[v2,3/3] builtin/repack.c: implement support for `--max-cruft-size`

Commit Message

Taylor Blau Oct. 3, 2023, 12:44 a.m. UTC

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 b757353676
(builtin/pack-objects.c: --cruft without expiration, 2022-05-20) and
a7d493833f (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 3d89a8c118
(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 <[e-mail]>
---
 Documentation/config/gc.txt  |   6 +
 Documentation/git-gc.txt     |   7 +
 Documentation/git-repack.txt |  11 ++
 builtin/gc.c                 |   7 +
 builtin/repack.c             | 134 +++++++++++++++++--
 t/t6500-gc.sh                |  27 ++++
 t/t7704-repack-cruft.sh      | 245 +++++++++++++++++++++++++++++++++++
 7 files changed, 426 insertions(+), 11 deletions(-)

Comments

Patrick Steinhardt Oct. 5, 2023, 12:08 p.m. UTC | #1

On Mon, Oct 02, 2023 at 08:44:32PM -0400, Taylor Blau wrote:
[snip]
> diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
> index 90806fd26a..fa0541b416 100644
> --- a/Documentation/git-gc.txt
> +++ b/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>`. Overrides any

We should probably mention the unit here, which is bytes.

[snip]
> @@ -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);

As far as I can see, the non-kept packs are passed to
git-pack-objects(1) both in the cases where we do collapse small cruft
packs and where we don't. Is there any particular reason why we handle
those in both code paths separately instead of merging that logic? Is
the ordering of packfiles important here?

> +	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);

Patrick

Taylor Blau Oct. 5, 2023, 5:35 p.m. UTC | #2

On Thu, Oct 05, 2023 at 02:08:11PM +0200, Patrick Steinhardt wrote:
> On Mon, Oct 02, 2023 at 08:44:32PM -0400, Taylor Blau wrote:
> [snip]
> > diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
> > index 90806fd26a..fa0541b416 100644
> > --- a/Documentation/git-gc.txt
> > +++ b/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>`. Overrides any
>
> We should probably mention the unit here, which is bytes.

Perhaps, though I'm OK with omitting it in the name of brevity, but only
since we link off to the relevant section in the git-repack(1)
documentation (which does include the units there).

> > +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);
>
> As far as I can see, the non-kept packs are passed to
> git-pack-objects(1) both in the cases where we do collapse small cruft
> packs and where we don't. Is there any particular reason why we handle
> those in both code paths separately instead of merging that logic? Is
> the ordering of packfiles important here?

No particularly good reason. The ordering isn't important, and you could do something like this:

--- 8< ---
diff --git a/builtin/repack.c b/builtin/repack.c
index 04770b15fe..6e17fc3f51 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -905,10 +905,6 @@ static void collapse_small_cruft_packs(FILE *in, size_t max_size,
 		}
 	}

-	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);
 }

@@ -959,14 +955,13 @@ 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);
-	if (args->max_pack_size && !cruft_expiration) {
+	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);
+	else
 		for_each_string_list_item(item, &existing->cruft_packs)
 			fprintf(in, "-%s.pack\n", 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->kept_packs)
 		fprintf(in, "%s.pack\n", item->string);
 	fclose(in);
--- >8 ---

But I think that having a small amount of duplication is a fair price to
pay for being able to see the whole input given to pack-objects outlined
in a single function.

Thanks,
Taylor

Junio C Hamano Oct. 5, 2023, 8:25 p.m. UTC | #3

Patrick Steinhardt <[e-mail]> writes:

> On Mon, Oct 02, 2023 at 08:44:32PM -0400, Taylor Blau wrote:
> [snip]
>> diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
>> index 90806fd26a..fa0541b416 100644
>> --- a/Documentation/git-gc.txt
>> +++ b/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>`. Overrides any
>
> We should probably mention the unit here, which is bytes.

Good suggestion.  I'll tweak "at most `<n>`" to "& bytes" or
something.

Patch

diff --git a/Documentation/config/gc.txt b/Documentation/config/gc.txt
index ca47eb2008..83ebc2de55 100644
--- a/Documentation/config/gc.txt
+++ b/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
diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
index 90806fd26a..fa0541b416 100644
--- a/Documentation/git-gc.txt
+++ b/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>`. 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`).
diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt
index 4017157949..fbfc72e1b2 100644
--- a/Documentation/git-repack.txt
+++ b/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
diff --git a/builtin/gc.c b/builtin/gc.c
index 00192ae5d3..c97c9fb046 100644
--- a/builtin/gc.c
+++ b/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),
diff --git a/builtin/repack.c b/builtin/repack.c
index 8a5bbb9cba..04770b15fe 100644
--- a/builtin/repack.c
+++ b/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,
diff --git a/t/t6500-gc.sh b/t/t6500-gc.sh
index 69509d0c11..0d6b5c3b27 100755
--- a/t/t6500-gc.sh
+++ b/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
diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index d91fcf1af1..dc86ca8269 100755
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -5,6 +5,7 @@  test_description='git repack works correctly'
 . ./test-lib.sh
 
 objdir=.git/objects
+packdir=$objdir/pack
 
 test_expect_success '--expire-to stores pruned objects (now)' '
 	git init expire-to-now &&
@@ -127,4 +128,248 @@  test_expect_success '--expire-to stores pruned objects (5.minutes.ago)' '
 	)
 '
 
