Subject: [PATCH v2 2/8] pack-objects: add --name-hash-version option

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Mon, 02 Dec 2024 23:21:16 +0000

From: Derrick Stolee <[email-masked]>

The previous change introduced a new pack_name_hash_v2() function that
intends to satisfy much of the hash locality features of the existing
pack_name_hash() function while also distinguishing paths with similar
final components of their paths.

This change adds a new --name-hash-version option for 'git pack-objects'
to allow users to select their preferred function version. This use of
an integer version allows for future expansion and a direct way to later
store a name hash version in the .bitmap format.

For now, let's consider how effective this mechanism is when repacking a
repository with different name hash versions. Specifically, we will
execute 'git pack-objects' the same way a 'git repack -adf' process
would, except we include --name-hash-version=<n> for testing.

On the Git repository, we do not expect much difference. All path names
are short. This is backed by our results:

| Stage                 | Pack Size | Repack Time |
|-----------------------|-----------|-------------|
| After clone           | 260 MB    | N/A         |
| --name-hash-version=1 | 127 MB    | 129s        |
| --name-hash-version=2 | 127 MB    | 112s        |

This example demonstrates how there is some natural overhead coming from
the cloned copy because the server is hosting many forks and has not
optimized for exactly this set of reachable objects. But the full repack
has similar characteristics for both versions.

Let's consider some repositories that are hitting too many collisions
with version 1. First, let's explore the kinds of paths that are
commonly causing these collisions:

 * "/CHANGELOG.json" is 15 characters, and is created by the beachball
   [1] tool. Only the final character of the parent directory can
   differentiate different versions of this file, but also only the two
   most-significant digits. If that character is a letter, then this is
   always a collision. Similar issues occur with the similar
   "/CHANGELOG.md" path, though there is more opportunity for
   differences In the parent directory.

 * Localization files frequently have common filenames but
   differentiates via parent directories. In C#, the name
   "/strings.resx.lcl" is used for these localization files and they
   will all collide in name-hash.

[1] https://github.com/microsoft/beachball

I've come across many other examples where some internal tool uses a
common name across multiple directories and is causing Git to repack
poorly due to name-hash collisions.

One open-source example is the fluentui [2] repo, which  uses beachball
to generate CHANGELOG.json and CHANGELOG.md files, and these files have
very poor delta characteristics when comparing against versions across
parent directories.

| Stage                 | Pack Size | Repack Time |
|-----------------------|-----------|-------------|
| After clone           | 694 MB    | N/A         |
| --name-hash-version=1 | 438 MB    | 728s        |
| --name-hash-version=2 | 168 MB    | 142s        |

[2] https://github.com/microsoft/fluentui

In this example, we see significant gains in the compressed packfile
size as well as the time taken to compute the packfile.

Using a collection of repositories that use the beachball tool, I was
able to make similar comparisions with dramatic results. While the
fluentui repo is public, the others are private so cannot be shared for
reproduction. The results are so significant that I find it important to
share here:

| Repo     | --name-hash-version=1 | --name-hash-version=2 |
|----------|-----------------------|-----------------------|
| fluentui |               440 MB  |               161 MB  |
| Repo B   |             6,248 MB  |               856 MB  |
| Repo C   |            37,278 MB  |             6,755 MB  |
| Repo D   |           131,204 MB  |             7,463 MB  |

Future changes could include making --name-hash-version implied by a config
value or even implied by default during a full repack.

It is important to point out that the name hash value is stored in the
.bitmap file format, so we must force --name-hash-version=1 when bitmaps
are being read or written. Later, the bitmap format could be updated to
be aware of the name hash version so deltas can be quickly computed
across the bitmapped/not-bitmapped boundary.

