Subject: Re: [PATCH v5 0/5] Speed up connectivity checks

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 9 Aug 2021 10:02:01 +0200

You can ignore this version. I realized I forgot to address some
feedback while I was sending mails out. I'll resend in some minutes.

Patrick

On Mon, Aug 09, 2021 at 10:00:39AM +0200, Patrick Steinhardt wrote:
> Hi,
> 
> this is the fifth version of my series to speed up connectivity checks
> in the context of repos with many refs. While the original goal has been
> to speed up connectivity checks only, the series is now optimizing
> git-rev-list(1) in general to be able to more efficiently load
> references. Like this, `--not --all` is a lot faster in the context of
> many refs, but other usecases benefit, too.
> 
> Changes compared to v4:
> 
>     - I've changed the interface to load commits via the commit-graph.
>       Instead of the previous version where you'd had to pass in a
>       `struct object`, which forced us to use `lookup_unknown_object()`,
>       one now only passes in an object ID. If the object ID is found in
>       the commit graph and if the corresponding object exists in the
>       ODB, then we return the parsed commit object.
> 
>       This also avoids a previous pitfal: we'd have parsed the commit
>       via the graph and thus had allocated the object even if the
>       corresponding object didn't exist. While we knew to handle this in
>       `get_reference()` by asserting object existence, any other caller
>       which executes `lookup_commit()` would get the parsed commit and
>       assume that it exists. This now cannot happen anymore given that
>       we only create the commit object in case we know the ID exists in
>       the ODB.
> 
>     - With this change, I could now drop the patch which avoided loading
>       objects multiple times: we don't need `lookup_unknown_object()`
>       anymore and thus don't thave the memory/perf tradeoff. And with
>       the new interface to load commits via the graph, the deduplication
>       only resulted in a ~1% speedup.
> 
> Patrick
> 
> Patrick Steinhardt (5):
>   revision: separate walk and unsorted flags
>   connected: do not sort input revisions
>   revision: stop retrieving reference twice
>   commit-graph: split out function to search commit position
>   revision: avoid hitting packfiles when commits are in commit-graph
> 
>  Documentation/rev-list-options.txt |  8 ++-
>  builtin/log.c                      |  2 +-
>  builtin/revert.c                   |  3 +-
>  commit-graph.c                     | 79 ++++++++++++++++++++----------
>  commit-graph.h                     |  8 +++
>  connected.c                        |  1 +
>  revision.c                         | 42 +++++++++-------
>  revision.h                         |  7 +--
>  t/t6000-rev-list-misc.sh           | 38 ++++++++++++++
>  9 files changed, 138 insertions(+), 50 deletions(-)
> 
> Range-diff against v4:
> 1:  67232910ac = 1:  67232910ac revision: separate walk and unsorted flags
> 2:  9d7f484907 = 2:  9d7f484907 connected: do not sort input revisions
> 3:  d8e63d0943 = 3:  d8e63d0943 revision: stop retrieving reference twice
> 4:  ba8df5cad0 < -:  ---------- revision: avoid loading object headers multiple times
> 5:  e33cd51ebf = 4:  549d85e5c2 commit-graph: split out function to search commit position
> 6:  900c5a9c60 ! 5:  4b893d943f revision: avoid hitting packfiles when commits are in commit-graph
>     @@ Metadata
>       ## Commit message ##
>          revision: avoid hitting packfiles when commits are in commit-graph
>      
>     -    When queueing references in git-rev-list(1), we try to either reuse an
>     -    already parsed object or alternatively we load the object header from
>     -    disk in order to determine its type. This is inefficient for commits
>     -    though in cases where we have a commit graph available: instead of
>     -    hitting the real object on disk to determine its type, we may instead
>     -    search the object graph for the object ID. In case it's found, we can
>     -    directly fill in the commit object, otherwise we can still hit the disk
>     -    to determine the object's type.
>     +    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 `parse_commit_in_graph_gently()`, which fills in
>     -    an object of unknown type in case we find its object ID in the graph.