+generate_random_blob() {
+	test-tool genrandom "$@" >blob &&
+	git hash-object -w -t blob blob &&
+	rm blob
+}
+
+pack_random_blob () {
+	generate_random_blob "$@" &&
+	git repack -d -q >/dev/null
+}
+
+generate_cruft_pack () {
+	pack_random_blob "$@" >/dev/null &&
+
+	ls $packdir/pack-*.pack | xargs -n 1 basename >in &&
+	pack="$(git pack-objects --cruft $packdir/pack <in)" &&
+	git prune-packed &&
+
+	echo "$packdir/pack-$pack.mtimes"
+}
+
+test_expect_success '--max-cruft-size creates new packs when above threshold' '
+	git init max-cruft-size-large &&
+	(
+		cd max-cruft-size-large &&
+		test_commit base &&
+
+		foo="$(pack_random_blob foo $((1*1024*1024)))" &&
+		git repack --cruft -d &&
+		cruft_foo="$(ls $packdir/pack-*.mtimes)" &&
+
+		bar="$(pack_random_blob bar $((1*1024*1024)))" &&
+		git repack --cruft -d --max-cruft-size=1M &&
+		cruft_bar="$(ls $packdir/pack-*.mtimes | grep -v $cruft_foo)" &&
+
+		test-tool pack-mtimes $(basename "$cruft_foo") >foo.objects &&
+		test-tool pack-mtimes $(basename "$cruft_bar") >bar.objects &&
+
+		grep "^$foo" foo.objects &&
+		test_line_count = 1 foo.objects &&
+		grep "^$bar" bar.objects &&
+		test_line_count = 1 bar.objects
+	)
+'
+
+test_expect_success '--max-cruft-size combines existing packs when below threshold' '
+	git init max-cruft-size-small &&
+	(
+		cd max-cruft-size-small &&
+		test_commit base &&
+
+		foo="$(pack_random_blob foo $((1*1024*1024)))" &&
+		git repack --cruft -d &&
+
+		bar="$(pack_random_blob bar $((1*1024*1024)))" &&
+		git repack --cruft -d --max-cruft-size=10M &&
+
+		cruft=$(ls $packdir/pack-*.mtimes) &&
+		test-tool pack-mtimes $(basename "$cruft") >cruft.objects &&
+
+		grep "^$foo" cruft.objects &&
+		grep "^$bar" cruft.objects &&
+		test_line_count = 2 cruft.objects
+	)
+'
+
+test_expect_success '--max-cruft-size combines smaller packs first' '
+	git init max-cruft-size-consume-small &&
+	(
+		cd max-cruft-size-consume-small &&
+
+		test_commit base &&
+		git repack -ad &&
+
+		cruft_foo="$(generate_cruft_pack foo 524288)" &&    # 0.5 MiB
+		cruft_bar="$(generate_cruft_pack bar 524288)" &&    # 0.5 MiB
+		cruft_baz="$(generate_cruft_pack baz 1048576)" &&   # 1.0 MiB
+		cruft_quux="$(generate_cruft_pack quux 1572864)" && # 1.5 MiB
+
+		test-tool pack-mtimes "$(basename $cruft_foo)" >expect.raw &&
+		test-tool pack-mtimes "$(basename $cruft_bar)" >>expect.raw &&
+		sort expect.raw >expect.objects &&
+
+		# repacking with `--max-cruft-size=2M` should combine
+		# both 0.5 MiB packs together, instead of, say, one of
+		# the 0.5 MiB packs with the 1.0 MiB pack
+		ls $packdir/pack-*.mtimes | sort >cruft.before &&
+		git repack -d --cruft --max-cruft-size=2M &&
+		ls $packdir/pack-*.mtimes | sort >cruft.after &&
+
+		comm -13 cruft.before cruft.after >cruft.new &&
+		comm -23 cruft.before cruft.after >cruft.removed &&
+
+		test_line_count = 1 cruft.new &&
+		test_line_count = 2 cruft.removed &&
+
+		# the two smaller packs should be rolled up first
+		printf "%s\n" $cruft_foo $cruft_bar | sort >expect.removed &&
+		test_cmp expect.removed cruft.removed &&
+
+		# ...and contain the set of objects rolled up
+		test-tool pack-mtimes "$(basename $(cat cruft.new))" >actual.raw &&
+		sort actual.raw >actual.objects &&
+
+		test_cmp expect.objects actual.objects
+	)
+'
+
+test_expect_success 'setup --max-cruft-size with freshened objects' '
+	git init max-cruft-size-freshen &&
+	(
+		cd max-cruft-size-freshen &&
+
+		test_commit base &&
+		git repack -ad &&
+
+		foo="$(generate_random_blob foo 64)" &&
+		test-tool chmtime --get -10000 \