Signed-off-by: Derrick Stolee <[email-masked]>
---
 Documentation/git-pack-objects.txt | 32 ++++++++++++++++++-
 builtin/pack-objects.c             | 49 +++++++++++++++++++++++++++---
 builtin/repack.c                   |  1 +
 t/t5300-pack-object.sh             | 31 +++++++++++++++++++
 4 files changed, 107 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt
index e32404c6aae..7f69ae4855f 100644
--- a/Documentation/git-pack-objects.txt
+++ b/Documentation/git-pack-objects.txt
@@ -15,7 +15,8 @@ SYNOPSIS
 	[--revs [--unpacked | --all]] [--keep-pack=<pack-name>]
 	[--cruft] [--cruft-expiration=<time>]
 	[--stdout [--filter=<filter-spec>] | <base-name>]
-	[--shallow] [--keep-true-parents] [--[no-]sparse] < <object-list>
+	[--shallow] [--keep-true-parents] [--[no-]sparse]
+	[--name-hash-version=<n>] < <object-list>

 DESCRIPTION
@@ -345,6 +346,35 @@ raise an error.
 	Restrict delta matches based on "islands". See DELTA ISLANDS
 	below.
 
+--name-hash-version=<n>::
+	While performing delta compression, Git groups objects that may be
+	similar based on heuristics using the path to that object. While
+	grouping objects by an exact path match is good for paths with
+	many versions, there are benefits for finding delta pairs across
+	different full paths. Git collects objects by type and then by a
+	"name hash" of the path and then by size, hoping to group objects
+	that will compress well together.
++
+The default name hash version is `1`, which prioritizes hash locality by
+considering the final bytes of the path as providing the maximum magnitude
+to the hash function. This version excels at distinguishing short paths
+and finding renames across directories. However, the hash function depends
+primarily on the final 16 bytes of the path. If there are many paths in
+the repo that have the same final 16 bytes and differ only by parent
+directory, then this name-hash may lead to too many collisions and cause
+poor results. At the moment, this version is required when writing
+reachability bitmap files with `--write-bitmap-index`.
++
+The name hash version `2` has similar locality features as version `1`,
+except it considers each path component separately and overlays the hashes
+with a shift. This still prioritizes the final bytes of the path, but also
+"salts" the lower bits of the hash using the parent directory names. This
+method allows for some of the locality benefits of version `1` while
+breaking most of the collisions from a similarly-named file appearing in
+many different directories. At the moment, this version is not allowed
+when writing reachability bitmap files with `--write-bitmap-index` and it
+will be automatically changed to version `1`.
+
 
 DELTA ISLANDS
 -------------
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 08007142671..e36a93a0082 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -266,6 +266,28 @@ struct configured_exclusion {
 static struct oidmap configured_exclusions;
 
 static struct oidset excluded_by_config;
+static int name_hash_version = 1;
+
+static void validate_name_hash_version(void)
+{
+	if (name_hash_version < 1 || name_hash_version > 2)
+		die(_("invalid --name-hash-version option: %d"), name_hash_version);
+}
+
+static inline uint32_t pack_name_hash_fn(const char *name)
+{
+	switch (name_hash_version)
+	{
+	case 1:
+		return pack_name_hash(name);
+
+	case 2:
+		return pack_name_hash_v2(name);
+
+	default:
+		BUG("invalid name-hash version: %d", name_hash_version);
+	}
+}
 
 /*
  * stats
@@ -1698,7 +1720,7 @@ static int add_object_entry(const struct object_id *oid, enum object_type type,
 		return 0;
 	}
 
-	create_object_entry(oid, type, pack_name_hash(name),
+	create_object_entry(oid, type, pack_name_hash_fn(name),
 			    exclude, name && no_try_delta(name),
 			    found_pack, found_offset);
 	return 1;
@@ -1912,7 +1934,7 @@ static void add_preferred_base_object(const char *name)
 {
 	struct pbase_tree *it;
 	size_t cmplen;
-	unsigned hash = pack_name_hash(name);
+	unsigned hash = pack_name_hash_fn(name);
 
 	if (!num_preferred_base || check_pbase_path(hash))
 		return;
@@ -3422,7 +3444,7 @@ static void show_object_pack_hint(struct object *object, const char *name,
 	 * here using a now in order to perhaps improve the delta selection
 	 * process.
 	 */
-	oe->hash = pack_name_hash(name);
+	oe->hash = pack_name_hash_fn(name);
 	oe->no_try_delta = name && no_try_delta(name);
 
 	stdin_packs_hints_nr++;
@@ -3572,7 +3594,7 @@ static void add_cruft_object_entry(const struct object_id *oid, enum object_type
 	entry = packlist_find(&to_pack, oid);
 	if (entry) {
 		if (name) {
-			entry->hash = pack_name_hash(name);
+			entry->hash = pack_name_hash_fn(name);
 			entry->no_try_delta = no_try_delta(name);
 		}
 	} else {
@@ -3595,7 +3617,7 @@ static void add_cruft_object_entry(const struct object_id *oid, enum object_type
 			return;
 		}
 
