refs/files: use heuristic to decide whether to repack with --auto

The `--auto` flag for git-pack-refs(1) allows the ref backend to decide
whether or not a repack is in order. This switch has been introduced
mostly with the "reftable" backend in mind, which already knows to
auto-compact its tables during normal operations. When the flag is set,
then it will use the same auto-compaction mechanism and thus end up
doing nothing in most cases.

The "files" backend does not have any such heuristic yet and instead
packs any loose references unconditionally. So we rewrite the complete
"packed-refs" file even if there's only a single loose reference to be
packed.

Even worse, starting with 9f6714a (builtin/gc: pack refs when using
`git maintenance run --auto`, 2024-03-25), `git pack-refs --auto` is
unconditionally executed via our auto maintenance, so we end up repacking
references every single time auto maintenance kicks in. And while that
commit already mentioned that the "files" backend unconditionally packs
refs now, the author obviously didn't quite think about the consequences
thereof. So while the idea was sound, we really should have added a
heuristic to the "files" backend before implementing it.

Introduce a heuristic that decides whether or not it is worth to pack
loose references. The important factors to decide here are the number of
loose references in comparison to the overall size of the "packed-refs"
file. The bigger the "packed-refs" file, the longer it takes to rewrite
it and thus we scale up the limit of allowed loose references before we
repack.

As is the nature of heuristics, this mechansim isn't obviously
"correct", but should rather be seen as a tradeoff between how much
resources we spend packing refs and how inefficient the ref store
becomes. For all I can say, we have successfully been using the exact
same heuristic in Gitaly for several years by now.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/files-backend.c

@@ -1300,6 +1300,68 @@ static int should_pack_ref(struct files_ref_store *refs,
 	return 0;
 }
 
+static int should_pack_refs(struct files_ref_store *refs,
+			    struct pack_refs_opts *opts)
+{
+	struct ref_iterator *iter;
+	size_t packed_size;
+	size_t refcount = 0;
+	size_t limit;
+	int ret;
+
+	if (!(opts->flags & PACK_REFS_AUTO))
+		return 1;
+
+	ret = packed_refs_size(refs->packed_ref_store, &packed_size);
+	if (ret < 0)
+		die("cannot determine packed-refs size");
+
+	/*
+	 * Packing loose references into the packed-refs file scales with the
+	 * number of references we're about to write. We thus decide whether we
+	 * repack refs by weighing the current size of the packed-refs file
+	 * against the number of loose references. This is done such that we do
+	 * not repack too often on repositories with a huge number of
+	 * references, where we can expect a lot of churn in the number of
+	 * references.
+	 *
+	 * As a heuristic, we repack if the number of loose references in the
+	 * repository exceeds `log2(nr_packed_refs) * 5`, where we estimate
+	 * `nr_packed_refs = packed_size / 100`, which scales as following:
+	 *
+	 * - 1kB ~ 10 packed refs: 16 refs
+	 * - 10kB ~ 100 packed refs: 33 refs
+	 * - 100kB ~ 1k packed refs: 49 refs
+	 * - 1MB ~ 10k packed refs: 66 refs
+	 * - 10MB ~ 100k packed refs: 82 refs
+	 * - 100MB ~ 1m packed refs: 99 refs
+	 *
+	 * We thus allow roughly 16 additional loose refs per factor of ten of
+	 * packed refs. This heuristic may be tweaked in the future, but should
+	 * serve as a sufficiently good first iteration.
+	 */
+	limit = log2u(packed_size / 100) * 5;
+	if (limit < 16)
+		limit = 16;
+
+	iter = cache_ref_iterator_begin(get_loose_ref_cache(refs, 0), NULL,
+					refs->base.repo, 0);
+	while ((ret = ref_iterator_advance(iter)) == ITER_OK) {
+		if (should_pack_ref(refs, iter->refname, iter->oid,
+				    iter->flags, opts))
+			refcount++;
+		if (refcount >= limit) {
+			ref_iterator_abort(iter);
+			return 1;
+		}
+	}
+
+	if (ret != ITER_DONE)
+		die("error while iterating over references");
+
+	return 0;
+}
+
 static int files_pack_refs(struct ref_store *ref_store,
 			   struct pack_refs_opts *opts)
 {
@@ -1312,6 +1374,9 @@ static int files_pack_refs(struct ref_store *ref_store,
 	struct strbuf err = STRBUF_INIT;
 	struct ref_transaction *transaction;
 
+	if (!should_pack_refs(refs, opts))
+		return 0;
+
 	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
 	if (!transaction)
 		return -1;

refs/packed-backend.c

@@ -1250,6 +1250,24 @@ int packed_refs_is_locked(struct ref_store *ref_store)
 	return is_lock_file_locked(&refs->lock);
 }
 
