builtin/repack.c: implement support for --max-cruft-size

Cruft packs are an alternative mechanism for storing a collection of
unreachable objects whose mtimes are recent enough to avoid being
pruned out of the repository.

When cruft packs were first introduced back in b757353
(builtin/pack-objects.c: --cruft without expiration, 2022-05-20) and
a7d4938 (builtin/pack-objects.c: --cruft with expiration,
2022-05-20), the recommended workflow consisted of:

  - Repacking periodically, either by packing anything loose in the
    repository (via `git repack -d`) or producing a geometric sequence
    of packs (via `git repack --geometric=<d> -d`).

  - Every so often, splitting the repository into two packs, one cruft
    to store the unreachable objects, and another non-cruft pack to
    store the reachable objects.

Repositories may (out of band with the above) choose periodically to
prune out some unreachable objects which have aged out of the grace
period by generating a pack with `--cruft-expiration=<approxidate>`.

This allowed repositories to maintain relatively few packs on average,
and quarantine unreachable objects together in a cruft pack, avoiding
the pitfalls of holding unreachable objects as loose while they age out
(for more, see some of the details in 3d89a8c
(Documentation/technical: add cruft-packs.txt, 2022-05-20)).

This all works, but can be costly from an I/O-perspective when
frequently repacking a repository that has many unreachable objects.
This problem is exacerbated when those unreachable objects are rarely
(if every) pruned.

Since there is at most one cruft pack in the above scheme, each time we
update the cruft pack it must be rewritten from scratch. Because much of
the pack is reused, this is a relatively inexpensive operation from a
CPU-perspective, but is very costly in terms of I/O since we end up
rewriting basically the same pack (plus any new unreachable objects that
have entered the repository since the last time a cruft pack was
generated).

At the time, we decided against implementing more robust support for
multiple cruft packs. This patch implements that support which we were
lacking.

Introduce a new option `--max-cruft-size` which allows repositories to
accumulate cruft packs up to a given size, after which point a new
generation of cruft packs can accumulate until it reaches the maximum
size, and so on. To generate a new cruft pack, the process works like
so:

  - Sort a list of any existing cruft packs in ascending order of pack
    size.

  - Starting from the beginning of the list, group cruft packs together
    while the accumulated size is smaller than the maximum specified
    pack size.

  - Combine the objects in these cruft packs together into a new cruft
    pack, along with any other unreachable objects which have since
    entered the repository.

Once a cruft pack grows beyond the size specified via `--max-cruft-size`
the pack is effectively frozen. This limits the I/O churn up to a
quadratic function of the value specified by the `--max-cruft-size`
option, instead of behaving quadratically in the number of total
unreachable objects.

When pruning unreachable objects, we bypass the new code paths which
combine small cruft packs together, and instead start from scratch,
passing in the appropriate `--max-pack-size` down to `pack-objects`,
putting it in charge of keeping the resulting set of cruft packs sized
correctly.

This may seem like further I/O churn, but in practice it isn't so bad.
We could prune old cruft packs for whom all or most objects are removed,
and then generate a new cruft pack with just the remaining set of
objects. But this additional complexity buys us relatively little,
because most objects end up being pruned anyway, so the I/O churn is
well contained.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

Documentation/config/gc.txt

@@ -86,6 +86,12 @@ gc.cruftPacks::
 	linkgit:git-repack[1]) instead of as loose objects. The default
 	is `true`.
 
+gc.maxCruftSize::
+	Limit the size of new cruft packs when repacking. When
+	specified in addition to `--max-cruft-size`, the command line
+	option takes priority. See the `--max-cruft-size` option of
+	linkgit:git-repack[1].
+
 gc.pruneExpire::
 	When 'git gc' is run, it will call 'prune --expire 2.weeks.ago'
 	(and 'repack --cruft --cruft-expiration 2.weeks.ago' if using

Documentation/git-gc.txt

@@ -59,6 +59,13 @@ be performed as well.
 	cruft pack instead of storing them as loose objects. `--cruft`
 	is on by default.
 
