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Mon, 24 Mar 2025 08:22:52 -0700 (PDT) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43d4fceae03sm128710235e9.8.2025.03.24.08.22.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Mar 2025 08:22:52 -0700 (PDT) Message-Id: In-Reply-To: References: From: "Derrick Stolee via GitGitGadget" Date: Mon, 24 Mar 2025 15:22:36 +0000 Subject: [PATCH v2 00/13] PATH WALK II: Add --path-walk option to 'git pack-objects' Fcc: Sent Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 To: git@vger.kernel.org Cc: christian.couder@gmail.com, gitster@pobox.com, johannes.schindelin@gmx.de, johncai86@gmail.com, jonathantanmy@google.com, karthik.188@gmail.com, kristofferhaugsbakk@fastmail.com, me@ttaylorr.com, newren@gmail.com, peff@peff.net, ps@pks.im, Derrick Stolee 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@gmail.com/ [2] https://lore.kernel.org/git/pull.1818.git.1730356023.gitgitgadget@gmail.com [3] https://lore.kernel.org/git/pull.1813.git.1728396723.gitgitgadget@gmail.com 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 Updates in v2 ============= * Re-added a dropped comment when moving code in patch 1. * Updated documentation to include interaction with --use-bitmap-index. * An UNUSED parameter is now used, reducing the use of global variables slightly. 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 | 26 +- Documentation/git-repack.adoc | 14 +- Documentation/technical/api-path-walk.adoc | 9 + builtin/pack-objects.c | 414 +++++++++++++++++++-- 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, 605 insertions(+), 67 deletions(-) base-commit: a36e024e989f4d35f35987a60e3af8022cac3420 Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1819%2Fderrickstolee%2Fpath-walk-upstream-v2 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1819/derrickstolee/path-walk-upstream-v2 Pull-Request: https://github.com/gitgitgadget/git/pull/1819 Range-diff vs v1: 1: a2ed1f2d4e3 ! 1: 57c1cc20de0 pack-objects: extract should_attempt_deltas() @@ builtin/pack-objects.c: static int add_ref_tag(const char *tag UNUSED, const cha +static int should_attempt_deltas(struct object_entry *entry) +{ + if (DELTA(entry)) ++ /* This happens if we decided to reuse existing ++ * delta from a pack. "reuse_delta &&" is implied. ++ */ + return 0; + + if (!entry->type_valid || 2: 9b31dc87bb6 ! 2: a271d6245c2 pack-objects: add --path-walk option @@ Documentation/git-pack-objects.adoc: many different directories. At the moment, + especially in the presence of filenames that cause collisions in + Git's default name-hash algorithm. Due to changing how the objects + are walked, this option is not compatible with `--delta-islands`, -+ `--shallow`, or `--filter`. ++ `--shallow`, or `--filter`. The `--use-bitmap-index` option will ++ be ignored in the presence of `--path-walk.` + DELTA ISLANDS 3: bc678acb109 = 3: dcff01392ff pack-objects: update usage to match docs 4: de848ebff74 = 4: 97a0b52ccee p5313: add performance tests for --path-walk 5: 4bfd438b5fa = 5: 0d49bb3d30a pack-objects: introduce GIT_TEST_PACK_PATH_WALK 6: cfee2136e92 = 6: ddf804e606a t5538: add tests to confirm deltas in shallow pushes 7: 1e75f068281 = 7: 11767e7653e repack: add --path-walk option 8: 073cb44a47c = 8: dd66a5b46f2 pack-objects: enable --path-walk via config 9: fd9a335329e = 9: e5624c379d5 scalar: enable path-walk during push via config 10: c047fbd7f27 ! 10: 622439d7855 pack-objects: refactor path-walk delta phase @@ builtin/pack-objects.c: static int should_attempt_deltas(struct object_entry *en return 1; } -+static void find_deltas_for_region(struct object_entry *list UNUSED, ++static void find_deltas_for_region(struct object_entry *list, + struct packing_region *region, + unsigned int *processed) +{ @@ builtin/pack-objects.c: static int should_attempt_deltas(struct object_entry *en + + ALLOC_ARRAY(delta_list, region->nr); + for (uint32_t i = 0; i < region->nr; i++) { -+ struct object_entry *entry = to_pack.objects + region->start + i; ++ struct object_entry *entry = list + region->start + i; + if (should_attempt_deltas(entry)) + delta_list[delta_list_nr++] = entry; + } @@ pack-objects.h: struct object_entry { uint32_t nr_objects, nr_alloc; + struct packing_region *regions; -+ uint32_t nr_regions, nr_regions_alloc; ++ uint64_t nr_regions, nr_regions_alloc; + int32_t *index; uint32_t index_size; 11: cf990b90b34 = 11: ae73d26319a pack-objects: thread the path-based compression 12: bc9383ef11c = 12: c45f35c204e path-walk: add new 'edge_aggressive' option 13: 2eb92507213 = 13: d5484ebd942 pack-objects: allow --shallow and --path-walk