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 440 </head>
 441 <body class="manpage">
 442 <div id="header">
 443 <h1>gitpacking(7) Manual Page</h1>
 444 <h2 id="_name">NAME</h2>
 445 <div class="sectionbody">
 446 <p>gitpacking - Advanced concepts related to packing in Git</p>
 447 </div>
 448 </div>
 449 <div id="content">
 450 <div class="sect1">
 451 <h2 id="_synopsis">SYNOPSIS</h2>
 452 <div class="sectionbody">
 453 <div class="paragraph">
 454 <p>gitpacking</p>
 455 </div>
 456 </div>
 457 </div>
 458 <div class="sect1">
 459 <h2 id="_description">DESCRIPTION</h2>
 460 <div class="sectionbody">
 461 <div class="paragraph">
 462 <p>This document aims to describe some advanced concepts related to packing
 463 in Git.</p>
 464 </div>
 465 <div class="paragraph">
 466 <p>Many concepts are currently described scattered between manual pages of
 467 various Git commands, including <a href="git-pack-objects.html">git-pack-objects(1)</a>,
 468 <a href="git-repack.html">git-repack(1)</a>, and others, as well as <a href="gitformat-pack.html">gitformat-pack(5)</a>,
 469 and parts of the <code>Documentation/technical</code> tree.</p>
 470 </div>
 471 <div class="paragraph">
 472 <p>There are many aspects of packing in Git that are not covered in this
 473 document that instead live in the aforementioned areas. Over time, those
 474 scattered bits may coalesce into this document.</p>
 475 </div>
 476 </div>
 477 </div>
 478 <div class="sect1">
 479 <h2 id="_pseudo_merge_bitmaps">Pseudo-merge bitmaps</h2>
 480 <div class="sectionbody">
 481 <div class="admonitionblock note">
 482 <table>
 483 <tr>
 484 <td class="icon">
 485 <div class="title">Note</div>
 486 </td>
 487 <td class="content">
 488 Pseudo-merge bitmaps are considered an experimental feature, so
 489 the configuration and many of the ideas are subject to change.
 490 </td>
 491 </tr>
 492 </table>
 493 </div>
 494 <div class="sect2">
 495 <h3 id="_background">Background</h3>
 496 <div class="paragraph">
 497 <p>Reachability bitmaps are most efficient when we have on-disk stored
 498 bitmaps for one or more of the starting points of a traversal. For this
 499 reason, Git prefers storing bitmaps for commits at the tips of refs,
 500 because traversals tend to start with those points.</p>
 501 </div>
 502 <div class="paragraph">
 503 <p>But if you have a large number of refs, it&#8217;s not feasible to store a
 504 bitmap for <em>every</em> ref tip. It takes up space, and just OR-ing all of
 505 those bitmaps together is expensive.</p>
 506 </div>
 507 <div class="paragraph">
 508 <p>One way we can deal with that is to create bitmaps that represent
 509 <em>groups</em> of refs. When a traversal asks about the entire group, then we
 510 can use this single bitmap instead of considering each ref individually.
 511 Because these bitmaps represent the set of objects which would be
 512 reachable in a hypothetical merge of all of the commits, we call them
 513 pseudo-merge bitmaps.</p>
 514 </div>
 515 </div>
 516 <div class="sect2">
 517 <h3 id="_overview">Overview</h3>
 518 <div class="paragraph">
 519 <p>A "pseudo-merge bitmap" is used to refer to a pair of bitmaps, as
 520 follows:</p>
 521 </div>
 522 <div class="dlist">
 523 <dl>
 524 <dt class="hdlist1">Commit bitmap</dt>
 525 <dd>
 526 <p>A bitmap whose set bits describe the set of commits included in the
 527 pseudo-merge&#8217;s "merge" bitmap (as below).</p>
 528 </dd>
 529 <dt class="hdlist1">Merge bitmap</dt>
 530 <dd>
 531 <p>A bitmap whose set bits describe the reachability closure over the set
 532 of commits in the pseudo-merge&#8217;s "commits" bitmap (as above). An
 533 identical bitmap would be generated for an octopus merge with the same
 534 set of parents as described in the commits bitmap.</p>
 535 </dd>
 536 </dl>
 537 </div>
 538 <div class="paragraph">
 539 <p>Pseudo-merge bitmaps can accelerate bitmap traversals when all commits
 540 for a given pseudo-merge are listed on either side of the traversal,
 541 either directly (by explicitly asking for them as part of the <code>HAVES</code>
 542 or <code>WANTS</code>) or indirectly (by encountering them during a fill-in
 543 traversal).</p>
 544 </div>
 545 </div>
 546 <div class="sect2">
 547 <h3 id="_use_cases">Use-cases</h3>
 548 <div class="paragraph">
 549 <p>For example, suppose there exists a pseudo-merge bitmap with a large
 550 number of commits, all of which are listed in the <code>WANTS</code> section of
 551 some bitmap traversal query. When pseudo-merge bitmaps are enabled, the
 552 bitmap machinery can quickly determine there is a pseudo-merge which
 553 satisfies some subset of the wanted objects on either side of the query.
