technical/parallel-checkout.html

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   8 <title>Parallel Checkout Design Notes</title>
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 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Parallel Checkout Design Notes</h1>
 439 <div class="details">
 440 <span id="revdate">2024-08-23</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div id="preamble">
 445 <div class="sectionbody">
 446 <div class="paragraph">
 447 <p>The "Parallel Checkout" feature attempts to use multiple processes to
 448 parallelize the work of uncompressing the blobs, applying in-core
 449 filters, and writing the resulting contents to the working tree during a
 450 checkout operation. It can be used by all checkout-related commands,
 451 such as <code>clone</code>, <code>checkout</code>, <code>reset</code>, <code>sparse-checkout</code>, and others.</p>
 452 </div>
 453 <div class="paragraph">
 454 <p>These commands share the following basic structure:</p>
 455 </div>
 456 <div class="ulist">
 457 <ul>
 458 <li>
 459 <p>Step 1: Read the current index file into memory.</p>
 460 </li>
 461 <li>
 462 <p>Step 2: Modify the in-memory index based upon the command, and
 463 temporarily mark all cache entries that need to be updated.</p>
 464 </li>
 465 <li>
 466 <p>Step 3: Populate the working tree to match the new candidate index.
 467 This includes iterating over all of the to-be-updated cache entries
 468 and delete, create, or overwrite the associated files in the working
 469 tree.</p>
 470 </li>
 471 <li>
 472 <p>Step 4: Write the new index to disk.</p>
 473 </li>
 474 </ul>
 475 </div>
 476 <div class="paragraph">
 477 <p>Step 3 is the focus of the "parallel checkout" effort described here.</p>
 478 </div>
 479 </div>
 480 </div>
 481 <div class="sect1">
 482 <h2 id="_sequential_implementation">Sequential Implementation</h2>
 483 <div class="sectionbody">
 484 <div class="paragraph">
 485 <p>For the purposes of discussion here, the current sequential
 486 implementation of Step 3 is divided in 3 parts, each one implemented in
 487 its own function:</p>
 488 </div>
 489 <div class="ulist">
 490 <ul>
 491 <li>
 492 <p>Step 3a: <code>unpack-trees.c:check_updates()</code> contains a series of
 493 sequential loops iterating over the <code>cache_entry</code>'s array. The main
 494 loop in this function calls the Step 3b function for each of the
 495 to-be-updated entries.</p>
 496 </li>
 497 <li>
 498 <p>Step 3b: <code>entry.c:checkout_entry()</code> examines the existing working tree
 499 for file conflicts, collisions, and unsaved changes. It removes files
 500 and creates leading directories as necessary. It calls the Step 3c
 501 function for each entry to be written.</p>
 502 </li>
 503 <li>
 504 <p>Step 3c: <code>entry.c:write_entry()</code> loads the blob into memory, smudges
 505 it if necessary, creates the file in the working tree, writes the
 506 smudged contents, calls <code>fstat()</code> or <code>lstat()</code>, and updates the
 507 associated <code>cache_entry</code> struct with the stat information gathered.</p>
 508 </li>
 509 </ul>
 510 </div>
 511 <div class="paragraph">
 512 <p>It wouldn&#8217;t be safe to perform Step 3b in parallel, as there could be
 513 race conditions between file creations and removals. Instead, the
 514 parallel checkout framework lets the sequential code handle Step 3b,
 515 and uses parallel workers to replace the sequential
 516 <code>entry.c:write_entry()</code> calls from Step 3c.</p>
 517 </div>
 518 </div>
 519 </div>
 520 <div class="sect1">
 521 <h2 id="_rejected_multi_threaded_solution">Rejected Multi-Threaded Solution</h2>
 522 <div class="sectionbody">
 523 <div class="paragraph">
 524 <p>The most "straightforward" implementation would be to spread the set of
 525 to-be-updated cache entries across multiple threads. But due to the
 526 thread-unsafe functions in the object database code, we would have to use locks to
 527 coordinate the parallel operation. An early prototype of this solution
 528 showed that the multi-threaded checkout would bring performance
 529 improvements over the sequential code, but there was still too much lock
 530 contention. A <code>perf</code> profiling indicated that around 20% of the runtime
 531 during a local Linux clone (on an SSD) was spent in locking functions.
