[PATCH v3 3/4] revision: avoid loading object headers multiple times

From: Patrick Steinhardt <ps@pks.im>
Date: Aug 2, 2021, 09:38 UTC
Message-ID: <b9897e102afbcab3bfee58ed8bda24257d8b54fb.1627896460.git.ps@pks.im>
In-Reply-To: <cover.1627896460.git.ps@pks.im>

When loading references, we try to optimize loading of commits by using the commit graph. To do so, we first need to determine whether the object actually is a commit or not, which is why we always execute `oid_object_info()` first. Like this, we'll unpack the object header of each object first.

This pattern can be quite inefficient in case many references point to the same commit: if the object didn't end up in the cached objects, then we'll repeatedly unpack the same object header, even if we've already seen the object before.

Optimize this pattern by using `lookup_unknown_object()` first in order to determine whether we've seen the object before. If so, then we don't need to re-parse the header but can directly use its object information and thus gain a modest performance improvement. Executed in a real-world repository with around 2.2 million references:

    Benchmark #1: HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.771 s ±  0.238 s    [User: 4.440 s, System: 0.330 s]
      Range (min … max):    4.539 s …  5.219 s    10 runs

    Benchmark #2: HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev
      Time (mean ± σ):      4.454 s ±  0.037 s    [User: 4.122 s, System: 0.332 s]
      Range (min … max):    4.375 s …  4.496 s    10 runs

    Summary
      'HEAD: rev-list --unsorted-input --objects --quiet --not --all --not $newrev' ran
        1.07 ± 0.05 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'

The downside is that `lookup_unknown_object()` is forced to always allocate an object such that it's big enough to host all object types' structs and thus we may waste memory here. This tradeoff is probably worth it though considering the following struct sizes:

    - commit: 72 bytes
    - tree: 56 bytes
    - blob: 40 bytes
    - tag: 64 bytes

Assuming that in almost all repositories, most references will point to either a tag or a commit, we'd have a modest increase in memory consumption of about 12.5% here.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 revision.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/revision.c b/revision.c
index f06a5d63a3..671b6d6513 100644
--- a/revision.c
+++ b/revision.c
@@ -359,14 +359,22 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
 				    const struct object_id *oid,
 				    unsigned int flags)
 {
-	struct object *object;
+	struct object *object = lookup_unknown_object(revs->repo, oid);
+
+	if (object->type == OBJ_NONE) {
+		int type = oid_object_info(revs->repo, oid, NULL);
+		if (type < 0 || !object_as_type(object, type, 1)) {
+			object = NULL;
+			goto out;
+		}
+	}
 
 	/*
 	 * If the repository has commit graphs, repo_parse_commit() avoids
 	 * reading the object buffer, so use it whenever possible.
 	 */
-	if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
-		struct commit *c = lookup_commit(revs->repo, oid);
+	if (object->type == OBJ_COMMIT) {
+		struct commit *c = (struct commit *) object;
 		if (!repo_parse_commit(revs->repo, c))
 			object = (struct object *) c;
 		else
@@ -375,6 +383,7 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
 		object = parse_object(revs->repo, oid);
 	}
 
+out:
 	if (!object) {
 		if (revs->ignore_missing)
 			return object;

-- 
2.32.0