 554 Then, we can inflate the EWAH-compressed bitmap, and <code>OR</code> it in to the
 555 resulting bitmap. By contrast, without pseudo-merge bitmaps, we would
 556 have to repeat the decompression and <code>OR</code>-ing step over a potentially
 557 large number of individual bitmaps, which can take proportionally more
 558 time.</p>
 559 </div>
 560 <div class="paragraph">
 561 <p>Another benefit of pseudo-merges arises when there is some combination
 562 of (a) a large number of references, with (b) poor bitmap coverage, and
 563 (c) deep, nested trees, making fill-in traversal relatively expensive.
 564 For example, suppose that there are a large enough number of tags where
 565 bitmapping each of the tags individually is infeasible. Without
 566 pseudo-merge bitmaps, computing the result of, say, <code>git</code> <code>rev-list</code>
 567 <code>--use-bitmap-index</code> <code>--count</code> <code>--objects</code> <code>--tags</code> would likely require a
 568 large amount of fill-in traversal. But when a large quantity of those
 569 tags are stored together in a pseudo-merge bitmap, the bitmap machinery
 570 can take advantage of the fact that we only care about the union of
 571 objects reachable from all of those tags, and answer the query much
 572 faster.</p>
 573 </div>
 574 </div>
 575 <div class="sect2">
 576 <h3 id="_configuration">Configuration</h3>
 577 <div class="paragraph">
 578 <p>Reference tips are grouped into different pseudo-merge groups according
 579 to two criteria. A reference name matches one or more of the defined
 580 pseudo-merge patterns, and optionally one or more capture groups within
 581 that pattern which further partition the group.</p>
 582 </div>
 583 <div class="paragraph">
 584 <p>Within a group, commits may be considered "stable", or "unstable"
 585 depending on their age. These are adjusted by setting the
 586 <code>bitmapPseudoMerge.</code><em>&lt;name&gt;</em><code>.stableThreshold</code> and
 587 <code>bitmapPseudoMerge.</code><em>&lt;name&gt;</em><code>.threshold</code> configuration values, respectively.</p>
 588 </div>
 589 <div class="paragraph">
 590 <p>All stable commits are grouped into pseudo-merges of equal size
 591 (<code>bitmapPseudoMerge.</code><em>&lt;name&gt;</em><code>.stableSize</code>). If the <code>stableSize</code>
 592 configuration is set to, say, 100, then the first 100 commits (ordered
 593 by committer date) which are older than the <code>stableThreshold</code> value will
 594 form one group, the next 100 commits will form another group, and so on.</p>
 595 </div>
 596 <div class="paragraph">
 597 <p>Among unstable commits, the pseudo-merge machinery will attempt to
 598 combine older commits into large groups as opposed to newer commits
 599 which will appear in smaller groups. This is based on the heuristic that
 600 references whose tip commit is older are less likely to be modified to
 601 point at a different commit than a reference whose tip commit is newer.</p>
 602 </div>
 603 <div class="paragraph">
 604 <p>The size of groups is determined by a power-law decay function, and the
 605 decay parameter roughly corresponds to "k" in <code>f</code>(<code>n</code>) <code>=</code> <code>C</code>*n^(<code>-k/100</code>),
 606 where <code>f</code>(<code>n</code>) describes the size of the <code>n</code>-th pseudo-merge group. The
 607 sample rate controls what percentage of eligible commits are considered
 608 as candidates. The threshold parameter indicates the minimum age (so as
 609 to avoid including too-recent commits in a pseudo-merge group, making it
 610 less likely to be valid). The "maxMerges" parameter sets an upper-bound
 611 on the number of pseudo-merge commits an individual group</p>
 612 </div>
 613 <div class="paragraph">
 614 <p>The "stable"-related parameters control "stable" pseudo-merge groups,
 615 comprised of a fixed number of commits which are older than the
 616 configured "stable threshold" value and may be grouped together in
 617 chunks of "stableSize" in order of age.</p>
 618 </div>
 619 <div class="paragraph">
 620 <p>The exact configuration for pseudo-merges is as follows:</p>
 621 </div>
 622 <div class="admonitionblock note">
 623 <table>
 624 <tr>
 625 <td class="icon">
 626 <div class="title">Note</div>
 627 </td>
 628 <td class="content">
 629 The configuration options in <code>bitmapPseudoMerge.</code>* are considered
 630 EXPERIMENTAL and may be subject to change or be removed entirely in the
 631 future. For more information about the pseudo-merge bitmap feature, see
 632 the "Pseudo-merge bitmaps" section of <a href="gitpacking.html">gitpacking(7)</a>.