 532 For this reason this approach was rejected in favor of using multiple
 533 child processes, which led to better performance.</p>
 534 </div>
 535 </div>
 536 </div>
 537 <div class="sect1">
 538 <h2 id="_multi_process_solution">Multi-Process Solution</h2>
 539 <div class="sectionbody">
 540 <div class="paragraph">
 541 <p>Parallel checkout alters the aforementioned Step 3 to use multiple
 542 <code>checkout--worker</code> background processes to distribute the work. The
 543 long-running worker processes are controlled by the foreground Git
 544 command using the existing run-command API.</p>
 545 </div>
 546 <div class="sect2">
 547 <h3 id="_overview">Overview</h3>
 548 <div class="paragraph">
 549 <p>Step 3b is only slightly altered; for each entry to be checked out, the
 550 main process performs the following steps:</p>
 551 </div>
 552 <div class="ulist">
 553 <ul>
 554 <li>
 555 <p>M1: Check whether there is any untracked or unclean file in the
 556 working tree which would be overwritten by this entry, and decide
 557 whether to proceed (removing the file(s)) or not.</p>
 558 </li>
 559 <li>
 560 <p>M2: Create the leading directories.</p>
 561 </li>
 562 <li>
 563 <p>M3: Load the conversion attributes for the entry&#8217;s path.</p>
 564 </li>
 565 <li>
 566 <p>M4: Check, based on the entry&#8217;s type and conversion attributes,
 567 whether the entry is eligible for parallel checkout (more on this
 568 later). If it is eligible, enqueue the entry and the loaded
 569 attributes to later write the entry in parallel. If not, write the
 570 entry right away, using the default sequential code.</p>
 571 </li>
 572 </ul>
 573 </div>
 574 <div class="paragraph">
 575 <p>Note: we save the conversion attributes associated with each entry
 576 because the workers don&#8217;t have access to the main process' index state,
 577 so they can&#8217;t load the attributes by themselves (and the attributes are
 578 needed to properly smudge the entry). Additionally, this has a positive
 579 impact on performance as (1) we don&#8217;t need to load the attributes twice
 580 and (2) the attributes machinery is optimized to handle paths in
 581 sequential order.</p>
 582 </div>
 583 <div class="paragraph">
 584 <p>After all entries have passed through the above steps, the main process
 585 checks if the number of enqueued entries is sufficient to spread among
 586 the workers. If not, it just writes them sequentially. Otherwise, it
 587 spawns the workers and distributes the queued entries uniformly in
 588 continuous chunks. This aims to minimize the chances of two workers
 589 writing to the same directory simultaneously, which could increase lock
 590 contention in the kernel.</p>
 591 </div>
 592 <div class="paragraph">
 593 <p>Then, for each assigned item, each worker:</p>
 594 </div>
 595 <div class="ulist">
 596 <ul>
 597 <li>
 598 <p>W1: Checks if there is any non-directory file in the leading part of
 599 the entry&#8217;s path or if there already exists a file at the entry' path.
 600 If so, mark the entry with <code>PC_ITEM_COLLIDED</code> and skip it (more on
 601 this later).</p>
 602 </li>
 603 <li>
 604 <p>W2: Creates the file (with O_CREAT and O_EXCL).</p>
 605 </li>
 606 <li>
 607 <p>W3: Loads the blob into memory (inflating and delta reconstructing
 608 it).</p>
 609 </li>
 610 <li>
 611 <p>W4: Applies any required in-process filter, like end-of-line
 612 conversion and re-encoding.</p>
 613 </li>
 614 <li>
 615 <p>W5: Writes the result to the file descriptor opened at W2.</p>
 616 </li>
 617 <li>
 618 <p>W6: Calls <code>fstat()</code> or <code>lstat()</code> on the just-written path, and sends
 619 the result back to the main process, together with the end status of
 620 the operation and the item&#8217;s identification number.</p>
 621 </li>
 622 </ul>
 623 </div>
 624 <div class="paragraph">
 625 <p>Note that, when possible, steps W3 to W5 are delegated to the streaming
 626 machinery, removing the need to keep the entire blob in memory.</p>
 627 </div>
 628 <div class="paragraph">
 629 <p>If the worker fails to read the blob or to write it to the working tree,
 630 it removes the created file to avoid leaving empty files behind. This is
 631 the <strong>only</strong> time a worker is allowed to remove a file.</p>
 632 </div>
 633 <div class="paragraph">
 634 <p>As mentioned earlier, it is the responsibility of the main process to
 635 remove any file that blocks the checkout operation (or abort if the
 636 removal(s) would cause data loss and the user didn&#8217;t ask to <code>--force</code>).