-		entry = create_object_entry(oid, type, pack_name_hash(name),
+		entry = create_object_entry(oid, type, pack_name_hash_fn(name),
 					    0, name && no_try_delta(name),
 					    pack, offset);
 	}
@@ -4069,6 +4091,15 @@ static int get_object_list_from_bitmap(struct rev_info *revs)
 	if (!(bitmap_git = prepare_bitmap_walk(revs, 0)))
 		return -1;
 
+	/*
+	 * For now, force the name-hash version to be 1 since that
+	 * is the version implied by the bitmap format. Later, the
+	 * format can include this version explicitly in its format,
+	 * allowing readers to know the version that was used during
+	 * the bitmap write.
+	 */
+	name_hash_version = 1;
+
 	if (pack_options_allow_reuse())
 		reuse_partial_packfile_from_bitmap(bitmap_git,
 						   &reuse_packfiles,
@@ -4429,6 +4460,8 @@ int cmd_pack_objects(int argc,
 		OPT_STRING_LIST(0, "uri-protocol", &uri_protocols,
 				N_("protocol"),
 				N_("exclude any configured uploadpack.blobpackfileuri with this protocol")),
+		OPT_INTEGER(0, "name-hash-version", &name_hash_version,
+			 N_("use the specified name-hash function to group similar objects")),
 		OPT_END(),
 	};
 
@@ -4576,6 +4609,12 @@ int cmd_pack_objects(int argc,
 	if (pack_to_stdout || !rev_list_all)
 		write_bitmap_index = 0;
 
+	validate_name_hash_version();
+	if (write_bitmap_index && name_hash_version != 1) {
+		warning(_("currently, --write-bitmap-index requires --name-hash-version=1"));
+		name_hash_version = 1;
+	}
+
 	if (use_delta_islands)
 		strvec_push(&rp, "--topo-order");
 
diff --git a/builtin/repack.c b/builtin/repack.c
index d6bb37e84ae..05e13adb87f 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -58,6 +58,7 @@ struct pack_objects_args {
 	int no_reuse_object;
 	int quiet;
 	int local;
+	int full_name_hash;
 	struct list_objects_filter_options filter_options;
 };
 
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 3b9dae331a5..4270eabe8b7 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -674,4 +674,35 @@ do
 	'
 done
 
+test_expect_success 'valid and invalid --name-hash-versions' '
+	# Valid values are hard to verify other than "do not fail".
+	# Performance tests will be more valuable to validate these versions.
+	for value in 1 2
+	do
+		git pack-objects base --all --name-hash-version=$value || return 1
+	done &&
+
+	# Invalid values have clear post-conditions.
+	for value in -1 0 3
+	do
+		test_must_fail git pack-objects base --all --name-hash-version=$value 2>err &&
+		test_grep "invalid --name-hash-version option" err || return 1
+	done
+'
+
+# The following test is not necessarily a permanent choice, but since we do not
+# have a "name hash version" bit in the .bitmap file format, we cannot write the
+# hash values into the .bitmap file without risking breakage later.
+#
+# TODO: Make these compatible in the future and replace this test with the
+# expected behavior when both are specified.
+test_expect_success '--name-hash-version=2 and --write-bitmap-index are incompatible' '
+	git pack-objects base --all --name-hash-version=2 --write-bitmap-index 2>err &&
+	test_grep "currently, --write-bitmap-index requires --name-hash-version=1" err &&
+
+	# --stdout option silently removes --write-bitmap-index
+	git pack-objects --stdout --all --name-hash-version=2 --write-bitmap-index >out 2>err &&
+	! test_grep "currently, --write-bitmap-index requires --name-hash-version=1" err
+'
+
 test_done
-- 
gitgitgadget