 633 </td>
 634 </tr>
 635 </table>
 636 </div>
 637 <div class="dlist">
 638 <dl>
 639 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.pattern</dt>
 640 <dd>
 641 <p>Regular expression used to match reference names. Commits
 642 pointed to by references matching this pattern (and meeting
 643 the below criteria, like <code>bitmapPseudoMerge.</code><em>&lt;name&gt;</em><code>.sampleRate</code>
 644 and <code>bitmapPseudoMerge.</code><em>&lt;name&gt;</em><code>.threshold</code>) will be considered
 645 for inclusion in a pseudo-merge bitmap.</p>
 646 <div class="paragraph">
 647 <p>Commits are grouped into pseudo-merge groups based on whether or not
 648 any reference(s) that point at a given commit match the pattern, which
 649 is an extended regular expression.</p>
 650 </div>
 651 <div class="paragraph">
 652 <p>Within a pseudo-merge group, commits may be further grouped into
 653 sub-groups based on the capture groups in the pattern. These
 654 sub-groupings are formed from the regular expressions by concatenating
 655 any capture groups from the regular expression, with a <em>-</em> dash in
 656 between.</p>
 657 </div>
 658 <div class="paragraph">
 659 <p>For example, if the pattern is <code>refs/tags/</code>, then all tags (provided
 660 they meet the below criteria) will be considered candidates for the
 661 same pseudo-merge group. However, if the pattern is instead
 662 <code>refs/remotes/</code>([<code>0-9</code>])<code>+/tags/</code>, then tags from different remotes will
 663 be grouped into separate pseudo-merge groups, based on the remote
 664 number.</p>
 665 </div>
 666 </dd>
 667 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.decay</dt>
 668 <dd>
 669 <p>Determines the rate at which consecutive pseudo-merge bitmap
 670 groups decrease in size. Must be non-negative. This parameter
 671 can be thought of as <code>k</code> in the function <code>f</code>(<code>n</code>) <code>=</code> <code>C</code> * <code>n^-k</code>,
 672 where <code>f</code>(<code>n</code>) is the size of the `n`th group.</p>
 673 <div class="paragraph">
 674 <p>Setting the decay rate equal to <code>0</code> will cause all groups to be the
 675 same size. Setting the decay rate equal to <code>1</code> will cause the <code>n</code>`th
 676 <code>group</code> <code>to</code> <code>be</code> `1/n the size of the initial group.  Higher values of the
 677 decay rate cause consecutive groups to shrink at an increasing rate.
 678 The default is <code>1</code>.</p>
 679 </div>
 680 <div class="paragraph">
 681 <p>If all groups are the same size, it is possible that groups containing
 682 newer commits will be able to be used less often than earlier groups,
 683 since it is more likely that the references pointing at newer commits
 684 will be updated more often than a reference pointing at an old commit.</p>
 685 </div>
 686 </dd>
 687 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.sampleRate</dt>
 688 <dd>
 689 <p>Determines the proportion of non-bitmapped commits (among
 690 reference tips) which are selected for inclusion in an
 691 unstable pseudo-merge bitmap. Must be between <code>0</code> and <code>1</code>
 692 (inclusive). The default is <code>1</code>.</p>
 693 </dd>
 694 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.threshold</dt>
 695 <dd>
 696 <p>Determines the minimum age of non-bitmapped commits (among
 697 reference tips, as above) which are candidates for inclusion
 698 in an unstable pseudo-merge bitmap. The default is
 699 <code>1.week.ago</code>.</p>
 700 </dd>
 701 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.maxMerges</dt>
 702 <dd>
 703 <p>Determines the maximum number of pseudo-merge commits among
 704 which commits may be distributed.</p>
 705 <div class="paragraph">
 706 <p>For pseudo-merge groups whose pattern does not contain any capture
 707 groups, this setting is applied for all commits matching the regular
 708 expression. For patterns that have one or more capture groups, this
 709 setting is applied for each distinct capture group.</p>
 710 </div>
 711 <div class="paragraph">
 712 <p>For example, if your capture group is <code>refs/tags/</code>, then this setting
 713 will distribute all tags into a maximum of <code>maxMerges</code> pseudo-merge
