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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/Makefile | 1 | ||||
| -rw-r--r-- | Documentation/technical/parallel-checkout.txt | 270 |
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diff --git a/Documentation/Makefile b/Documentation/Makefile index 874a01d7a8..c2baad0bd8 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -91,6 +91,7 @@ TECH_DOCS += technical/multi-pack-index TECH_DOCS += technical/pack-format TECH_DOCS += technical/pack-heuristics TECH_DOCS += technical/pack-protocol +TECH_DOCS += technical/parallel-checkout TECH_DOCS += technical/partial-clone TECH_DOCS += technical/protocol-capabilities TECH_DOCS += technical/protocol-common diff --git a/Documentation/technical/parallel-checkout.txt b/Documentation/technical/parallel-checkout.txt new file mode 100644 index 0000000000..e790258a1a --- /dev/null +++ b/Documentation/technical/parallel-checkout.txt @@ -0,0 +1,270 @@ +Parallel Checkout Design Notes +============================== + +The "Parallel Checkout" feature attempts to use multiple processes to +parallelize the work of uncompressing the blobs, applying in-core +filters, and writing the resulting contents to the working tree during a +checkout operation. It can be used by all checkout-related commands, +such as `clone`, `checkout`, `reset`, `sparse-checkout`, and others. + +These commands share the following basic structure: + +* Step 1: Read the current index file into memory. + +* Step 2: Modify the in-memory index based upon the command, and + temporarily mark all cache entries that need to be updated. + +* Step 3: Populate the working tree to match the new candidate index. + This includes iterating over all of the to-be-updated cache entries + and delete, create, or overwrite the associated files in the working + tree. + +* Step 4: Write the new index to disk. + +Step 3 is the focus of the "parallel checkout" effort described here. + +Sequential Implementation +------------------------- + +For the purposes of discussion here, the current sequential +implementation of Step 3 is divided in 3 parts, each one implemented in +its own function: + +* Step 3a: `unpack-trees.c:check_updates()` contains a series of + sequential loops iterating over the `cache_entry`'s array. The main + loop in this function calls the Step 3b function for each of the + to-be-updated entries. + +* Step 3b: `entry.c:checkout_entry()` examines the existing working tree + for file conflicts, collisions, and unsaved changes. It removes files + and creates leading directories as necessary. It calls the Step 3c + function for each entry to be written. + +* Step 3c: `entry.c:write_entry()` loads the blob into memory, smudges + it if necessary, creates the file in the working tree, writes the + smudged contents, calls `fstat()` or `lstat()`, and updates the + associated `cache_entry` struct with the stat information gathered. + +It wouldn't be safe to perform Step 3b in parallel, as there could be +race conditions between file creations and removals. Instead, the +parallel checkout framework lets the sequential code handle Step 3b, +and uses parallel workers to replace the sequential +`entry.c:write_entry()` calls from Step 3c. + +Rejected Multi-Threaded Solution +-------------------------------- + +The most "straightforward" implementation would be to spread the set of +to-be-updated cache entries across multiple threads. But due to the +thread-unsafe functions in the ODB code, we would have to use locks to +coordinate the parallel operation. An early prototype of this solution +showed that the multi-threaded checkout would bring performance +improvements over the sequential code, but there was still too much lock +contention. A `perf` profiling indicated that around 20% of the runtime +during a local Linux clone (on an SSD) was spent in locking functions. +For this reason this approach was rejected in favor of using multiple +child processes, which led to a better performance. + +Multi-Process Solution +---------------------- + +Parallel checkout alters the aforementioned Step 3 to use multiple +`checkout--worker` background processes to distribute the work. The +long-running worker processes are controlled by the foreground Git +command using the existing run-command API. + +Overview +~~~~~~~~ + +Step 3b is only slightly altered; for each entry to be checked out, the +main process performs the following steps: + +* M1: Check whether there is any untracked or unclean file in the + working tree which would be overwritten by this entry, and decide + whether to proceed (removing the file(s)) or not. + +* M2: Create the leading directories. + +* M3: Load the conversion attributes for the entry's path. + +* M4: Check, based on the entry's type and conversion attributes, + whether the entry is eligible for parallel checkout (more on this + later). If it is eligible, enqueue the entry and the loaded + attributes to later write the entry in parallel. If not, write the + entry right away, using the default sequential code. + +Note: we save the conversion attributes associated with each entry +because the workers don't have access to the main process' index state, +so they can't load the attributes by themselves (and the attributes are +needed to properly smudge the entry). Additionally, this has a positive +impact on performance as (1) we don't need to load the attributes twice +and (2) the attributes machinery is optimized to handle paths in +sequential order. + +After all entries have passed through the above steps, the main process +checks if the number of enqueued entries is sufficient to spread among +the workers. If not, it just writes them sequentially. Otherwise, it +spawns the workers and distributes the queued entries uniformly in +continuous chunks. This aims to minimize the chances of two workers +writing to the same directory simultaneously, which could increase lock +contention in the kernel. + +Then, for each assigned item, each worker: + +* W1: Checks if there is any non-directory file in the leading part of + the entry's path or if there already exists a file at the entry' path. + If so, mark the entry with `PC_ITEM_COLLIDED` and skip it (more on + this later). + +* W2: Creates the file (with O_CREAT and O_EXCL). + +* W3: Loads the blob into memory (inflating and delta reconstructing + it). + +* W4: Applies any required in-process filter, like end-of-line + conversion and re-encoding. + +* W5: Writes the result to the file descriptor opened at W2. + +* W6: Calls `fstat()` or lstat()` on the just-written path, and sends + the result back to the main process, together with the end status of + the operation and the item's identification number. + +Note