pack-objects: add --path-walk option for better deltas #1813

In anticipation of a few planned applications, introduce the most basic form of a path-walk API. It currently assumes that there are no UNINTERESTING objects, and does not include any complicated filters. It calls a function pointer on groups of tree and blob objects as grouped by path. This only includes objects the first time they are discovered, so an object that appears at multiple paths will not be included in two batches. These batches are collected in 'struct type_and_oid_list' objects, which store an object type and an oid_array of objects. The data structures are documented in 'struct path_walk_context', but in summary the most important are: * 'paths_to_lists' is a strmap that connects a path to a type_and_oid_list for that path. To avoid conflicts in path names, we make sure that tree paths end in "/" (except the root path with is an empty string) and blob paths do not end in "/". * 'path_stack' is a string list that is added to in an append-only way. This stores the stack of our depth-first search on the heap instead of using recursion. * 'path_stack_pushed' is a strmap that stores path names that were already added to 'path_stack', to avoid repeating paths in the stack. Mostly, this saves us from quadratic lookups from doing unsorted checks into the string_list. The coupling of 'path_stack' and 'path_stack_pushed' is protected by the push_to_stack() method. Call this instead of inserting into these structures directly. The walk_objects_by_path() method initializes these structures and starts walking commits from the given rev_info struct. The commits are used to find the list of root trees which populate the start of our depth-first search. The core of our depth-first search is in a while loop that continues while we have not indicated an early exit and our 'path_stack' still has entries in it. The loop body pops a path off of the stack and "visits" the path via the walk_path() method. The walk_path() method gets the list of OIDs from the 'path_to_lists' strmap and executes the callback method on that list with the given path and type. If the OIDs correspond to tree objects, then iterate over all trees in the list and run add_children() to add the child objects to their own lists, adding new entries to the stack if necessary. In testing, this depth-first search approach was the one that used the least memory while iterating over the object lists. There is still a chance that repositories with too-wide path patterns could cause memory pressure issues. Limiting the stack size could be done in the future by limiting how many objects are being considered in-progress, or by visiting blob paths earlier than trees. There are many future adaptations that could be made, but they are left for future updates when consumers are ready to take advantage of those features. Signed-off-by: Derrick Stolee <[email protected]>

This test helper will be helpful to reduce repeated logic in t6601-path-walk.sh, but may be helpful elsewhere, too. Signed-off-by: Derrick Stolee <[email protected]>

Add some tests based on the current behavior, doing interesting checks for different sets of branches, ranges, and the --boundary option. This sets a baseline for the behavior and we can extend it as new options are introduced. Store and output a 'batch_nr' value so we can demonstrate that the paths are grouped together in a batch and not following some other ordering. This allows us to test the depth-first behavior of the path-walk API. However, we purposefully do not test the order of the objects in the batch, so the output is compared to the expected output through a sort. It is important to mention that the behavior of the API will change soon as we start to handle UNINTERESTING objects differently, but these tests will demonstrate the change in behavior. Signed-off-by: Derrick Stolee <[email protected]>

We add the ability to filter the object types in the path-walk API so the callback function is called fewer times. This adds the ability to ask for the commits in a list, as well. We re-use the empty string for this set of objects because these are passed directly to the callback function instead of being part of the 'path_stack'. Future changes will add the ability to visit annotated tags. Signed-off-by: Derrick Stolee <[email protected]>

The rev_info that is specified for a path-walk traversal may specify visiting tag refs (both lightweight and annotated) and also may specify indexed objects (blobs and trees). Update the path-walk API to walk these objects as well. When walking tags, we need to peel the annotated objects until reaching a non-tag object. If we reach a commit, then we can add it to the pending objects to make sure we visit in the commit walk portion. If we reach a tree, then we will assume that it is a root tree. If we reach a blob, then we have no good path name and so add it to a new list of "tagged blobs". When the rev_info includes the "--indexed-objects" flag, then the pending set includes blobs and trees found in the cache entries and cache-tree. The cache entries are usually blobs, though they could be trees in the case of a sparse index. The cache-tree stores previously-hashed tree objects but these are cleared out when staging objects below those paths. We add tests that demonstrate this. The indexed objects come with a non-NULL 'path' value in the pending item. This allows us to prepopulate the 'path_to_lists' strmap with lists for these paths. The tricky thing about this walk is that we will want to combine the indexed objects walk with the commit walk, especially in the future case of walking objects during a command like 'git repack'. Whenever possible, we want the objects from the index to be grouped with similar objects in history. We don't want to miss any paths that appear only in the index and not in the commit history. Thus, we need to be careful to let the path stack be populated initially with only the root tree path (and possibly tags and tagged blobs) and go through the normal depth-first search. Afterwards, if there are other paths that are remaining in the paths_to_lists strmap, we should then iterate through the stack and visit those objects recursively. Signed-off-by: Derrick Stolee <[email protected]>