 714 commits. However, if your capture group is, say,
 715 <code>refs/remotes/</code>([<code>0-9</code>]<code>+</code>)<code>/tags/</code>, then this setting will be applied to
 716 each remote&#8217;s set of tags individually.</p>
 717 </div>
 718 <div class="paragraph">
 719 <p>Must be non-negative. The default value is 64.</p>
 720 </div>
 721 </dd>
 722 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.stableThreshold</dt>
 723 <dd>
 724 <p>Determines the minimum age of commits (among reference tips,
 725 as above, however stable commits are still considered
 726 candidates even when they have been covered by a bitmap) which
 727 are candidates for a stable a pseudo-merge bitmap. The default
 728 is <code>1.month.ago</code>.</p>
 729 <div class="paragraph">
 730 <p>Setting this threshold to a smaller value (e.g., 1.week.ago) will cause
 731 more stable groups to be generated (which impose a one-time generation
 732 cost) but those groups will likely become stale over time. Using a
 733 larger value incurs the opposite penalty (fewer stable groups which are
 734 more useful).</p>
 735 </div>
 736 </dd>
 737 <dt class="hdlist1">bitmapPseudoMerge.&lt;name&gt;.stableSize</dt>
 738 <dd>
 739 <p>Determines the size (in number of commits) of a stable
 740 psuedo-merge bitmap. The default is <code>512</code>.</p>
 741 </dd>
 742 </dl>
 743 </div>
 744 </div>
 745 <div class="sect2">
 746 <h3 id="_examples">Examples</h3>
 747 <div class="paragraph">
 748 <p>Suppose that you have a repository with a large number of references,
 749 and you want a bare-bones configuration of pseudo-merge bitmaps that
 750 will enhance bitmap coverage of the <code>refs/</code> namespace. You may start
 751 with a configuration like so:</p>
 752 </div>
 753 <div class="listingblock">
 754 <div class="content">
 755 <pre>[bitmapPseudoMerge "all"]
 756         pattern = "refs/"
 757         threshold = now
 758         stableThreshold = never
 759         sampleRate = 100
 760         maxMerges = 64</pre>
 761 </div>
 762 </div>
 763 <div class="paragraph">
 764 <p>This will create pseudo-merge bitmaps for all references, regardless of
 765 their age, and group them into 64 pseudo-merge commits.</p>
 766 </div>
 767 <div class="paragraph">
 768 <p>If you wanted to separate tags from branches when generating
 769 pseudo-merge commits, you would instead define the pattern with a
 770 capture group, like so:</p>
 771 </div>
 772 <div class="listingblock">
 773 <div class="content">
 774 <pre>[bitmapPseudoMerge "all"]
 775         pattern = "refs/(heads/tags)/"</pre>
 776 </div>
 777 </div>
 778 <div class="paragraph">
 779 <p>Suppose instead that you are working in a fork-network repository, with
 780 each fork specified by some numeric ID, and whose refs reside in
 781 <code>refs/virtual/NNN/</code> (where <code>NNN</code> is the numeric ID corresponding to some
 782 fork) in the network. In this instance, you may instead write something
 783 like:</p>
 784 </div>
 785 <div class="listingblock">
 786 <div class="content">
 787 <pre>[bitmapPseudoMerge "all"]
 788         pattern = "refs/virtual/([0-9]+)/(heads|tags)/"
 789         threshold = now
 790         stableThreshold = never
 791         sampleRate = 100
 792         maxMerges = 64</pre>
 793 </div>
 794 </div>
 795 <div class="paragraph">
 796 <p>Which would generate pseudo-merge group identifiers like "1234-heads",
 797 and "5678-tags" (for branches in fork "1234", and tags in remote "5678",
 798 respectively).</p>
 799 </div>
 800 </div>
 801 </div>
 802 </div>
 803 <div class="sect1">
 804 <h2 id="_see_also">SEE ALSO</h2>
 805 <div class="sectionbody">
 806 <div class="paragraph">
 807 <p><a href="git-pack-objects.html">git-pack-objects(1)</a>
 808 <a href="git-repack.html">git-repack(1)</a></p>
 809 </div>
 810 </div>
 811 </div>
 812 <div class="sect1">
 813 <h2 id="_git">GIT</h2>
 814 <div class="sectionbody">
 815 <div class="paragraph">
 816 <p>Part of the <a href="git.html">git(1)</a> suite</p>
 817 </div>
 818 </div>
 819 </div>
 820 </div>
 821 <div id="footer">
 822 <div id="footer-text">
 823 Last updated 2024-07-18 09:52:44 -0700
 824 </div>
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 826 </body>
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