that, when possible, steps W3 to W5 are delegated to the streaming +machinery, removing the need to keep the entire blob in memory. + +If the worker fails to read the blob or to write it to the working tree, +it removes the created file to avoid leaving empty files behind. This is +the *only* time a worker is allowed to remove a file. + +As mentioned earlier, it is the responsibility of the main process to +remove any file that blocks the checkout operation (or abort if the +removal(s) would cause data loss and the user didn't ask to `--force`). +This is crucial to avoid race conditions and also to properly detect +path collisions at Step W1. + +After the workers finish writing the items and sending back the required +information, the main process handles the results in two steps: + +- First, it updates the in-memory index with the `lstat()` information + sent by the workers. (This must be done first as this information + might me required in the following step.) + +- Then it writes the items which collided on disk (i.e. items marked + with `PC_ITEM_COLLIDED`). More on this below. + +Path Collisions +--------------- + +Path collisions happen when two different paths correspond to the same +entry in the file system. E.g. the paths 'a' and 'A' would collide in a +case-insensitive file system. + +The sequential checkout deals with collisions in the same way that it +deals with files that were already present in the working tree before +checkout. Basically, it checks if the path that it wants to write +already exists on disk, makes sure the existing file doesn't have +unsaved data, and then overwrites it. (To be more pedantic: it deletes +the existing file and creates the new one.) So, if there are multiple +colliding files to be checked out, the sequential code will write each +one of them but only the last will actually survive on disk. + +Parallel checkout aims to reproduce the same behavior. However, we +cannot let the workers racily write to the same file on disk. Instead, +the workers detect when the entry that they want to check out would +collide with an existing file, and mark it with `PC_ITEM_COLLIDED`. +Later, the main process can sequentially feed these entries back to +`checkout_entry()` without the risk of race conditions. On clone, this +also has the effect of marking the colliding entries to later emit a +warning for the user, like the classic sequential checkout does. + +The workers are able to detect both collisions among the entries being +concurrently written and collisions between a parallel-eligible entry +and an ineligible entry. The general idea for collision detection is +quite straightforward: for each parallel-eligible entry, the main +process must remove all files that prevent this entry from being written +(before enqueueing it). This includes any non-directory file in the +leading path of the entry. Later, when a worker gets assigned the entry, +it looks again for the non-directories files and for an already existing +file at the entry's path. If any of these checks finds something, the +worker knows that there was a path collision. + +Because parallel checkout can distinguish path collisions from the case +where the file was already present in the working tree before checkout, +we could alternatively choose to skip the checkout of colliding entries. +However, each entry that doesn't get written would have NULL `lstat()` +fields on the index. This could cause performance penalties for +subsequent commands that need to refresh the index, as they would have +to go to the file system to see if the entry is dirty. Thus, if we have +N entries in a colliding group and we decide to write and `lstat()` only +one of them, every subsequent `git-status` will have to read, convert, +and hash the written file N - 1 times. By checking out all colliding +entries (like the sequential code does), we only pay the overhead once, +during checkout. + +Eligible Entries for Parallel Checkout +-------------------------------------- + +As previously mentioned, not all entries passed to `checkout_entry()` +will be considered eligible for parallel checkout. More specifically, we +exclude: + +- Symbolic links; to avoid race conditions that, in combination with + path collisions, could cause workers to write files at the wrong + place. For example, if we were to concurrently check out a symlink + 'a' -> 'b' and a regular file 'A/f' in a case-insensitive file system, + we could potentially end up writing the file 'A/f' at 'a/f', due to a + race condition. + +- Regular files that require external filters (either "one shot" filters + or long-running process filters). These filters are black-boxes to Git + and may have their own internal locking or non-concurrent assumptions. + So it might not be safe to run multiple instances in parallel. ++ +Besides, long-running filters may use the delayed checkout feature to +postpone the return of some filtered blobs. The delayed checkout queue +and the parallel checkout queue are not compatible and should remain +separate. ++ +Note: regular files that only require internal filters, like end-of-line +conversion and re-encoding, are eligible for parallel checkout. + +Ineligible entries are checked out by the classic sequential codepath +*before* spawning workers. + +Note: submodules's files are also eligible for parallel checkout (as +long as they don't fall into any of the excluding categories mentioned +above). But since each submodule is checked out in its own child +process, we don't mix the superproject's and the submodules' files in +the same parallel checkout process or queue. + +The API +------- + +The parallel checkout API was designed with the goal of minimizing +changes to the current users of the checkout machinery. This means that +they don't have to call a different function for sequential or parallel +checkout. As already mentioned, `checkout_entry()` will automatically +insert the given entry in the parallel checkout queue when this feature +is enabled and the entry is eligible; otherwise, it will just write the +entry right away, using the sequential code. In general, callers of the +parallel checkout API should look similar to this: + +---------------------------------------------- +int pc_workers, pc_threshold, err = 0; +struct checkout state; + +get_parallel_checkout_configs(&pc_workers, &pc_threshold); + +/* + * This check is not strictly required, but it + * should save some time in sequential mode. + */ +if (pc_workers > 1) + init_parallel_checkout(); + +for (each cache_entry ce to-be-updated) + err |= checkout_entry(ce, &state, NULL, NULL); + +err |= run_parallel_checkout(&state, pc_workers, pc_threshold, NULL, NULL); +---------------------------------------------- |