When the input rev_info has UNINTERESTING starting points, we want to be sure that the UNINTERESTING flag is passed appropriately through the objects. To match how this is done in places such as 'git pack-objects', we use the mark_edges_uninteresting() method. This method has an option for using the "sparse" walk, which is similar in spirit to the path-walk API's walk. To be sure to keep it independent, add a new 'prune_all_uninteresting' option to the path_walk_info struct. To check how the UNINTERSTING flag is spread through our objects, extend the 'test-tool path-walk' command to output whether or not an object has that flag. This changes our tests significantly, including the removal of some objects that were previously visited due to the incomplete implementation. Signed-off-by: Derrick Stolee <[email protected]>

This will be helpful in a future change. Signed-off-by: Derrick Stolee <[email protected]>

In order to more easily compute delta bases among objects that appear at the exact same path, add a --path-walk option to 'git pack-objects'. This option will use the path-walk API instead of the object walk given by the revision machinery. Since objects will be provided in batches representing a common path, those objects can be tested for delta bases immediately instead of waiting for a sort of the full object list by name-hash. This has multiple benefits, including avoiding collisions by name-hash. The objects marked as UNINTERESTING are included in these batches, so we are guaranteeing some locality to find good delta bases. After the individual passes are done on a per-path basis, the default name-hash is used to find other opportunistic delta bases that did not match exactly by the full path name. The current implementation performs delta calculations while walking objects, which is not ideal for a few reasons. First, this will cause the "Enumerating objects" phase to be much longer than usual. Second, it does not take advantage of threading during the path-scoped delta calculations. Even with this lack of threading, the path-walk option is sometimes faster than the usual approach. Future changes will refactor this code to allow for threading, but that complexity is deferred until later to keep this patch as simple as possible. This new walk is incompatible with some features and is ignored by others: * Object filters are not currently integrated with the path-walk API, such as sparse-checkout or tree depth. A blobless packfile could be integrated easily, but that is deferred for later. * Server-focused features such as delta islands, shallow packs, and using a bitmap index are incompatible with the path-walk API. * The path walk API is only compatible with the --revs option, not taking object lists or pack lists over stdin. These alternative ways to specify the objects currently ignores the --path-walk option without even a warning. Future changes will create performance tests that demonstrate the power of this approach. Signed-off-by: Derrick Stolee <[email protected]>

The t0450 test script verifies that builtin usage matches the synopsis in the documentation. Adjust the builtin to match and then remove 'git pack-objects' from the exception list. Signed-off-by: Derrick Stolee <[email protected]>

The previous change added a --path-walk option to 'git pack-objects'. Create a performance test that demonstrates the time and space benefits of the feature. In order to get an appropriate comparison, we need to avoid reusing deltas and recompute them from scratch. Compare the creation of a thin pack representing a small push and the creation of a relatively large non-thin pack. Running on my copy of the Git repository results in this data: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.00(0.00+0.00) 5313.3: thin pack size 589 5313.4: thin pack with --path-walk 0.00(0.00+0.00) 5313.5: thin pack size with --path-walk 589 5313.6: big pack 2.76(7.19+0.27) 5313.7: big pack size 14.0M 5313.8: big pack with --path-walk 5.76(6.72+0.16) 5313.9: big pack size with --path-walk 13.2M Note that the timing is slower because there is no threading in the --path-walk case (yet). The cases where the --path-walk option really shines is when the default name-hash is overwhelmed with collisions. An open source example can be found in the microsoft/fluentui repo [1] at a certain commit [2]. [1] https://github.com/microsoft/fluentui [2] e70848ebac1cd720875bccaa3026f4a9ed700e08 Running the tests on this repo results in the following output: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.28(0.40+0.03) 5313.3: thin pack size 1.2M 5313.4: thin pack with --path-walk 0.07(0.06+0.00) 5313.5: thin pack size with --path-walk 18.4K 5313.6: big pack 4.05(29.48+0.41) 5313.7: big pack size 19.7M 5313.8: big pack with --path-walk 6.01(9.17+0.20) 5313.9: big pack size with --path-walk 16.5M Notice in particular that in the small thin pack, the time performance has improved from 0.28s to 0.08s and this is likely due to the improved size of the resulting pack: 18.4K instead of 1.2M. Finally, running this on a copy of the Linux kernel repository results in these data points: Test HEAD -------------------------------------------------------------- 5313.2: thin pack 0.03(0.02+0.00) 5313.3: thin pack size 4.6K 5313.4: thin pack with --path-walk 0.03(0.01+0.01) 5313.5: thin pack size with --path-walk 4.6K 5313.6: big pack 21.06(60.57+1.45) 5313.7: big pack size 158.3M 5313.8: big pack with --path-walk 37.65(57.83+0.67) 5313.9: big pack size with --path-walk 152.3M Signed-off-by: Derrick Stolee <[email protected]>

