Here is a full submission of the --path-walk feature for 'git pack-objects' and 'git repack'. It's been discussed in an RFC [1], as a future application for the path walk API [2], and is updated now that --name-hash-version=2 exists (as a replacement for the --full-name-hash option from the RFC) [3]. [1] https://lore.kernel.org/git/pull.1813.v2.git.1729431810.gitgitgadget@xxxxxxxxx/ [2] https://lore.kernel.org/git/pull.1818.git.1730356023.gitgitgadget@xxxxxxxxx [3] https://lore.kernel.org/git/pull.1813.git.1728396723.gitgitgadget@xxxxxxxxx This patch series does the following: 1. Add a new '--path-walk' option to 'git pack-objects' that uses the path-walk API instead of the revision API to collect objects for delta compression. 2. Add a new '--path-walk' option to 'git repack' to pass this option along to 'git pack-objects'. 3. Add a new 'pack.usePathWalk' config option to opt into this option implicitly, such as in 'git push'. 4. Optimize the '--path-walk' option using threading so it better competes with the existing multi-threaded delta compression mechanism. 5. Update the path-walk API with a new 'edge_aggressive' option that pairs close to the --edge-aggressive option in the revision API. This is useful when creating thin packs inside shallow clones. This feature works by using the path-walk API to emit groups of objects that appear at the same path. These groups are tracked so they can be tested for delta compression with each other, and then after those groups are tested a second pass using the name-hash attempts to find better (or first time) deltas across path boundaries. This second pass is much faster than a fresh pass since the existing deltas are used as a limit for the size of potentially new deltas, short-circuiting the checks when the delta size exceeds the current-best. The benefits of the --path-walk feature first come into play when the name hash functions have many collisions, so sorting by name hash value leads to unhelpful groupings of objects. Many of these benefits are improved by --name-hash-version=2, but collisions still exist with any hash-based approach. There are also performance benefits in some cases due to the isolation of delta compression testing within path groups. All of the benefits of the --path-walk feature are less dramatic when compared to --name-hash-version=2, but they can still exist in many cases. I have also seen some cases where --name-hash-version=2 compresses better than --path-walk with --name-hash-version=1, but these options can be combined to get the best of both worlds. Detailed statistics are provided within patch messages, but a few are highlighted here: The microsoft/fluentui is a public Javascript repo that suffers from many of the name hash collisions as internal repositories I've worked with. Here is a comparison of the compressed size and end-to-end time of the repack: Repack Method Pack Size Time --------------------------------------- Hash v1 439.4M 87.24s Hash v2 161.7M 21.51s Path Walk 142.5M 28.16s Less dramatic, but perhaps more standardly structured is the nodejs/node repository, with these stats: Repack Method Pack Size Time ------------------------------------------ Hash v1 739.9M 71.18s Hash v2 764.6M 67.82s Path Walk 698.0M 75.10s Even the Linux kernel repository gains some benefits, even though the number of hash collisions is relatively low due to a preference for short filenames: Repack Method Pack Size Time ------------------------------------------ Hash v1 2.5G 554.41s Hash v2 2.5G 549.62s Path Walk 2.2G 559.00s The drawbacks of the --path-walk feature is that it will be harder to integrate it with bitmap features, specifically delta islands. This is not insurmountable, but would require more work, such as a revision walk to paint objects with reachability information before using that during delta computations. However, there should still be significant benefits to Git clients trying to save space and improve local performance. This feature was shipped with similar features in microsoft/git as of v2.47.0.vfs.0.3 [4]. This was used in CI machines for an internal monorepo that had significant repository growth due to constructing a batch of beachball [5] CHANGELOG.[md|json] files and pushing them to a release branch. These pushes were frequently 70-200 MB due to poor delta compression. Using the 'pack.usePathWalk=true' config, these pushes dropped in size by 100x while improving performance. Since these CI machines were working with a shallow clone, the 'edge_aggressive' changes were required to enable the path-walk option. [4] https://github.com/microsoft/git/releases/tag/v2.47.0.vfs.0.3 [5] https://github.com/microsoft/beachball This version incorporates feedback from previous RFCs and reviewed patch series whenever possible. It also benefits from learned experience, much of which was already applied in the original path-walk API submission. Thanks, -Stolee Derrick Stolee (13): pack-objects: extract should_attempt_deltas() pack-objects: add --path-walk option pack-objects: update usage to match docs p5313: add performance tests for --path-walk pack-objects: introduce GIT_TEST_PACK_PATH_WALK t5538: add tests to confirm deltas in shallow pushes repack: add --path-walk option pack-objects: enable --path-walk via config scalar: enable path-walk during push via config pack-objects: refactor path-walk delta phase pack-objects: thread the path-based compression path-walk: add new 'edge_aggressive' option pack-objects: allow --shallow and --path-walk Documentation/config/feature.adoc | 4 + Documentation/config/pack.adoc | 8 + Documentation/git-pack-objects.adoc | 25 +- Documentation/git-repack.adoc | 14 +- Documentation/technical/api-path-walk.adoc | 9 + builtin/pack-objects.c | 411 +++++++++++++++++++-- builtin/repack.c | 7 +- pack-objects.h | 12 + path-walk.c | 6 +- path-walk.h | 7 + repo-settings.c | 3 + repo-settings.h | 1 + scalar.c | 1 + t/README | 4 + t/helper/test-path-walk.c | 2 + t/perf/p5313-pack-objects.sh | 37 +- t/t0411-clone-from-partial.sh | 6 + t/t0450/adoc-help-mismatches | 1 - t/t5300-pack-object.sh | 19 + t/t5306-pack-nobase.sh | 5 + t/t5310-pack-bitmaps.sh | 13 +- t/t5316-pack-delta-depth.sh | 9 +- t/t5332-multi-pack-reuse.sh | 7 + t/t5538-push-shallow.sh | 34 ++ t/t6601-path-walk.sh | 20 + t/t7406-submodule-update.sh | 3 + 26 files changed, 601 insertions(+), 67 deletions(-) base-commit: a36e024e989f4d35f35987a60e3af8022cac3420 Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1819%2Fderrickstolee%2Fpath-walk-upstream-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1819/derrickstolee/path-walk-upstream-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/1819 -- gitgitgadget
