@matheusbsbs / matheus-bagliano / commits / f559d6d45e

revision: avoid hitting packfiles when commits are in commit-graph

When queueing references in git-rev-list(1), we try to optimize parsing of commits via the commit-graph. To do so, we first look up the object's type, and if it is a commit we call `repo_parse_commit()` instead of `parse_object()`. This is quite inefficient though given that we're always uncompressing the object header in order to determine the type. Instead, we can opportunistically search the commit-graph for the object ID: in case it's found, we know it's a commit and can directly fill in the commit object without having to uncompress the object header. Expose a new function `lookup_commit_in_graph()`, which tries to find a commit in the commit-graph by ID, and convert `get_reference()` to use this function. This provides a big performance win in cases where we load references in a repository with lots of references pointing to commits. The following has been executed in a real-world repository with about 2.2 million refs: Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev Time (mean ± σ): 4.458 s ± 0.044 s [User: 4.115 s, System: 0.342 s] Range (min … max): 4.409 s … 4.534 s 10 runs Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev Time (mean ± σ): 3.089 s ± 0.015 s [User: 2.768 s, System: 0.321 s] Range (min … max): 3.061 s … 3.105 s 10 runs Summary 'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran 1.44 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Patrick Steinhardt committed Aug 9, 2021 at 10:12 UTC f559d6d45e7e58ae1f922213948723de77ea77bd
3 files changed +40 -10
commit-graph.c
+24
@@ -891,6 +891,30 @@ static int find_commit_pos_in_graph(struct commit *item, struct commit_graph *g,
891 }
892 }
893
894 +struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
895 +{
896 + struct commit *commit;
897 + uint32_t pos;
898 +
899 + if (!repo->objects->commit_graph)
900 + return NULL;
901 + if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
902 + return NULL;
903 + if (!repo_has_object_file(repo, id))
904 + return NULL;
905 +
906 + commit = lookup_commit(repo, id);
907 + if (!commit)
908 + return NULL;
909 + if (commit->object.parsed)
910 + return commit;
911 +
912 + if (!fill_commit_in_graph(repo, commit, repo->objects->commit_graph, pos))
913 + return NULL;
914 +
915 + return commit;
916 +}
917 +
918 static int parse_commit_in_graph_one(struct repository *r,
919 struct commit_graph *g,
920 struct commit *item)
commit-graph.h
+8
@@ -40,6 +40,14 @@ int open_commit_graph(const char *graph_file, int *fd, struct stat *st);
40 */
41 int parse_commit_in_graph(struct repository *r, struct commit *item);
42
43 +/*
44 + * Look up the given commit ID in the commit-graph. This will only return a
45 + * commit if the ID exists both in the graph and in the object database such
46 + * that we don't return commits whose object has been pruned. Otherwise, this
47 + * function returns `NULL`.
48 + */
49 +struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id);
50 +
51 /*
52 * It is possible that we loaded commit contents from the commit buffer,
53 * but we also want to ensure the commit-graph content is correctly
revision.c
+8 -10
@@ -360,20 +360,18 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
360 unsigned int flags)
361 {
362 struct object *object;
363 + struct commit *commit;
364
365 /*
365 - * If the repository has commit graphs, repo_parse_commit() avoids
366 - * reading the object buffer, so use it whenever possible.
366 + * If the repository has commit graphs, we try to opportunistically
367 + * look up the object ID in those graphs. Like this, we can avoid
368 + * parsing commit data from disk.
369 */
368 - if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
369 - struct commit *c = lookup_commit(revs->repo, oid);
370 - if (!repo_parse_commit(revs->repo, c))
371 - object = (struct object *) c;
372 - else
373 - object = NULL;
374 - } else {
370 + commit = lookup_commit_in_graph(revs->repo, oid);
371 + if (commit)
372 + object = &commit->object;
373 + else
374 object = parse_object(revs->repo, oid);
376 - }
375
376 if (!object) {
377 if (revs->ignore_missing)