There are many tests that validate whether 'git pack-objects' works as expected. Instead of duplicating these tests, add a new test environment variable, GIT_TEST_PACK_PATH_WALK, that implies --path-walk by default when specified. This was useful in testing the implementation of the --path-walk implementation, especially in conjunction with test such as: - t0411-clone-from-partial.sh : One test fetches from a repo that does not have the boundary objects. This causes the path-based walk to fail. Disable the variable for this test. - t5306-pack-nobase.sh : Similar to t0411, one test fetches from a repo without a boundary object. - t5310-pack-bitmaps.sh : One test compares the case when packing with bitmaps to the case when packing without them. Since we disable the test variable when writing bitmaps, this causes a difference in the object list (the --path-walk option adds an extra object). Specify --no-path-walk in both processes for the comparison. Another test checks for a specific delta base, but when computing dynamically without using bitmaps, the base object it too small to be considered in the delta calculations so no base is used. - t5316-pack-delta-depth.sh : This script cares about certain delta choices and their chain lengths. The --path-walk option changes how these chains are selected, and thus changes the results of this test. - t5322-pack-objects-sparse.sh : This demonstrates the effectiveness of the --sparse option and how it combines with --path-walk. - t5332-multi-pack-reuse.sh : This test verifies that the preferred pack is used for delta reuse when possible. The --path-walk option is not currently aware of the preferred pack at all, so finds a different delta base. - t7406-submodule-update.sh : When using the variable, the --depth option collides with the --path-walk feature, resulting in a warning message. Disable the variable so this warning does not appear. I want to call out one specific test change that is only temporary: - t5530-upload-pack-error.sh : One test cares specifically about an "unable to read" error message. Since the current implementation performs delta calculations within the path-walk API callback, a different "unable to get size" error message appears. When this is changed in a future refactoring, this test change can be reverted. Signed-off-by: Derrick Stolee <[email protected]>

It can be notoriously difficult to detect if delta bases are being computed properly during 'git push'. Construct an example where it will make a kilobyte worth of difference when a delta base is not found. We can then use the progress indicators to distinguish between bytes and KiB depending on whether the delta base is found and used. Signed-off-by: Derrick Stolee <[email protected]>

