master
5 Commits
| Author | SHA1 | Message | Date | |
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Derrick Stolee
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bff4555767 |
backfill: add --sparse option
One way to significantly reduce the cost of a Git clone and later fetches is to use a blobless partial clone and combine that with a sparse-checkout that reduces the paths that need to be populated in the working directory. Not only does this reduce the cost of clones and fetches, the sparse-checkout reduces the number of objects needed to download from a promisor remote. However, history investigations can be expensive as computing blob diffs will trigger promisor remote requests for one object at a time. This can be avoided by downloading the blobs needed for the given sparse-checkout using 'git backfill' and its new '--sparse' mode, at a time that the user is willing to pay that extra cost. Note that this is distinctly different from the '--filter=sparse:<oid>' option, as this assumes that the partial clone has all reachable trees and we are using client-side logic to avoid downloading blobs outside of the sparse-checkout cone. This avoids the server-side cost of walking trees while also achieving a similar goal. It also downloads in batches based on similar path names, presenting a resumable download if things are interrupted. This augments the path-walk API to have a possibly-NULL 'pl' member that may point to a 'struct pattern_list'. This could be more general than the sparse-checkout definition at HEAD, but 'git backfill --sparse' is currently the only consumer. Be sure to test this in both cone mode and not cone mode. Cone mode has the benefit that the path-walk can skip certain paths once they would expand beyond the sparse-checkout. Non-cone mode can describe the included files using both positive and negative patterns, which changes the possible return values of path_matches_pattern_list(). Test both kinds of matches for increased coverage. To test this, we can create a blobless sparse clone, expand the sparse-checkout slightly, and then run 'git backfill --sparse' to see how much data is downloaded. The general steps are 1. git clone --filter=blob:none --sparse <url> 2. git sparse-checkout set <dir1> ... <dirN> 3. git backfill --sparse For the Git repository with the 'builtin' directory in the sparse-checkout, we get these results for various batch sizes: | Batch Size | Pack Count | Pack Size | Time | |-----------------|------------|-----------|-------| | (Initial clone) | 3 | 110 MB | | | 10K | 12 | 192 MB | 17.2s | | 15K | 9 | 192 MB | 15.5s | | 20K | 8 | 192 MB | 15.5s | | 25K | 7 | 192 MB | 14.7s | This case matters less because a full clone of the Git repository from GitHub is currently at 277 MB. Using a copy of the Linux repository with the 'kernel/' directory in the sparse-checkout, we get these results: | Batch Size | Pack Count | Pack Size | Time | |-----------------|------------|-----------|------| | (Initial clone) | 2 | 1,876 MB | | | 10K | 11 | 2,187 MB | 46s | | 25K | 7 | 2,188 MB | 43s | | 50K | 5 | 2,194 MB | 44s | | 100K | 4 | 2,194 MB | 48s | This case is more meaningful because a full clone of the Linux repository is currently over 6 GB, so this is a valuable way to download a fraction of the repository and no longer need network access for all reachable objects within the sparse-checkout. Choosing a batch size will depend on a lot of factors, including the user's network speed or reliability, the repository's file structure, and how many versions there are of the file within the sparse-checkout scope. There will not be a one-size-fits-all solution. Signed-off-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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Derrick Stolee
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6333e7ae0b |
path-walk: mark trees and blobs as UNINTERESTING
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 <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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Derrick Stolee
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9145660979 |
path-walk: visit tags and cached objects
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 <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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Derrick Stolee
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c8dba310d7 |
path-walk: allow consumer to specify object types
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 <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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Derrick Stolee
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9d46bc791b |
path-walk: introduce an object walk by path
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 <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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