+--max-cruft-size=<n>::
+	When packing unreachable objects into a cruft pack, limit the
+	size of new cruft packs to be at most `<n>` bytes. Overrides any
+	value specified via the `gc.maxCruftSize` configuration. See
+	the `--max-cruft-size` option of linkgit:git-repack[1] for
+	more.
+
 --prune=<date>::
 	Prune loose objects older than date (default is 2 weeks ago,
 	overridable by the config variable `gc.pruneExpire`).

Documentation/git-repack.txt

@@ -74,6 +74,17 @@ to the new separate pack will be written.
 	immediately instead of waiting for the next `git gc` invocation.
 	Only useful with `--cruft -d`.
 
+--max-cruft-size=<n>::
+	Repack cruft objects into packs as large as `<n>` bytes before
+	creating new packs. As long as there are enough cruft packs
+	smaller than `<n>`, repacking will cause a new cruft pack to
+	be created containing objects from any combined cruft packs,
+	along with any new unreachable objects. Cruft packs larger than
+	`<n>` will not be modified. When the new cruft pack is larger
+	than `<n>` bytes, it will be split into multiple packs, all of
+	which are guaranteed to be at most `<n>` bytes in size. Only
+	useful with `--cruft -d`.
+
 --expire-to=<dir>::
 	Write a cruft pack containing pruned objects (if any) to the
 	directory `<dir>`. This option is useful for keeping a copy of

builtin/gc.c

@@ -52,6 +52,7 @@ static const char * const builtin_gc_usage[] = {
 static int pack_refs = 1;
 static int prune_reflogs = 1;
 static int cruft_packs = 1;
+static unsigned long max_cruft_size;
 static int aggressive_depth = 50;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
@@ -163,6 +164,7 @@ static void gc_config(void)
 	git_config_get_int("gc.autopacklimit", &gc_auto_pack_limit);
 	git_config_get_bool("gc.autodetach", &detach_auto);
 	git_config_get_bool("gc.cruftpacks", &cruft_packs);
+	git_config_get_ulong("gc.maxcruftsize", &max_cruft_size);
 	git_config_get_expiry("gc.pruneexpire", &prune_expire);
 	git_config_get_expiry("gc.worktreepruneexpire", &prune_worktrees_expire);
 	git_config_get_expiry("gc.logexpiry", &gc_log_expire);
@@ -347,6 +349,9 @@ static void add_repack_all_option(struct string_list *keep_pack)
 		strvec_push(&repack, "--cruft");
 		if (prune_expire)
 			strvec_pushf(&repack, "--cruft-expiration=%s", prune_expire);
+		if (max_cruft_size)
+			strvec_pushf(&repack, "--max-cruft-size=%lu",
+				     max_cruft_size);
 	} else {
 		strvec_push(&repack, "-A");
 		if (prune_expire)
@@ -575,6 +580,8 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			N_("prune unreferenced objects"),
 			PARSE_OPT_OPTARG, NULL, (intptr_t)prune_expire },
 		OPT_BOOL(0, "cruft", &cruft_packs, N_("pack unreferenced objects separately")),
+		OPT_MAGNITUDE(0, "max-cruft-size", &max_cruft_size,
+			      N_("with --cruft, limit the size of new cruft packs")),
 		OPT_BOOL(0, "aggressive", &aggressive, N_("be more thorough (increased runtime)")),
 		OPT_BOOL_F(0, "auto", &auto_gc, N_("enable auto-gc mode"),
 			   PARSE_OPT_NOCOMPLETE),

builtin/repack.c

@@ -27,6 +27,7 @@
 #define PACK_CRUFT 4
 
 #define DELETE_PACK 1
+#define RETAIN_PACK 2
 
 static int pack_everything;
 static int delta_base_offset = 1;
@@ -116,11 +117,26 @@ static void pack_mark_for_deletion(struct string_list_item *item)
 	item->util = (void*)((uintptr_t)item->util | DELETE_PACK);
 }
 
+static void pack_unmark_for_deletion(struct string_list_item *item)
+{
+	item->util = (void*)((uintptr_t)item->util & ~DELETE_PACK);
+}
+
 static int pack_is_marked_for_deletion(struct string_list_item *item)
 {
 	return (uintptr_t)item->util & DELETE_PACK;
 }
 