Since 'git pack-objects' supports a --path-walk option, allow passing it through in 'git repack'. This presents interesting testing opportunities for comparing the different repacking strategies against each other. Add the --path-walk option to the performance tests in p5313. For the microsoft/fluentui repo [1] checked out at a specific commit [2], the results are very interesting: Test HEAD ---------------------------------------------------------- 5313.2: thin pack 0.41(0.48+0.03) 5313.3: thin pack size 1.2M 5313.4: thin pack with --path-walk 0.08(0.05+0.02) 5313.5: thin pack size with --path-walk 18.4K 5313.6: big pack 4.47(30.62+0.40) 5313.7: big pack size 19.6M 5313.8: big pack with --path-walk 6.76(9.87+0.23) 5313.9: big pack size with --path-walk 16.5M 5313.10: repack 96.87(664.29+2.75) 5313.11: repack size 439.5M 5313.12: repack with --path-walk 95.68(109.90+0.92) 5313.13: repack size with --path-walk 122.6M [1] https://github.com/microsoft/fluentui [2] e70848ebac1cd720875bccaa3026f4a9ed700e08 This repo suffers from having a lot of paths that collide in the name hash, so examining them in groups by path leads to better deltas. Also, in this case, the single-threaded implementation is competitive with the full repack. This is saving time diffing files that have significant differences from each other. A similar, but private, repo has even more extremes during repacking: Test HEAD ----------------------------------------------------------------- 5313.10: repack 2138.22(11961.00+17.67) 5313.11: repack size 6.4G 5313.12: repack with --path-walk 1351.46(1418.28+3.96) 5313.13: repack size with --path-walk 804.1M There are small benefits in size for my copy of the Git repository: Test HEAD ----------------------------------------------------------- 5313.10: repack 22.11(98.37+1.64) 5313.11: repack size 126.4M 5313.12: repack with --path-walk 66.89(75.61+0.58) 5313.13: repack size with --path-walk 109.6M As well as in the nodejs/node repository [3]: Test HEAD ------------------------------------------------------------- 5313.2: thin pack 0.01(0.01+0.00) 5313.3: thin pack size 1.6K 5313.4: thin pack with --path-walk 0.02(0.01+0.00) 5313.5: thin pack size with --path-walk 1.6K 5313.6: big pack 5.35(12.43+0.32) 5313.7: big pack size 52.2M 5313.8: big pack with --path-walk 7.12(11.97+0.27) 5313.9: big pack size with --path-walk 52.1M 5313.10: repack 87.74(342.90+4.24) 5313.11: repack size 739.7M 5313.12: repack with --path-walk 212.79(245.05+1.78) 5313.13: repack size with --path-walk 697.6M [3] https://github.com/nodejs/node This benefit also repeats in this instance in the Linux kernel repository: Test HEAD --------------------------------------------------------------- 5313.2: thin pack 0.04(0.00+0.03) 5313.3: thin pack size 4.6K 5313.4: thin pack with --path-walk 0.03(0.01+0.01) 5313.5: thin pack size with --path-walk 4.6K 5313.6: big pack 21.16(62.81+1.35) 5313.7: big pack size 158.3M 5313.8: big pack with --path-walk 36.09(55.25+0.67) 5313.9: big pack size with --path-walk 152.2M 5313.10: repack 734.26(2149.62+31.24) 5313.11: repack size 2.5G 5313.12: repack with --path-walk 1457.23(1618.15+7.00) 5313.13: repack size with --path-walk 2.2G It is important to see that even when the repository shape does not have many name-hash collisions, there is a slight space boost to be found using this method. Also, there is no known case where the space is worse with --path-walk. This is of course due to the second pass where all objects to be packed are sorted in the usual way and checked for deltas. This second pass is usually very fast as the path-walk has primed many objects with quality deltas that short-circuit other delta computation attempts. Signed-off-by: Derrick Stolee <[email protected]>

The t0450 test script verifies that the builtin usage matches the synopsis in the documentation. Update 'git repack' to match and remove it from the exception list. Signed-off-by: Derrick Stolee <[email protected]>

Users may want to enable the --path-walk option for 'git pack-objects' by default, especially underneath commands like 'git push' or 'git repack'. This should be limited to client repositories, since the --path-walk option disables bitmap walks, so would be bad to include in Git servers when serving fetches and clones. There is potential that it may be helpful to consider when repacking the repository, to take advantage of improved deltas across historical versions of the same files. Much like how "pack.useSparse" was introduced and included in "feature.experimental" before being enabled by default, use the repository settings infrastructure to make the new "pack.usePathWalk" config enabled by "feature.experimental" and "feature.manyFiles". Signed-off-by: Derrick Stolee <[email protected]>

Repositories registered with Scalar are expected to be client-only repositories that are rather large. This means that they are more likely to be good candidates for using the --path-walk option when running 'git pack-objects', especially under the hood of 'git push'. Enable this config in Scalar repositories. Signed-off-by: Derrick Stolee <[email protected]>

Previously, the --path-walk option to 'git pack-objects' would compute deltas inline with the path-walk logic. This would make the progress indicator look like it is taking a long time to enumerate objects, and then very quickly computed deltas. Instead of computing deltas on each region of objects organized by tree, store a list of regions corresponding to these groups. These can later be pulled from the list for delta compression before doing the "global" delta search. This presents a new progress indicator that can be used in tests to verify that this stage is happening. The current implementation is not integrated with threads, but could be done in a future update. Since we do not attempt to sort objects by size until after exploring all trees, we can remove the previous change to t5530 due to a different error message appearing first. Signed-off-by: Derrick Stolee <[email protected]>