+			"$objdir/$(test_oid_to_path "$foo")" >foo.mtime &&
+
+		git repack --cruft -d &&
+
+		cruft="$(ls $packdir/pack-*.mtimes)" &&
+		test-tool pack-mtimes "$(basename $cruft)" >actual &&
+		echo "$foo $(cat foo.mtime)" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--max-cruft-size with freshened objects (loose)' '
+	(
+		cd max-cruft-size-freshen &&
+
+		# regenerate the object, setting its mtime to be more recent
+		foo="$(generate_random_blob foo 64)" &&
+		test-tool chmtime --get -100 \
+			"$objdir/$(test_oid_to_path "$foo")" >foo.mtime &&
+
+		git repack --cruft -d &&
+
+		cruft="$(ls $packdir/pack-*.mtimes)" &&
+		test-tool pack-mtimes "$(basename $cruft)" >actual &&
+		echo "$foo $(cat foo.mtime)" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--max-cruft-size with freshened objects (packed)' '
+	(
+		cd max-cruft-size-freshen &&
+
+		# regenerate the object and store it in a packfile,
+		# setting its mtime to be more recent
+		#
+		# store it alongside another cruft object so that we
+		# do not create an identical copy of the existing
+		# cruft pack (which contains $foo).
+		foo="$(generate_random_blob foo 64)" &&
+		bar="$(generate_random_blob bar 64)" &&
+		foo_pack="$(printf "%s\n" $foo $bar | git pack-objects $packdir/pack)" &&
+		git prune-packed &&
+
+		test-tool chmtime --get -10 \
+			"$packdir/pack-$foo_pack.pack" >foo.mtime &&
+
+		git repack --cruft -d &&
+
+		cruft="$(ls $packdir/pack-*.mtimes)" &&
+		test-tool pack-mtimes "$(basename $cruft)" >actual &&
+		echo "$foo $(cat foo.mtime)" >expect.raw &&
+		echo "$bar $(cat foo.mtime)" >>expect.raw &&
+		sort expect.raw >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--max-cruft-size with pruning' '
+	git init max-cruft-size-prune &&
+	(
+		cd max-cruft-size-prune &&
+
+		test_commit base &&
+		foo="$(generate_random_blob foo $((1024*1024)))" &&
+		bar="$(generate_random_blob bar $((1024*1024)))" &&
+		baz="$(generate_random_blob baz $((1024*1024)))" &&
+
+		test-tool chmtime -10000 "$objdir/$(test_oid_to_path "$foo")" &&
+
+		git repack -d --cruft --max-cruft-size=1M &&
+
+		# backdate the mtimes of all cruft packs to validate
+		# that they were rewritten as a result of pruning
+		ls $packdir/pack-*.mtimes | sort >cruft.before &&
+		for cruft in $(cat cruft.before)
+		do
+			mtime="$(test-tool chmtime --get -10000 "$cruft")" &&
+			echo $cruft $mtime >>mtimes || return 1
+		done &&
+
+		# repack (and prune) with a --max-cruft-size to ensure
+		# that we appropriately split the resulting set of packs
+		git repack -d --cruft --max-cruft-size=1M \
+			--cruft-expiration=10.seconds.ago &&
+		ls $packdir/pack-*.mtimes | sort >cruft.after &&
+
+		for cruft in $(cat cruft.after)
+		do
+			old_mtime="$(grep $cruft mtimes | cut -d" " -f2)" &&
+			new_mtime="$(test-tool chmtime --get $cruft)" &&
+			test $old_mtime -lt $new_mtime || return 1
+		done &&
+
+		test_line_count = 3 cruft.before &&
+		test_line_count = 2 cruft.after &&
+		test_must_fail git cat-file -e $foo &&
+		git cat-file -e $bar &&
+		git cat-file -e $baz
+	)
+'
+
+test_expect_success '--max-cruft-size ignores non-local packs' '
+	repo="max-cruft-size-non-local" &&
+	git init $repo &&
+	(
+		cd $repo &&
+		test_commit base &&
+		generate_random_blob foo 64 &&
+		git repack --cruft -d
+	) &&
+
+	git clone --reference=$repo $repo $repo-alt &&
+	(
+		cd $repo-alt &&
+
+		test_commit other &&
+		generate_random_blob bar 64 &&
+
+		# ensure that we do not attempt to pick up packs from
+		# the non-alternated repository, which would result in a
+		# crash
+		git repack --cruft --max-cruft-size=1M -d
+	)
+'
+
 test_done