+static void pack_mark_retained(struct string_list_item *item)
+{
+	item->util = (void*)((uintptr_t)item->util | RETAIN_PACK);
+}
+
+static int pack_is_retained(struct string_list_item *item)
+{
+	return (uintptr_t)item->util & RETAIN_PACK;
+}
+
 static void mark_packs_for_deletion_1(struct string_list *names,
 				      struct string_list *list)
 {
@@ -133,17 +149,39 @@ static void mark_packs_for_deletion_1(struct string_list *names,
 		if (len < hexsz)
 			continue;
 		sha1 = item->string + len - hexsz;
-		/*
-		 * Mark this pack for deletion, which ensures that this
-		 * pack won't be included in a MIDX (if `--write-midx`
-		 * was given) and that we will actually delete this pack
-		 * (if `-d` was given).
-		 */
-		if (!string_list_has_string(names, sha1))
+
+		if (pack_is_retained(item)) {
+			pack_unmark_for_deletion(item);
+		} else if (!string_list_has_string(names, sha1)) {
+			/*
+			 * Mark this pack for deletion, which ensures
+			 * that this pack won't be included in a MIDX
+			 * (if `--write-midx` was given) and that we
+			 * will actually delete this pack (if `-d` was
+			 * given).
+			 */
 			pack_mark_for_deletion(item);
+		}
 	}
 }
 
+static void retain_cruft_pack(struct existing_packs *existing,
+			      struct packed_git *cruft)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct string_list_item *item;
+
+	strbuf_addstr(&buf, pack_basename(cruft));
+	strbuf_strip_suffix(&buf, ".pack");
+
+	item = string_list_lookup(&existing->cruft_packs, buf.buf);
+	if (!item)
+		BUG("could not find cruft pack '%s'", pack_basename(cruft));
+
+	pack_mark_retained(item);
+	strbuf_release(&buf);
+}
+
 static void mark_packs_for_deletion(struct existing_packs *existing,
 				    struct string_list *names)
 
@@ -225,6 +263,8 @@ static void collect_pack_filenames(struct existing_packs *existing,
 	}
 
 	string_list_sort(&existing->kept_packs);
+	string_list_sort(&existing->non_kept_packs);
+	string_list_sort(&existing->cruft_packs);
 	strbuf_release(&buf);
 }
 
@@ -806,6 +846,72 @@ static void remove_redundant_bitmaps(struct string_list *include,
 	strbuf_release(&path);
 }
 
