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>

diff --git a/commit-graph.c b/commit-graph.c
index 8c4c7262c892531815f6f97c4abd11117e751586..00614acd65d8417792e4f3060b87113f2699347e 100644 (file)
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -891,6 +891,30 @@ static int find_commit_pos_in_graph(struct commit *item, struct commit_graph *g,
        }
 }
 
+struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
+{
+       struct commit *commit;
+       uint32_t pos;
+
+       if (!repo->objects->commit_graph)
+               return NULL;
+       if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
+               return NULL;
+       if (!repo_has_object_file(repo, id))
+               return NULL;
+
+       commit = lookup_commit(repo, id);
+       if (!commit)
+               return NULL;
+       if (commit->object.parsed)
+               return commit;
+
+       if (!fill_commit_in_graph(repo, commit, repo->objects->commit_graph, pos))
+               return NULL;
+
+       return commit;
+}
+
 static int parse_commit_in_graph_one(struct repository *r,
                                     struct commit_graph *g,
                                     struct commit *item)

diff --git a/commit-graph.h b/commit-graph.h
index 96c24fb5777de48ccb5d5144543b3c2ff6f7efae..04a94e18302d8d2e9b91933497cfb868a3cf3c12 100644 (file)
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -40,6 +40,14 @@ int open_commit_graph(const char *graph_file, int *fd, struct stat *st);
  */
 int parse_commit_in_graph(struct repository *r, struct commit *item);
 
+/*
+ * Look up the given commit ID in the commit-graph. This will only return a
+ * commit if the ID exists both in the graph and in the object database such
+ * that we don't return commits whose object has been pruned. Otherwise, this
+ * function returns `NULL`.
+ */
+struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id);
+
 /*
  * It is possible that we loaded commit contents from the commit buffer,
  * but we also want to ensure the commit-graph content is correctly

diff --git a/revision.c b/revision.c
index 80a59896b96ddd5d37bc6abb70cc5c44e2dd0209..0dabb5a0bcfe929124677a34d0fd94831d346c57 100644 (file)
--- a/revision.c
+++ b/revision.c
@@ -360,20 +360,18 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
                                    unsigned int flags)
 {
        struct object *object;
+       struct commit *commit;
 
        /*
-        * If the repository has commit graphs, repo_parse_commit() avoids
-        * reading the object buffer, so use it whenever possible.
+        * If the repository has commit graphs, we try to opportunistically
+        * look up the object ID in those graphs. Like this, we can avoid
+        * parsing commit data from disk.
         */
-       if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
-               struct commit *c = lookup_commit(revs->repo, oid);
-               if (!repo_parse_commit(revs->repo, c))
-                       object = (struct object *) c;
-               else
-                       object = NULL;
-       } else {
+       commit = lookup_commit_in_graph(revs->repo, oid);
+       if (commit)
+               object = &commit->object;
+       else
                object = parse_object(revs->repo, oid);
-       }
 
        if (!object) {
                if (revs->ignore_missing)