+int packed_refs_size(struct ref_store *ref_store,
+		     size_t *out)
+{
+	struct packed_ref_store *refs = packed_downcast(ref_store, REF_STORE_READ,
+							"packed_refs_size");
+	struct stat st;
+
+	if (stat(refs->path, &st) < 0) {
+		if (errno != ENOENT)
+			return -1;
+		*out = 0;
+		return 0;
+	}
+
+	*out = st.st_size;
+	return 0;
+}
+
 /*
  * The packed-refs header line that we write out. Perhaps other traits
  * will be added later.

refs/packed-backend.h

@@ -27,6 +27,13 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 void packed_refs_unlock(struct ref_store *ref_store);
 int packed_refs_is_locked(struct ref_store *ref_store);
 
+/*
+ * Obtain the size of the `packed-refs` file. Reports `0` as size in case there
+ * is no packed-refs file. Returns 0 on success, negative otherwise.
+ */
+int packed_refs_size(struct ref_store *ref_store,
+		     size_t *out);
+
 /*
  * Return true if `transaction` really needs to be carried out against
  * the specified packed_ref_store, or false if it can be skipped

t/t0601-reffiles-pack-refs.sh

@@ -362,21 +362,86 @@ test_expect_success 'pack-refs does not drop broken refs during deletion' '
 
 for command in "git pack-refs --all --auto" "git maintenance run --task=pack-refs --auto"
 do
-	test_expect_success "$command unconditionally packs loose refs" '
+	test_expect_success "$command does not repack below 16 refs without packed-refs" '
 		test_when_finished "rm -rf repo" &&
 		git init repo &&
 		(
 			cd repo &&
 			git config set maintenance.auto false &&
-			test_commit initial &&
-
-			git update-ref refs/heads/something HEAD &&
-			test_path_is_file .git/refs/heads/something &&
-			git rev-parse refs/heads/something >expect &&
-			$command &&
-			test_path_is_missing .git/refs/heads/something &&
-			git rev-parse refs/heads/something >actual &&
-			test_cmp expect actual
+			git commit --allow-empty --message "initial" &&
+
+			# Create 14 additional references, which brings us to
+			# 15 together with the default branch.
+			printf "create refs/heads/loose-%d HEAD\n" $(test_seq 14) >stdin &&
+			git update-ref --stdin <stdin &&
+			test_path_is_missing .git/packed-refs &&
+			git pack-refs --auto --all &&
+			test_path_is_missing .git/packed-refs &&
+
+			# Create the 16th reference, which should cause us to repack.
+			git update-ref refs/heads/loose-15 HEAD &&
+			git pack-refs --auto --all &&
+			test_path_is_file .git/packed-refs
+		)
+	'
+
+	test_expect_success "$command does not repack below 16 refs with small packed-refs" '
+		test_when_finished "rm -rf repo" &&
+		git init repo &&
+		(
+			cd repo &&
+			git config set maintenance.auto false &&
+			git commit --allow-empty --message "initial" &&
+
+			git pack-refs --all &&
+			test_line_count = 2 .git/packed-refs &&
+
+			# Create 15 loose references.
+			printf "create refs/heads/loose-%d HEAD\n" $(test_seq 15) >stdin &&
+			git update-ref --stdin <stdin &&
+			git pack-refs --auto --all &&
+			test_line_count = 2 .git/packed-refs &&
+
+			# Create the 16th loose reference, which should cause us to repack.
+			git update-ref refs/heads/loose-17 HEAD &&
+			git pack-refs --auto --all &&
+			test_line_count = 18 .git/packed-refs
+		)
+	'
+
+	test_expect_success "$command scales with size of packed-refs" '
+		test_when_finished "rm -rf repo" &&
+		git init repo &&
+		(
+			cd repo &&
+			git config set maintenance.auto false &&
+			git commit --allow-empty --message "initial" &&
+
+			# Create 99 packed refs. This should cause the heuristic
+			# to require more than the minimum amount of loose refs.
+			test_seq 99 |
+			while read i
+			do
+				printf "create refs/heads/packed-%d HEAD\n" $i || return 1
+			done >stdin &&
+			git update-ref --stdin <stdin &&
+			git pack-refs --all &&
+			test_line_count = 101 .git/packed-refs &&
+
+			# Create 24 loose refs, which should not yet cause us to repack.
+			printf "create refs/heads/loose-%d HEAD\n" $(test_seq 24) >stdin &&
+			git update-ref --stdin <stdin &&
+			git pack-refs --auto --all &&
+			test_line_count = 101 .git/packed-refs &&
+
+			# Create another handful of refs to cross the border.
+			# Note that we explicitly do not check for strict
+			# boundaries here, as this also depends on the size of
+			# the object hash.
+			printf "create refs/heads/addn-%d HEAD\n" $(test_seq 10) >stdin &&
+			git update-ref --stdin <stdin &&
+			git pack-refs --auto --all &&
+			test_line_count = 135 .git/packed-refs
 		)
 	'
 done