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| <title>Parallel Checkout Design Notes</title> | |
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| // Footnotes generator | |
| ///////////////////////////////////////////////////////////////////// | |
| /* Based on footnote generation code from: | |
| * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html | |
| */ | |
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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 a 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 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. | |
| </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 me 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-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.</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’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.</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><code>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);</code></pre> | |
| </div></div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footnotes"><hr /></div> | |
| <div id="footer"> | |
| <div id="footer-text"> | |
| Last updated | |
| 2023-04-17 18:02:32 PDT | |
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