fetch.writeCommitGraph is enabled, the commit-graph is rebuilt from all reachable refs after every fetch. This is unnecessarily expensive on repositories with many refs, since add_ref_to_set() validates each ref against the odb. This series optimizes the commit-graph write by using only the newly updated refs as seeds instead of scanning all refs. Since fetch writes the commit-graph in split mode, the newly fetched history is added as a new layer on top of the existing chain. A three-mode enum (REACHABLE / TIPS / SKIP) makes the policy explicit: * No-op fetch: skip the commit-graph write entirely * Updated refs + existing graph: write incrementally from updated tips only * No existing graph or multi-remote fetch: fall back to full reachable scan Patch 1 adds a commit-info subcommand to test-tool read-graph for verifying graph contents in tests. Patch 2 implements the optimization in builtin/fetch.c with tests covering the incremental, unrelated-commit, no-op, fallback, and shallow-rejected cases. Benchmark 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. Times 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 Changes since v1: * Explain in the commit message that fetch writes in split mode, so the new tips are added as a new layer on top of the existing chain, and that the incremental path relies on this (a non-split write would replace the graph with only the closure of the seeds). * Extend the incremental test to check that a local-only commit, which was in the graph before the fetch but is not reachable from the fetched tips, is still in the graph afterwards. * Add benchmark numbers to the commit message. * Add a comment explaining why shallow-rejected refs are skipped when collecting the updated tips (like store_updated_refs(), since their history is incomplete), and mention it in the commit message. * Add a test in t5537 for a fetch with fetch.writeCommitGraph where a ref is rejected because it would require changes to .git/shallow. Without the check, the commit-graph write dies on the missing parent. Kristofer Karlsson (2): test-tool read-graph: add commit-info subcommand fetch: write commit-graph using updated refs only builtin/fetch.c | 69 +++++++++++++++++++++++++++++++++----- commit-graph.c | 2 +- commit-graph.h | 1 + t/helper/test-read-graph.c | 23 ++++++++++++- t/t5510-fetch.sh | 59 ++++++++++++++++++++++++++++++++ t/t5537-fetch-shallow.sh | 28 ++++++++++++++++ 6 files changed, 171 insertions(+), 11 deletions(-) base-commit: 0f8e75abebff0877cae681a3d5ff31ac47f54220 Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2239%2Fspkrka%2Fkrka%2Fincremental-commit-graph-v2 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2239/spkrka/krka/incremental-commit-graph-v2 Pull-Request: https://github.com/gitgitgadget/git/pull/2239 Range-diff vs v1: 1: 36cc3ff3b3 = 1: 36cc3ff3b3 test-tool read-graph: add commit-info subcommand 2: fee92f3c20 ! 2: 7507354cc9 fetch: write commit-graph using updated refs only @@ Commit message Since do_fetch() already knows which refs were updated, collect them into an oidset and then pass them directly to write_commit_graph(). - In split mode, close_reachable() walks from the updated tips and - stops at commits already present in the graph, efficiently adding - the newly fetched history. This reachability closure also covers - auto-followed tags, since their targets are reachable from the - fetched tips that caused them to be auto-followed. + fetch always writes the commit-graph in split mode, so this adds a + new layer on top of the existing chain rather than replacing it: + close_reachable() walks from the updated tips and stops at commits + already present in the graph, so the new layer only contains the + newly fetched history, and commits covered by the existing layers + remain covered. This relies on split mode; a non-split write would + replace the graph with just the closure of the seeds. + + The reachability closure also covers auto-followed tags, since their + targets are reachable from the fetched tips that caused them to be + auto-followed. + + Refs that are rejected because they would require changes to + .git/shallow are skipped, just like store_updated_refs() does. Their + objects are received but their history is incomplete, so walking from + them would make the commit-graph write fail. After fetch_one() returns, call prepare_commit_graph() (which is made non-static by this commit) to determine the graph-write mode: @@ Commit message 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 ## builtin/fetch.c ## @@ builtin/fetch.c: out: + struct ref *rm; + for (rm = ref_map; rm; rm = rm->next) { + struct commit *commit; ++ /* ++ * Like store_updated_refs(), skip shallow-rejected refs: ++ * they are not stored, and their history is incomplete. ++ */ + if (rm->status == REF_STATUS_REJECT_SHALLOW) + continue; + if (is_null_oid(&rm->old_oid)) @@ t/t5510-fetch.sh: test_expect_success 'fetch.writeCommitGraph' ' + git init incremental-source && + test_commit -C incremental-source one && + git clone incremental-source incremental-dest && ++ test_commit -C incremental-dest local && + git -C incremental-dest commit-graph write --reachable --split && + test_commit -C incremental-source two && + test_commit -C incremental-source three && + ( + cd incremental-dest && + git -c fetch.writeCommitGraph=true fetch origin && -+ test-tool read-graph commit-info three two ++ test-tool read-graph commit-info three two local + ) +' + @@ t/t5510-fetch.sh: test_expect_success 'fetch.writeCommitGraph' ' test_expect_success 'fetch.writeCommitGraph with submodules' ' test_config_global protocol.file.allow always && git clone dups super && + + ## t/t5537-fetch-shallow.sh ## +@@ t/t5537-fetch-shallow.sh: test_expect_success 'fetch that requires changes in .git/shallow is filtered' ' + ) + ' + ++test_expect_success 'fetch.writeCommitGraph skips refs that require changes in .git/shallow' ' ++ git clone --no-local --depth=2 .git shallow-graph && ++ ( ++ cd shallow-graph && ++ git checkout --orphan no-shallow && ++ commit no-shallow ++ ) && ++ git init notshallow-graph && ++ git -C notshallow-graph -c fetch.writeCommitGraph=true \ ++ fetch ../shallow-graph/.git "refs/heads/*:refs/remotes/shallow/*" && ++ ( ++ cd shallow-graph && ++ commit no-shallow-2 ++ ) && ++ rejected=$(git -C shallow-graph rev-parse main) && ++ ( ++ cd notshallow-graph && ++ git -c fetch.writeCommitGraph=true \ ++ fetch ../shallow-graph/.git "refs/heads/*:refs/remotes/shallow/*" && ++ git for-each-ref --format="%(refname)" >actual.refs && ++ echo refs/remotes/shallow/no-shallow >expect.refs && ++ test_cmp expect.refs actual.refs && ++ test-tool read-graph commit-info shallow/no-shallow && ++ test_expect_code 1 \ ++ test-tool read-graph commit-info $rejected 2>/dev/null ++ ) ++' ++ + test_expect_success 'fetch --update-shallow' ' + ( + cd shallow && -- gitgitgadget