From 0ad6090bddbf01cb5778dec726fe8e799d340a6f Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Tue, 30 Mar 2021 13:10:44 +0000 Subject: sparse-index: design doc and format update This begins a long effort to update the index format to allow sparse directory entries. This should result in a significant improvement to Git commands when HEAD contains millions of files, but the user has selected many fewer files to keep in their sparse-checkout definition. Currently, the index format is only updated in the presence of extensions.sparseIndex instead of increasing a file format version number. This is temporary, and index v5 is part of the plan for future work in this area. The design document details many of the reasons for embarking on this work, and also the plan for completing it safely. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/technical/index-format.txt | 7 ++ Documentation/technical/sparse-index.txt | 175 +++++++++++++++++++++++++++++++ 2 files changed, 182 insertions(+) create mode 100644 Documentation/technical/sparse-index.txt (limited to 'Documentation/technical') diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt index d363a71c37..3b74c05647 100644 --- a/Documentation/technical/index-format.txt +++ b/Documentation/technical/index-format.txt @@ -44,6 +44,13 @@ Git index format localization, no special casing of directory separator '/'). Entries with the same name are sorted by their stage field. + An index entry typically represents a file. However, if sparse-checkout + is enabled in cone mode (`core.sparseCheckoutCone` is enabled) and the + `extensions.sparseIndex` extension is enabled, then the index may + contain entries for directories outside of the sparse-checkout definition. + These entries have mode `040000`, include the `SKIP_WORKTREE` bit, and + the path ends in a directory separator. + 32-bit ctime seconds, the last time a file's metadata changed this is stat(2) data diff --git a/Documentation/technical/sparse-index.txt b/Documentation/technical/sparse-index.txt new file mode 100644 index 0000000000..8d3d808046 --- /dev/null +++ b/Documentation/technical/sparse-index.txt @@ -0,0 +1,175 @@ +Git Sparse-Index Design Document +================================ + +The sparse-checkout feature allows users to focus a working directory on +a subset of the files at HEAD. The cone mode patterns, enabled by +`core.sparseCheckoutCone`, allow for very fast pattern matching to +discover which files at HEAD belong in the sparse-checkout cone. + +Three important scale dimensions for a Git working directory are: + +* `HEAD`: How many files are present at `HEAD`? + +* Populated: How many files are within the sparse-checkout cone. + +* Modified: How many files has the user modified in the working directory? + +We will use big-O notation -- O(X) -- to denote how expensive certain +operations are in terms of these dimensions. + +These dimensions are ordered by their magnitude: users (typically) modify +fewer files than are populated, and we can only populate files at `HEAD`. + +Problems occur if there is an extreme imbalance in these dimensions. For +example, if `HEAD` contains millions of paths but the populated set has +only tens of thousands, then commands like `git status` and `git add` can +be dominated by operations that require O(`HEAD`) operations instead of +O(Populated). Primarily, the cost is in parsing and rewriting the index, +which is filled primarily with files at `HEAD` that are marked with the +`SKIP_WORKTREE` bit. + +The sparse-index intends to take these commands that read and modify the +index from O(`HEAD`) to O(Populated). To do this, we need to modify the +index format in a significant way: add "sparse directory" entries. + +With cone mode patterns, it is possible to detect when an entire +directory will have its contents outside of the sparse-checkout definition. +Instead of listing all of the files it contains as individual entries, a +sparse-index contains an entry with the directory name, referencing the +object ID of the tree at `HEAD` and marked with the `SKIP_WORKTREE` bit. +If we need to discover the details for paths within that directory, we +can parse trees to find that list. + +At time of writing, sparse-directory entries violate expectations about the +index format and its in-memory data structure. There are many consumers in +the codebase that expect to iterate through all of the index entries and +see only files. In fact, these loops expect to see a reference to every +staged file. One way to handle this is to parse trees to replace a +sparse-directory entry with all of the files within that tree as the index +is loaded. However, parsing trees is slower than parsing the index format, +so that is a slower operation than if we left the index alone. The plan is +to make all of these integrations "sparse aware" so this expansion through +tree parsing is unnecessary and they use fewer resources than when using a +full index. + +The implementation plan below follows four phases to slowly integrate with +the sparse-index. The intention is to incrementally update Git commands to +interact safely with the sparse-index without significant slowdowns. This +may not always be possible, but the hope is that the primary commands that +users need in their daily work are dramatically improved. + +Phase I: Format and initial speedups +------------------------------------ + +During this phase, Git learns to enable the sparse-index and safely parse +one. Protections are put in place