Adapting the implementation of ll_find_deltas(), create a threaded version of the --path-walk compression step in 'git pack-objects'. This involves adding a 'regions' member to the thread_params struct, allowing each thread to own a section of paths. We can simplify the way jobs are split because there is no value in extending the batch based on name-hash the way sections of the object entry array are attempted to be grouped. We re-use the 'list_size' and 'remaining' items for the purpose of borrowing work in progress from other "victim" threads when a thread has finished its batch of work more quickly. Using the Git repository as a test repo, the p5313 performance test shows that the resulting size of the repo is the same, but the threaded implementation gives gains of varying degrees depending on the number of objects being packed. (This was tested on a 16-core machine.) Test HEAD~1 HEAD ----------------------------------------------------------------- 5313.2: thin pack 0.00 0.00 = 5313.3: thin pack size 589 589 +0.0% 5313.4: thin pack with --path-walk 0.00 0.00 = 5313.5: thin pack size with --path-walk 589 589 +0.0% 5313.6: big pack 2.84 2.80 -1.4% 5313.7: big pack size 14.0M 14.1M +0.3% 5313.8: big pack with --path-walk 5.46 3.77 -31.0% 5313.9: big pack size with --path-walk 13.2M 13.2M -0.0% 5313.10: repack 22.11 21.50 -2.8% 5313.11: repack size 126.4M 126.2M -0.2% 5313.12: repack with --path-walk 66.89 26.41 -60.5% 5313.13: repack size with --path-walk 109.6M 109.6M +0.0% This 60% reduction in 'git repack --path-walk' time is typical across all repos I used for testing. What is interesting is to compare when the overall time improves enough to outperform the standard case. These time improvements correlate with repositories with data shapes that significantly improve their data size as well. For example, the microsoft/fluentui repo has a 439M to 122M size reduction, and the repack time is now 36.6 seconds with --path-walk compared to 95+ seconds without it: Test HEAD~! HEAD ----------------------------------------------------------------- 5313.2: thin pack 0.41 0.42 +2.4% 5313.3: thin pack size 1.2M 1.2M +0.0% 5313.4: thin pack with --path-walk 0.08 0.05 -37.5% 5313.5: thin pack size with --path-walk 18.4K 18.4K +0.0% 5313.6: big pack 4.47 4.53 +1.3% 5313.7: big pack size 19.6M 19.7M +0.3% 5313.8: big pack with --path-walk 6.76 3.51 -48.1% 5313.9: big pack size with --path-walk 16.5M 16.4M -0.2% 5313.10: repack 96.87 99.05 +2.3% 5313.11: repack size 439.5M 439.0M -0.1% 5313.12: repack with --path-walk 95.68 36.55 -61.8% 5313.13: repack size with --path-walk 122.6M 122.6M +0.0% In a more extreme example, an internal repository that has a similar name-hash collision issue to microsoft/fluentui reduces its size from 6.4G to 805M with the --path-walk option. This also reduces the repacking time from 2,138 seconds to 478 seconds. Test HEAD~1 HEAD ------------------------------------------------------------------ 5313.10: repack 2138.22 2138.19 -0.0% 5313.11: repack size 6.4G 6.4G -0.0% 5313.12: repack with --path-walk 1351.46 477.91 -64.6% 5313.13: repack size with --path-walk 804.1M 804.1M -0.0% Finally, the Linux kernel repository is a good test for this repacking time change, even though the space savings is more reasonable: Test HEAD~1 HEAD ---------------------------------------------------------------- 5313.10: repack 734.26 735.11 +0.1% 5313.11: repack size 2.5G 2.5G -0.0% 5313.12: repack with --path-walk 1457.23 598.17 -59.0% 5313.13: repack size with --path-walk 2.2G 2.2G +0.0% Signed-off-by: Derrick Stolee <[email protected]>

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pack-objects: add --path-walk option for better deltas #1813

pack-objects: add --path-walk option for better deltas #1813

Commits on Nov 8, 2024

pack-objects: add --path-walk option for better deltas #1813

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pack-objects: add --path-walk option for better deltas #1813

Commits on Nov 8, 2024