 637 This is crucial to avoid race conditions and also to properly detect
 638 path collisions at Step W1.</p>
 639 </div>
 640 <div class="paragraph">
 641 <p>After the workers finish writing the items and sending back the required
 642 information, the main process handles the results in two steps:</p>
 643 </div>
 644 <div class="ulist">
 645 <ul>
 646 <li>
 647 <p>First, it updates the in-memory index with the <code>lstat()</code> information
 648 sent by the workers. (This must be done first as this information
 649 might be required in the following step.)</p>
 650 </li>
 651 <li>
 652 <p>Then it writes the items which collided on disk (i.e. items marked
 653 with <code>PC_ITEM_COLLIDED</code>). More on this below.</p>
 654 </li>
 655 </ul>
 656 </div>
 657 </div>
 658 </div>
 659 </div>
 660 <div class="sect1">
 661 <h2 id="_path_collisions">Path Collisions</h2>
 662 <div class="sectionbody">
 663 <div class="paragraph">
 664 <p>Path collisions happen when two different paths correspond to the same
 665 entry in the file system. E.g. the paths <em>a</em> and <em>A</em> would collide in a
 666 case-insensitive file system.</p>
 667 </div>
 668 <div class="paragraph">
 669 <p>The sequential checkout deals with collisions in the same way that it
 670 deals with files that were already present in the working tree before
 671 checkout. Basically, it checks if the path that it wants to write
 672 already exists on disk, makes sure the existing file doesn&#8217;t have
 673 unsaved data, and then overwrites it. (To be more pedantic: it deletes
 674 the existing file and creates the new one.) So, if there are multiple
 675 colliding files to be checked out, the sequential code will write each
 676 one of them but only the last will actually survive on disk.</p>
 677 </div>
 678 <div class="paragraph">
 679 <p>Parallel checkout aims to reproduce the same behavior. However, we
 680 cannot let the workers racily write to the same file on disk. Instead,
 681 the workers detect when the entry that they want to check out would
 682 collide with an existing file, and mark it with <code>PC_ITEM_COLLIDED</code>.
 683 Later, the main process can sequentially feed these entries back to
 684 <code>checkout_entry()</code> without the risk of race conditions. On clone, this
 685 also has the effect of marking the colliding entries to later emit a
 686 warning for the user, like the classic sequential checkout does.</p>
 687 </div>
 688 <div class="paragraph">
 689 <p>The workers are able to detect both collisions among the entries being
 690 concurrently written and collisions between a parallel-eligible entry
 691 and an ineligible entry. The general idea for collision detection is
 692 quite straightforward: for each parallel-eligible entry, the main
 693 process must remove all files that prevent this entry from being written
 694 (before enqueueing it). This includes any non-directory file in the
 695 leading path of the entry. Later, when a worker gets assigned the entry,
 696 it looks again for the non-directory files and for an already existing
 697 file at the entry&#8217;s path. If any of these checks finds something, the
 698 worker knows that there was a path collision.</p>
 699 </div>
 700 <div class="paragraph">
 701 <p>Because parallel checkout can distinguish path collisions from the case
 702 where the file was already present in the working tree before checkout,
 703 we could alternatively choose to skip the checkout of colliding entries.