so that every consumer of the in-memory +data structure can operate with its current assumption of every file at +`HEAD`. + +At first, every index parse will call a helper method, +`ensure_full_index()`, which scans the index for sparse-directory entries +(pointing to trees) and replaces them with the full list of paths (with +blob contents) by parsing tree objects. This will be slower in all cases. +The only noticeable change in behavior will be that the serialized index +file contains sparse-directory entries. + +To start, we use a new required index extension, `sdir`, to allow +inserting sparse-directory entries into indexes with file format +versions 2, 3, and 4. This prevents Git versions that do not understand +the sparse-index from operating on one, while allowing tools that do not +understand the sparse-index to operate on repositories as long as they do +not interact with the index. A new format, index v5, will be introduced +that includes sparse-directory entries by default. It might also +introduce other features that have been considered for improving the +index, as well. + +Next, consumers of the index will be guarded against operating on a +sparse-index by inserting calls to `ensure_full_index()` or +`expand_index_to_path()`. After these guards are in place, we can begin +leaving sparse-directory entries in the in-memory index structure. + +Even after inserting these guards, we will keep expanding sparse-indexes +for most Git commands using the `command_requires_full_index` repository +setting. This setting will be on by default and disabled one builtin at a +time until we have sufficient confidence that all of the index operations +are properly guarded. + +To complete this phase, the commands `git status` and `git add` will be +integrated with the sparse-index so that they operate with O(Populated) +performance. They will be carefully tested for operations within and +outside the sparse-checkout definition. + +Phase II: Careful integrations +------------------------------ + +This phase focuses on ensuring that all index extensions and APIs work +well with a sparse-index. This requires significant increases to our test +coverage, especially for operations that interact with the working +directory outside of the sparse-checkout definition. Some of these +behaviors may not be the desirable ones, such as some tests already +marked for failure in `t1092-sparse-checkout-compatibility.sh`. + +The index extensions that may require special integrations are: + +* FS Monitor +* Untracked cache + +While integrating with these features, we should look for patterns that +might lead to better APIs for interacting with the index. Coalescing +common usage patterns into an API call can reduce the number of places +where sparse-directories need to be handled carefully. + +Phase III: Important command speedups +------------------------------------- + +At this point, the patterns for testing and implementing sparse-directory +logic should be relatively stable. This phase focuses on updating some of +the most common builtins that use the index to operate as O(Populated). +Here is a potential list of commands that could be valuable to integrate +at this point: + +* `git commit` +* `git checkout` +* `git merge` +* `git rebase` + +Hopefully, commands such as `git merge` and `git rebase` can benefit +instead from merge algorithms that do not use the index as a data +structure, such as the merge-ORT strategy. As these topics mature, we +may enable the ORT strategy by default for repositories using the +sparse-index feature. + +Along with `git status` and `git add`, these commands cover the majority +of users' interactions with the working directory. In addition, we can +integrate with these commands: + +* `git grep` +* `git rm` + +These have been proposed as some whose behavior could change when in a +repo with a sparse-checkout definition. It would be good to include this +behavior automatically when using a sparse-index. Some clarity is needed +to make the behavior switch clear to the user. + +This phase is the first where parallel work might be possible without too +much conflicts between topics. + +Phase IV: The long tail +----------------------- + +This last phase is less a "phase" and more "the new normal" after all of +the previous work. + +To start, the `command_requires_full_index` option could be removed in +favor of expanding only when hitting an API guard. + +There are many Git commands that could use special attention to operate as +O(Populated), while some might be so rare that it is acceptable to leave +them with additional overhead when a sparse-index is present. + +Here are some commands that might be useful to update: + +* `git sparse-checkout set` +* `git am` +* `git clean` +* `git stash` -- cgit v1.2.3 From cd42415fb4c9680f308ebf8f9f85ba90ae6014be Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Tue, 30 Mar 2021 13:10:54 +0000 Subject: sparse-index: add 'sdir' index extension The index format does not currently allow for sparse directory entries. This violates some expectations that older versions of Git or third-party tools might not understand. We need an indicator inside the index file to warn these tools to not interact with a sparse index unless they are aware of sparse directory entries. Add a new _required_ index extension, 'sdir', that indicates that the index may contain sparse directory entries. This allows us to continue to use the differences in index formats 2, 3, and 4 before we create a new index version 5 in a later change. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano --- Documentation/technical/index-format.txt | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'Documentation/technical') diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt index 3b74c05647..65da0daaa5 100644 --- a/Documentation/technical/index-format.txt +++ b/Documentation/technical/index-format.txt @@ -392,3 +392,15 @@ The remaining data of each directory block is grouped by type: in this block of entries. - 32-bit count of cache entries in this block + +== Sparse Directory Entries + + When using sparse-checkout in cone mode, some entire directories within + the index can be summarized by pointing to a tree object instead of the + entire expanded list of paths within that tree. An index containing such + entries is a "sparse index". Index format versions 4 and less were not + implemented with such entries in mind. Thus, for these versions, an + index containing sparse directory entries will include this extension + with signature { 's', 'd', 'i', 'r' }. Like the split-index extension, + tools should avoid interacting with a sparse index unless they understand + this extension. -- cgit v1.2.3 From 839a66349e094a28f29577a8b311491b9d6b907b Mon Sep 17 00:00:00 2001 From: Derrick Stolee Date: Thu, 1 Apr 2021 01:49:38 +0000 Subject: sparse-index: API protection strategy Edit and expand the sparse-index design document with the plan for guarding index operations with ensure_full_index(). Notably, the plan has changed to not have an expand_to_path() method in favor of checking for a sparse-directory hit inside of the index_path_pos() API. The changes that follow this one will incrementally add ensure_full_index() guards to iterations over all cache entries. Some iterations over the cache entries are not protected due to a few categories listed in the document. Since these are not being modified, here is a short list of the files and methods that will not receive these guards: Looking for non-zero stage: * builtin/add.c:chmod_pathspec() * builtin/merge.c:count_unmerged_entries() * merge-ort.c:record_conflicted_index_entries() * read-cache.c:unmerged_index() * rerere.c:check_one_conflict(), find_conflict(), rerere_remaining() * revision.c:prepare_show_merge() * sequencer.c:append_conflicts_hint() * wt-status.c:wt_status_collect_changes_initial() Looking for submodules: * builtin/submodule--helper.c:module_list_compute() * submodule.c: several methods * worktree.c:validate_no_submodules() Part of the index API: * name-hash.c: lazy init methods * preload-index.c:preload_thread(), preload_index() * read-cache.c: file format methods Checking for correct order of cache entries: * read-cache.c:check_ce_order() Ignores SKIP_WORKTREE entries or already aware: * unpack-trees.c:mark_new_skip_worktree() * wt-status.c:wt_status_check_sparse_checkout() Signed-off-by: Derrick Stolee Reviewed-by: Elijah Newren Signed-off-by: Junio C Hamano --- Documentation/technical/sparse-index.txt | 37 ++++++++++++++++++++++++++++++-- 1 file changed, 35 insertions(+), 2 deletions(-) (limited to 'Documentation/technical') diff --git a/Documentation/technical/sparse-index.txt b/Documentation/technical/sparse-index.txt index 8d3d808046..3b24c1a219 100644 --- a/Documentation/technical/sparse-index.txt +++ b/Documentation/technical/sparse-index.txt @@ -85,8 +85,41 @@ index, as well. Next, consumers of the index will be guarded against operating on a sparse-index by inserting calls to `ensure_full_index()` or -`expand_index_to_path()`. After these guards are in place, we can begin -leaving sparse-directory entries in the in-memory index structure. +`expand_index_to_path()`. If a specific path is requested, then those will +be protected from within the `index_file_exists()` and `index_name_pos()` +API calls: they will call `ensure_full_index()` if necessary. The +intention here is to preserve existing behavior when interacting with a +sparse-checkout. We don't want a change to happen by accident, without +tests. Many of these locations may not need any change before removing the +guards, but we should not do so without tests to ensure the expected +behavior happens. + +It may be desirable to _change_ the behavior of some commands in the +presence of a sparse index or more generally in any sparse-checkout +scenario. In such cases, these should be carefully communicated and +tested. No such behavior changes are intended during this phase. + +During a scan of the codebase, not every iteration of the cache entries +needs an `ensure_full_index()` check. The basic reasons include: + +1. The loop is scanning for entries with non-zero stage. These entries + are not collapsed into a sparse-directory entry. + +2. The loop is scanning for submodules. These entries are not collapsed + into a sparse-directory entry. + +3. The loop is part of the index API, especially around reading or + writing the format. + +4. The loop is checking for correct order of cache entries and that is + correct if and only if the sparse-directory entries are in the correct + location. + +5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is + otherwise already aware of sparse directory entries. + +6. The sparse-index is disabled at this point when using the split-index + feature, so no effort is made to protect the split-index API. Even after inserting these guards, we will keep expanding sparse-indexes for most Git commands using the `command_requires_full_index` repository -- cgit v1.2.3