+static int existing_cruft_pack_cmp(const void *va, const void *vb)
+{
+	struct packed_git *a = *(struct packed_git **)va;
+	struct packed_git *b = *(struct packed_git **)vb;
+
+	if (a->pack_size < b->pack_size)
+		return -1;
+	if (a->pack_size > b->pack_size)
+		return 1;
+	return 0;
+}
+
+static void collapse_small_cruft_packs(FILE *in, size_t max_size,
+				       struct existing_packs *existing)
+{
+	struct packed_git **existing_cruft, *p;
+	struct strbuf buf = STRBUF_INIT;
+	size_t total_size = 0;
+	size_t existing_cruft_nr = 0;
+	size_t i;
+
+	ALLOC_ARRAY(existing_cruft, existing->cruft_packs.nr);
+
+	for (p = get_all_packs(the_repository); p; p = p->next) {
+		if (!(p->is_cruft && p->pack_local))
+			continue;
+
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, pack_basename(p));
+		strbuf_strip_suffix(&buf, ".pack");
+
+		if (!string_list_has_string(&existing->cruft_packs, buf.buf))
+			continue;
+
+		if (existing_cruft_nr >= existing->cruft_packs.nr)
+			BUG("too many cruft packs (found %"PRIuMAX", but knew "
+			    "of %"PRIuMAX")",
+			    (uintmax_t)existing_cruft_nr + 1,
+			    (uintmax_t)existing->cruft_packs.nr);
+		existing_cruft[existing_cruft_nr++] = p;
+	}
+
+	QSORT(existing_cruft, existing_cruft_nr, existing_cruft_pack_cmp);
+
+	for (i = 0; i < existing_cruft_nr; i++) {
+		size_t proposed;
+
+		p = existing_cruft[i];
+		proposed = st_add(total_size, p->pack_size);
+
+		if (proposed <= max_size) {
+			total_size = proposed;
+			fprintf(in, "-%s\n", pack_basename(p));
+		} else {
+			retain_cruft_pack(existing, p);
+			fprintf(in, "%s\n", pack_basename(p));
+		}
+	}
+
+	for (i = 0; i < existing->non_kept_packs.nr; i++)
+		fprintf(in, "-%s.pack\n",
+			existing->non_kept_packs.items[i].string);
+
+	strbuf_release(&buf);
+}
+
 static int write_cruft_pack(const struct pack_objects_args *args,
 			    const char *destination,
 			    const char *pack_prefix,
@@ -853,10 +959,14 @@ static int write_cruft_pack(const struct pack_objects_args *args,
 	in = xfdopen(cmd.in, "w");
 	for_each_string_list_item(item, names)
 		fprintf(in, "%s-%s.pack\n", pack_prefix, item->string);
-	for_each_string_list_item(item, &existing->non_kept_packs)
-		fprintf(in, "-%s.pack\n", item->string);
-	for_each_string_list_item(item, &existing->cruft_packs)
-		fprintf(in, "-%s.pack\n", item->string);
+	if (args->max_pack_size && !cruft_expiration) {
+		collapse_small_cruft_packs(in, args->max_pack_size, existing);
+	} else {
+		for_each_string_list_item(item, &existing->non_kept_packs)
+			fprintf(in, "-%s.pack\n", item->string);
+		for_each_string_list_item(item, &existing->cruft_packs)
+			fprintf(in, "-%s.pack\n", item->string);
+	}
 	for_each_string_list_item(item, &existing->kept_packs)
 		fprintf(in, "%s.pack\n", item->string);
 	fclose(in);
@@ -919,6 +1029,8 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 				   PACK_CRUFT),
 		OPT_STRING(0, "cruft-expiration", &cruft_expiration, N_("approxidate"),
 				N_("with --cruft, expire objects older than this")),
+		OPT_MAGNITUDE(0, "max-cruft-size", &cruft_po_args.max_pack_size,
+				N_("with --cruft, limit the size of new cruft packs")),
 		OPT_BOOL('d', NULL, &delete_redundant,
 				N_("remove redundant packs, and run git-prune-packed")),
 		OPT_BOOL('f', NULL, &po_args.no_reuse_delta,

t/t6500-gc.sh

@@ -303,6 +303,33 @@ test_expect_success 'gc.bigPackThreshold ignores cruft packs' '
 	)
 '
 
+cruft_max_size_opts="git repack -d -l --cruft --cruft-expiration=2.weeks.ago"
+
+test_expect_success 'setup for --max-cruft-size tests' '
+	git init cruft--max-size &&
+	(
+		cd cruft--max-size &&
+		prepare_cruft_history
+	)
+'
+
+test_expect_success '--max-cruft-size sets appropriate repack options' '
+	GIT_TRACE2_EVENT=$(pwd)/trace2.txt git -C cruft--max-size \
+		gc --cruft --max-cruft-size=1M &&
+	test_subcommand $cruft_max_size_opts --max-cruft-size=1048576 <trace2.txt
+'
+
+test_expect_success 'gc.maxCruftSize sets appropriate repack options' '
+	GIT_TRACE2_EVENT=$(pwd)/trace2.txt \
+		git -C cruft--max-size -c gc.maxCruftSize=2M gc --cruft &&
+	test_subcommand $cruft_max_size_opts --max-cruft-size=2097152 <trace2.txt &&
+
+	GIT_TRACE2_EVENT=$(pwd)/trace2.txt \
+		git -C cruft--max-size -c gc.maxCruftSize=2M gc --cruft \
+		--max-cruft-size=3M &&
+	test_subcommand $cruft_max_size_opts --max-cruft-size=3145728 <trace2.txt
+'
+
 run_and_wait_for_auto_gc () {
 	# We read stdout from gc for the side effect of waiting until the
 	# background gc process exits, closing its fd 9.  Furthermore, the