commit fc62e033cd93ff6b93e312d89bfb5683a4c6f90c
author Derrick Stolee <stolee@gmail.com> Mon Jan 27 19:02:29 2025 +0000
committer Junio C Hamano <gitster@pobox.com> Mon Jan 27 13:21:41 2025 -0800
tree bbb36212fc126dffc1bde3a2b0a0641f3047f82d
parent dca924b4508e3147e82be5bba10cfc8aefa7ecf0

pack-objects: add --name-hash-version option

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. To promote the safety of
this parameter, the validate_name_hash_version() method will die() if
the given name-hash version is incorrect and will disable newer versions
if not yet compatible with other features, such as --write-bitmap-index.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>