>     -    This provides a big performance win in cases where there is a
>     -    commit-graph available in the repository in case we load lots of
>     -    references. The following has been executed in a real-world repository
>     -    with about 2.2 million refs:
>     +    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):      4.508 s ±  0.039 s    [User: 4.131 s, System: 0.377 s]
>     -          Range (min … max):    4.455 s …  4.576 s    10 runs
>     +          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):      3.072 s ±  0.031 s    [User: 2.707 s, System: 0.365 s]
>     -          Range (min … max):    3.040 s …  3.144 s    10 runs
>     +          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
>     -            1.47 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'
>     +            1.44 ± 0.02 times faster than 'HEAD~: rev-list --unsorted-input --objects --quiet --not --all --not $newrev'
>      
>          Signed-off-by: Patrick Steinhardt <[email-masked]>
>      
>     @@ commit-graph.c: static int find_commit_pos_in_graph(struct commit *item, struct
>       	}
>       }
>       
>     -+int parse_commit_in_graph_gently(struct repository *repo, struct object *object)
>     ++struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
>      +{
>      +	struct commit *commit;
>      +	uint32_t pos;
>      +
>     -+	if (object->parsed) {
>     -+		if (object->type != OBJ_COMMIT)
>     -+			return -1;
>     -+		return 0;
>     -+	}
>     -+
>      +	if (!repo->objects->commit_graph)
>     -+		return -1;
>     ++		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;
>      +
>     -+	if (!search_commit_pos_in_graph(&object->oid, repo->objects->commit_graph, &pos))
>     -+		return -1;
>     -+
>     -+	commit = object_as_type(object, OBJ_COMMIT, 1);
>     ++	commit = lookup_commit(repo, id);
>      +	if (!commit)
>     -+		return -1;
>     -+	if (!fill_commit_in_graph(repo, commit, repo->objects->commit_graph, pos))
>     -+		return -1;
>     ++		return NULL;
>     ++	if (commit->object.parsed)
>     ++		return commit;
>      +
>     -+	return 0;
>     ++	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,
>     @@ commit-graph.h: int open_commit_graph(const char *graph_file, int *fd, struct st
>       int parse_commit_in_graph(struct repository *r, struct commit *item);
>       
>      +/*
>     -+ * Given an object of unknown type, try to fill in the object in case it is a
>     -+ * commit part of the commit-graph. Returns 0 if the object is a parsed commit
>     -+ * or if it could be filled in via the commit graph, otherwise it returns -1.
>     ++ * 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`.
>      + */
>     -+int parse_commit_in_graph_gently(struct repository *repo, struct object *object);
>     ++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,
>     @@ commit-graph.h: int open_commit_graph(const char *graph_file, int *fd, struct st
>      
>       ## revision.c ##
>      @@ revision.c: static struct object *get_reference(struct rev_info *revs, const char *name,
>     - 	struct object *object = lookup_unknown_object(revs->repo, oid);
>     + 				    unsigned int flags)
>     + {
>     + 	struct object *object;
>     ++	struct commit *commit;
>       
>     - 	if (object->type == OBJ_NONE) {
>     --		int type = oid_object_info(revs->repo, oid, NULL);
>     --		if (type < 0 || !object_as_type(object, type, 1)) {
>     -+		/*
>     -+		 * It's likely that the reference points to a commit, so we
>     -+		 * first try to look it up via the commit-graph. If successful,
>     -+		 * then we know it's a commit and don't have to unpack the
>     -+		 * object header. We still need to assert that the object
>     -+		 * exists, but given that we don't request any info about the
>     -+		 * object this is a lot faster than `oid_object_info()`.
>     -+		 */
>     -+		if (parse_commit_in_graph_gently(revs->repo, object) < 0) {
>     -+			int type = oid_object_info(revs->repo, oid, NULL);
>     -+			if (type < 0 || !object_as_type(object, type, 1)) {
>     -+				object = NULL;
>     -+				goto out;
>     -+			}
>     -+		} else if (!repo_has_object_file(revs->repo, oid)) {
>     - 			object = NULL;
>     - 			goto out;
>     - 		}
>     + 	/*
>     +-	 * 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)
> -- 
> 2.32.0
>