Re: [RFC PATCH] index-pack: improve performance on NFS

From: Jeff King <peff@peff.net>
Date: Oct 27, 2018, 09:33 UTC
Message-ID: <20181027093300.GA23974@sigill.intra.peff.net>
In-Reply-To: <xmqq1s8bc0jp.fsf@gitster-ct.c.googlers.com>

On Sat, Oct 27, 2018 at 04:26:50PM +0900, Junio C Hamano wrote:

Show 15 quoted lines

> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
> 
> > But as Junio notes the devil's in the details, another one I thought of
> > is:
> >
> >     GIT_OBJECT_DIRECTORY=/some/other/repository git clone ...
> >
> > It seems to me that ...
> 
> Actually I take all of that back ;-)
> 
> For the purpose of this patch, use of existing .cloning field in the
> transport is fine, as the sole existing user of the field wants the
> field to mean "Are we starting with an empty object store?", and not
> "Are we running the command whose name is 'git clone'?".

Taking one step back, the root problem in this thread is that stat() on non-existing files is slow (which makes has_sha1_file slow).

One solution there is to cache the results of looking in .git/objects (or any alternate object store) for loose files. And indeed, this whole scheme is just a specialized form of that: it's a flag to say "hey, we do not have any objects yet, so do not bother looking".

Could we implement that in a more direct and central way? And could we implement it in a way that catches more cases? E.g., if I have _one_ object, that defeats this specialized optimization, but it is probably still beneficial to cache that knowledge (and the reasonable cutoff is probably not 1, but some value of N loose objects).

Of course any cache raises questions of cache invalidation, but I think we've already dealt with that for this case. When we use OBJECT_INFO_QUICK, that is a sign that we want to make this kind of accuracy/speed tradeoff (which does a similar caching thing with packfiles).

So putting that all together, could we have something like:

diff --git a/object-store.h b/object-store.h
index 63b7605a3e..28cde568a0 100644
--- a/object-store.h
+++ b/object-store.h
@@ -135,6 +135,18 @@ struct raw_object_store {
 	 */
 	struct packed_git *all_packs;
 
+	/*
+	 * A set of all loose objects we have. This probably ought to be split
+	 * into a set of 256 caches so that we can fault in one directory at a
+	 * time.
+	 */
+	struct oid_array loose_cache;
+	enum {
+		LOOSE_CACHE_UNFILLED = 0,
+		LOOSE_CACHE_INVALID,
+		LOOSE_CACHE_VALID
+	} loose_cache_status;
+
 	/*
 	 * A fast, rough count of the number of objects in the repository.
 	 * These two fields are not meant for direct access. Use
diff --git a/packfile.c b/packfile.c
index 86074a76e9..68ca4fff0e 100644
--- a/packfile.c
+++ b/packfile.c
@@ -990,6 +990,8 @@ void reprepare_packed_git(struct repository *r)
 	r->objects->approximate_object_count_valid = 0;
 	r->objects->packed_git_initialized = 0;
 	prepare_packed_git(r);
+	oid_array_clear(&r->objects->loose_cache);
+	r->objects->loose_cache_status = LOOSE_CACHE_UNFILLED;
 }
 
 struct packed_git *get_packed_git(struct repository *r)
diff --git a/sha1-file.c b/sha1-file.c
index dd0b6aa873..edbe037eaa 100644
--- a/sha1-file.c
+++ b/sha1-file.c
@@ -1172,6 +1172,40 @@ int parse_sha1_header(const char *hdr, unsigned long *sizep)
 	return parse_sha1_header_extended(hdr, &oi, 0);
 }
 
+/* probably should be configurable? */
+#define LOOSE_OBJECT_CACHE_MAX 65536
+
+static int fill_loose_cache(const struct object_id *oid,
+			    const char *path,
+			    void *data)
+{
+	struct oid_array *cache = data;
+
+	if (cache->nr == LOOSE_OBJECT_CACHE_MAX)
+		return -1;
+
+	oid_array_append(data, oid);
+	return 0;
+}
+
+static int quick_has_loose(struct raw_object_store *r,
+			   struct object_id *oid)
+{
+	struct oid_array *cache = &r->loose_cache;
+
+	if (r->loose_cache_status == LOOSE_CACHE_UNFILLED) {
+		if (for_each_loose_object(fill_loose_cache, cache, 0) < 0)
+			r->loose_cache_status = LOOSE_CACHE_INVALID;
+		else
+			r->loose_cache_status = LOOSE_CACHE_VALID;
+	}
+
+	if (r->loose_cache_status == LOOSE_CACHE_INVALID)
+		return -1;
+
+	return oid_array_lookup(cache, oid) >= 0;
+}
+
 static int sha1_loose_object_info(struct repository *r,
 				  const unsigned char *sha1,
 				  struct object_info *oi, int flags)
@@ -1198,6 +1232,19 @@ static int sha1_loose_object_info(struct repository *r,
 	if (!oi->typep && !oi->type_name && !oi->sizep && !oi->contentp) {
 		const char *path;
 		struct stat st;
+		if (!oi->disk_sizep && (flags & OBJECT_INFO_QUICK)) {
+			struct object_id oid;
+			hashcpy(oid.hash, sha1);
+			switch (quick_has_loose(r->objects, &oid)) {
+			case 0:
+				return -1; /* missing: error */
+			case 1:
+				return 0; /* have: 0 == success */
+			default:
+				/* unknown; fall back to stat */
+				break;
+			}
+		}
 		if (stat_sha1_file(r, sha1, &st, &path) < 0)
 			return -1;
 		if (oi->disk_sizep)

That's mostly untested, but it might be enough to run some timing tests
with. I think if we want to pursue this, we'd want to address the bits I
mentioned in the comments, and look at unifying this with the loose
cache from cc817ca3ef (which if I had remembered we added, probably
would have saved some time writing the above ;) ).

-Peff