 704 However, each entry that doesn&#8217;t get written would have NULL <code>lstat()</code>
 705 fields on the index. This could cause performance penalties for
 706 subsequent commands that need to refresh the index, as they would have
 707 to go to the file system to see if the entry is dirty. Thus, if we have
 708 N entries in a colliding group and we decide to write and <code>lstat()</code> only
 709 one of them, every subsequent <code>git-status</code> will have to read, convert,
 710 and hash the written file N - 1 times. By checking out all colliding
 711 entries (like the sequential code does), we only pay the overhead once,
 712 during checkout.</p>
 713 </div>
 714 </div>
 715 </div>
 716 <div class="sect1">
 717 <h2 id="_eligible_entries_for_parallel_checkout">Eligible Entries for Parallel Checkout</h2>
 718 <div class="sectionbody">
 719 <div class="paragraph">
 720 <p>As previously mentioned, not all entries passed to <code>checkout_entry()</code>
 721 will be considered eligible for parallel checkout. More specifically, we
 722 exclude:</p>
 723 </div>
 724 <div class="ulist">
 725 <ul>
 726 <li>
 727 <p>Symbolic links; to avoid race conditions that, in combination with
 728 path collisions, could cause workers to write files at the wrong
 729 place. For example, if we were to concurrently check out a symlink
 730 <em>a</em> &#8594; <em>b</em> and a regular file <em>A/f</em> in a case-insensitive file system,
 731 we could potentially end up writing the file <em>A/f</em> at <em>a/f</em>, due to a
 732 race condition.</p>
 733 </li>
 734 <li>
 735 <p>Regular files that require external filters (either "one shot" filters
 736 or long-running process filters). These filters are black-boxes to Git
 737 and may have their own internal locking or non-concurrent assumptions.
 738 So it might not be safe to run multiple instances in parallel.</p>
 739 <div class="paragraph">
 740 <p>Besides, long-running filters may use the delayed checkout feature to
 741 postpone the return of some filtered blobs. The delayed checkout queue
 742 and the parallel checkout queue are not compatible and should remain
 743 separate.</p>
 744 </div>
 745 <div class="paragraph">
 746 <p>Note: regular files that only require internal filters, like end-of-line
 747 conversion and re-encoding, are eligible for parallel checkout.</p>
 748 </div>
 749 </li>
 750 </ul>
 751 </div>
 752 <div class="paragraph">
 753 <p>Ineligible entries are checked out by the classic sequential codepath
 754 <strong>before</strong> spawning workers.</p>
 755 </div>
 756 <div class="paragraph">
 757 <p>Note: submodules' files are also eligible for parallel checkout (as
 758 long as they don&#8217;t fall into any of the excluding categories mentioned
 759 above). But since each submodule is checked out in its own child
 760 process, we don&#8217;t mix the superproject&#8217;s and the submodules' files in
 761 the same parallel checkout process or queue.</p>
 762 </div>
 763 </div>
 764 </div>
 765 <div class="sect1">
 766 <h2 id="_the_api">The API</h2>
 767 <div class="sectionbody">
 768 <div class="paragraph">
 769 <p>The parallel checkout API was designed with the goal of minimizing
 770 changes to the current users of the checkout machinery. This means that
 771 they don&#8217;t have to call a different function for sequential or parallel
 772 checkout. As already mentioned, <code>checkout_entry()</code> will automatically
 773 insert the given entry in the parallel checkout queue when this feature
 774 is enabled and the entry is eligible; otherwise, it will just write the
 775 entry right away, using the sequential code. In general, callers of the
 776 parallel checkout API should look similar to this:</p>
 777 </div>
 778 <div class="listingblock">
 779 <div class="content">
 780 <pre>int pc_workers, pc_threshold, err = 0;
 781 struct checkout state;
 782
 783 get_parallel_checkout_configs(&amp;pc_workers, &amp;pc_threshold);
 784
 785 /*
 786  * This check is not strictly required, but it
 787  * should save some time in sequential mode.
 788  */
 789 if (pc_workers &gt; 1)
 790         init_parallel_checkout();
 791
 792 for (each cache_entry ce to-be-updated)
 793         err |= checkout_entry(ce, &amp;state, NULL, NULL);
 794
 795 err |= run_parallel_checkout(&amp;state, pc_workers, pc_threshold, NULL, NULL);</pre>
 796 </div>
 797 </div>
 798 </div>
 799 </div>
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