[PATCH 1/7] pack-objects: add --full-name-hash option

From: Derrick Stolee <stolee@gmail.com>

The pack_name_hash() method has not been materially changed since it was introduced in ce0bd64299a (pack-objects: improve path grouping heuristics., 2006-06-05). The intention here is to group objects by path name, but also attempt to group similar file types together by making the most-significant digits of the hash be focused on the final characters.

Here's the crux of the implementation:

	/*
	 * This effectively just creates a sortable number from the
	 * last sixteen non-whitespace characters. Last characters
	 * count "most", so things that end in ".c" sort together.
	 */
	while ((c = *name++) != 0) {
		if (isspace(c))
			continue;
		hash = (hash >> 2) + (c << 24);
	}

As the comment mentions, this only cares about the last sixteen non-whitespace characters. This cause some filenames to collide more than others. Here are some examples that I've seen while investigating repositories that are growing more than they should be:

 * "/CHANGELOG.json" is 15 characters, and is created by the beachball
   [1] tool. Only the final character of the parent directory can
   differntiate 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 differentiate
   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.

It is clear that the existing name-hash algorithm is optimized for repositories with short path names, but also is optimized for packing a single snapshot of a repository, not a repository with many versions of the same file. In my testing, this has proven out where the name-hash algorithm does a good job of finding peer files as delta bases when unable to use a historical version of that exact file.

However, for repositories that have many versions of most files and directories, it is more important that the objects that appear at the same path are grouped together.

Create a new pack_full_name_hash() method and a new --full-name-hash option for 'git pack-objects' to call that method instead. Add a simple pass-through for 'git repack --full-name-hash' for additional testing in the context of a full repack, where I expect this will be most effective.

The hash algorithm is as simple as possible to be reasonably effective: for each character of the path string, add a multiple of that character and a large prime number (chosen arbitrarily, but intended to be large relative to the size of a uint32_t). Then, shift the current hash value to the right by 5, with overlap. The addition and shift parameters are standard mechanisms for creating hard-to-predict behaviors in the bits of the resulting hash.

This is not meant to be cryptographic at all, but uniformly distributed across the possible hash values. This creates a hash that appears pseudorandom. There is no ability to consider similar file types as being close to each other.

In a later change, a test-tool will be added so the effectiveness of this hash can be demonstrated directly.

For now, let's consider how effective this mechanism is when repacking a repository with and without the --full-name-hash option. Specifically, let's use 'git repack -adf [--full-name-hash]' as our test.

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         |
| Standard Repack       | 127MB     | 106s        |
| With --full-name-hash | 126 MB    | 99s         |

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 with and without --full-name-hash.

However, we can test this in a repository that uses one of the problematic naming conventions above. The fluentui [2] repo 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         |
| Standard Repack       | 438 MB    | 728s        |
| With --full-name-hash | 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     | Standard Repack | With --full-name-hash |
|----------|-----------------|-----------------------|
| fluentui |         438 MB  |               168 MB  |
| Repo B   |       6,255 MB  |               829 MB  |
| Repo C   |      37,737 MB  |             7,125 MB  |
| Repo D   |     130,049 MB  |             6,190 MB  |

Future changes could include making --full-name-hash 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 disable the --full-name-hash option 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 <stolee@gmail.com>
---
 Documentation/git-pack-objects.txt |  3 ++-
 builtin/pack-objects.c             | 25 ++++++++++++++++++++-----
 builtin/repack.c                   |  5 +++++
 pack-objects.h                     | 21 +++++++++++++++++++++
 t/t5300-pack-object.sh             | 15 +++++++++++++++
 5 files changed, 63 insertions(+), 6 deletions(-)