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| <title>Parallel Checkout Design Notes</title> |
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| </head> |
| <body class="article"> |
| <div id="header"> |
| <h1>Parallel Checkout Design Notes</h1> |
| </div> |
| <div id="content"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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 <code>clone</code>, <code>checkout</code>, <code>reset</code>, <code>sparse-checkout</code>, and others.</p> |
| </div> |
| <div class="paragraph"> |
| <p>These commands share the following basic structure:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Step 1: Read the current index file into memory.</p> |
| </li> |
| <li> |
| <p>Step 2: Modify the in-memory index based upon the command, and |
| temporarily mark all cache entries that need to be updated.</p> |
| </li> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>Step 4: Write the new index to disk.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Step 3 is the focus of the "parallel checkout" effort described here.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_sequential_implementation">Sequential Implementation</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Step 3a: <code>unpack-trees.c:check_updates</code>() contains a series of |
| sequential loops iterating over the <code>cache_entry</code>'s array. The main |
| loop in this function calls the Step 3b function for each of the |
| to-be-updated entries.</p> |
| </li> |
| <li> |
| <p>Step 3b: <code>entry.c:checkout_entry</code>() 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.</p> |
| </li> |
| <li> |
| <p>Step 3c: <code>entry.c:write_entry</code>() loads the blob into memory, smudges |
| it if necessary, creates the file in the working tree, writes the |
| smudged contents, calls <code>fstat</code>() or <code>lstat</code>(), and updates the |
| associated <code>cache_entry</code> struct with the stat information gathered.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>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 |
| <code>entry.c:write_entry</code>() calls from Step 3c.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_rejected_multi_threaded_solution">Rejected Multi-Threaded Solution</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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 object database 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 <code>perf</code> 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 better performance.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_multi_process_solution">Multi-Process Solution</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Parallel checkout alters the aforementioned Step 3 to use multiple |
| <code>checkout--worker</code> background processes to distribute the work. The |
| long-running worker processes are controlled by the foreground Git |
| command using the existing run-command API.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_overview">Overview</h3> |
| <div class="paragraph"> |
| <p>Step 3b is only slightly altered; for each entry to be checked out, the |
| main process performs the following steps:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>M2: Create the leading directories.</p> |
| </li> |
| <li> |
| <p>M3: Load the conversion attributes for the entry’s path.</p> |
| </li> |
| <li> |
| <p>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.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Then, for each assigned item, each worker:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>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 <code>PC_ITEM_COLLIDED</code> and skip it (more on |
| this later).</p> |
| </li> |
| <li> |
| <p>W2: Creates the file (with O_CREAT and O_EXCL).</p> |
| </li> |
| <li> |
| <p>W3: Loads the blob into memory (inflating and delta reconstructing |
| it).</p> |
| </li> |
| <li> |
| <p>W4: Applies any required in-process filter, like end-of-line |
| conversion and re-encoding.</p> |
| </li> |
| <li> |
| <p>W5: Writes the result to the file descriptor opened at W2.</p> |
| </li> |
| <li> |
| <p>W6: Calls <code>fstat</code>() or <code>lstat</code>() 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.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Note that, when possible, steps W3 to W5 are delegated to the streaming |
| machinery, removing the need to keep the entire blob in memory.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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 <strong>only</strong> time a worker is allowed to remove a file.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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 <code>--force</code>). |
| This is crucial to avoid race conditions and also to properly detect |
| path collisions at Step W1.</p> |
| </div> |
| <div class="paragraph"> |
| <p>After the workers finish writing the items and sending back the required |
| information, the main process handles the results in two steps:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>First, it updates the in-memory index with the <code>lstat</code>() information |
| sent by the workers. (This must be done first as this information |
| might be required in the following step.)</p> |
| </li> |
| <li> |
| <p>Then it writes the items which collided on disk (i.e. items marked |
| with <code>PC_ITEM_COLLIDED</code>). More on this below.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_path_collisions">Path Collisions</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Path collisions happen when two different paths correspond to the same |
| entry in the file system. E.g. the paths <em>a</em> and <em>A</em> would collide in a |
| case-insensitive file system.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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 <code>PC_ITEM_COLLIDED</code>. |
| Later, the main process can sequentially feed these entries back to |
| <code>checkout_entry</code>() 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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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-directory 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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>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 <code>lstat</code>() |
| 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 <code>lstat</code>() only |
| one of them, every subsequent <code>git-status</code> 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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_eligible_entries_for_parallel_checkout">Eligible Entries for Parallel Checkout</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>As previously mentioned, not all entries passed to <code>checkout_entry</code>() |
| will be considered eligible for parallel checkout. More specifically, we |
| exclude:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>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 |
| <em>a</em> → <em>b</em> and a regular file <em>A/f</em> in a case-insensitive file system, |
| we could potentially end up writing the file <em>A/f</em> at <em>a/f</em>, due to a |
| race condition.</p> |
| </li> |
| <li> |
| <p>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.</p> |
| <div class="paragraph"> |
| <p>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.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Note: regular files that only require internal filters, like end-of-line |
| conversion and re-encoding, are eligible for parallel checkout.</p> |
| </div> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Ineligible entries are checked out by the classic sequential codepath |
| <strong>before</strong> spawning workers.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Note: submodules' 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.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_the_api">The API</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>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, <code>checkout_entry</code>() 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:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>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);</pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
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