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GITPACKING(7) Git Manual GITPACKING(7)

NAME top

   gitpacking - Advanced concepts related to packing in Git

SYNOPSIS top

   gitpacking

DESCRIPTION top

   This document aims to describe some advanced concepts related to
   packing in Git.

   Many concepts are currently described scattered between manual
   pages of various Git commands, including [git-pack-objects(1)](../man1/git-pack-objects.1.html),
   [git-repack(1)](../man1/git-repack.1.html), and others, as well as [gitformat-pack(5)](../man5/gitformat-pack.5.html), and parts
   of the **Documentation/technical** tree.

   There are many aspects of packing in Git that are not covered in
   this document that instead live in the aforementioned areas. Over
   time, those scattered bits may coalesce into this document.

PSEUDO-MERGE BITMAPS top

       **Note**

       Pseudo-merge bitmaps are considered an experimental feature,
       so the configuration and many of the ideas are subject to
       change.

Background Reachability bitmaps are most efficient when we have on-disk stored bitmaps for one or more of the starting points of a traversal. For this reason, Git prefers storing bitmaps for commits at the tips of refs, because traversals tend to start with those points.

   But if you have a large number of refs, it’s not feasible to store
   a bitmap for _every_ ref tip. It takes up space, and just OR-ing all
   of those bitmaps together is expensive.

   One way we can deal with that is to create bitmaps that represent
   _groups_ of refs. When a traversal asks about the entire group, then
   we can use this single bitmap instead of considering each ref
   individually. Because these bitmaps represent the set of objects
   which would be reachable in a hypothetical merge of all of the
   commits, we call them pseudo-merge bitmaps.

Overview A "pseudo-merge bitmap" is used to refer to a pair of bitmaps, as follows:

   Commit bitmap
       A bitmap whose set bits describe the set of commits included
       in the pseudo-merge’s "merge" bitmap (as below).

   Merge bitmap
       A bitmap whose set bits describe the reachability closure over
       the set of commits in the pseudo-merge’s "commits" bitmap (as
       above). An identical bitmap would be generated for an octopus
       merge with the same set of parents as described in the commits
       bitmap.

   Pseudo-merge bitmaps can accelerate bitmap traversals when all
   commits for a given pseudo-merge are listed on either side of the
   traversal, either directly (by explicitly asking for them as part
   of the **HAVES** or **WANTS**) or indirectly (by encountering them during
   a fill-in traversal).

Use-cases For example, suppose there exists a pseudo-merge bitmap with a large number of commits, all of which are listed in the WANTS section of some bitmap traversal query. When pseudo-merge bitmaps are enabled, the bitmap machinery can quickly determine there is a pseudo-merge which satisfies some subset of the wanted objects on either side of the query. Then, we can inflate the EWAH-compressed bitmap, and OR it in to the resulting bitmap. By contrast, without pseudo-merge bitmaps, we would have to repeat the decompression and OR-ing step over a potentially large number of individual bitmaps, which can take proportionally more time.

   Another benefit of pseudo-merges arises when there is some
   combination of (a) a large number of references, with (b) poor
   bitmap coverage, and (c) deep, nested trees, making fill-in
   traversal relatively expensive. For example, suppose that there
   are a large enough number of tags where bitmapping each of the
   tags individually is infeasible. Without pseudo-merge bitmaps,
   computing the result of, say, **git rev-list --use-bitmap-index**
   **--count --objects --tags** would likely require a large amount of
   fill-in traversal. But when a large quantity of those tags are
   stored together in a pseudo-merge bitmap, the bitmap machinery can
   take advantage of the fact that we only care about the union of
   objects reachable from all of those tags, and answer the query
   much faster.

Configuration Reference tips are grouped into different pseudo-merge groups according to two criteria. A reference name matches one or more of the defined pseudo-merge patterns, and optionally one or more capture groups within that pattern which further partition the group.

   Within a group, commits may be considered "stable", or "unstable"
   depending on their age. These are adjusted by setting the
   **bitmapPseudoMerge.**_<name>_**.stableThreshold** and
   **bitmapPseudoMerge.**_<name>_**.threshold** configuration values,
   respectively.

   All stable commits are grouped into pseudo-merges of equal size
   (**bitmapPseudoMerge.**_<name>_**.stableSize**). If the **stableSize**
   configuration is set to, say, 100, then the first 100 commits
   (ordered by committer date) which are older than the
   **stableThreshold** value will form one group, the next 100 commits
   will form another group, and so on.

   Among unstable commits, the pseudo-merge machinery will attempt to
   combine older commits into large groups as opposed to newer
   commits which will appear in smaller groups. This is based on the
   heuristic that references whose tip commit is older are less
   likely to be modified to point at a different commit than a
   reference whose tip commit is newer.

   The size of groups is determined by a power-law decay function,
   and the decay parameter roughly corresponds to "k" in **f**(**n**) **=**
   **C***n^(**-k/100**), where **f**(**n**) describes the size of the **n**-th
   pseudo-merge group. The sample rate controls what percentage of
   eligible commits are considered as candidates. The threshold
   parameter indicates the minimum age (so as to avoid including
   too-recent commits in a pseudo-merge group, making it less likely
   to be valid). The "maxMerges" parameter sets an upper-bound on the
   number of pseudo-merge commits an individual group

   The "stable"-related parameters control "stable" pseudo-merge
   groups, comprised of a fixed number of commits which are older
   than the configured "stable threshold" value and may be grouped
   together in chunks of "stableSize" in order of age.

   The exact configuration for pseudo-merges is as follows:

       **Note**

       The configuration options in **bitmapPseudoMerge.*** are
       considered EXPERIMENTAL and may be subject to change or be
       removed entirely in the future. For more information about the
       pseudo-merge bitmap feature, see the "Pseudo-merge bitmaps"
       section of [gitpacking(7)](../man7/gitpacking.7.html).

   bitmapPseudoMerge.<name>.pattern
       Regular expression used to match reference names. Commits
       pointed to by references matching this pattern (and meeting
       the below criteria, like **bitmapPseudoMerge.**_<name>_**.sampleRate**
       and **bitmapPseudoMerge.**_<name>_**.threshold**) will be considered for
       inclusion in a pseudo-merge bitmap.

       Commits are grouped into pseudo-merge groups based on whether
       or not any reference(s) that point at a given commit match the
       pattern, which is an extended regular expression.

       Within a pseudo-merge group, commits may be further grouped
       into sub-groups based on the capture groups in the pattern.
       These sub-groupings are formed from the regular expressions by
       concatenating any capture groups from the regular expression,
       with a _-_ dash in between.

       For example, if the pattern is **refs/tags/**, then all tags
       (provided they meet the below criteria) will be considered
       candidates for the same pseudo-merge group. However, if the
       pattern is instead **refs/remotes/**([**0-9**])**+/tags/**, then tags from
       different remotes will be grouped into separate pseudo-merge
       groups, based on the remote number.

   bitmapPseudoMerge.<name>.decay
       Determines the rate at which consecutive pseudo-merge bitmap
       groups decrease in size. Must be non-negative. This parameter
       can be thought of as **k** in the function **f**(**n**) **= C** * **n^-k**, where
       **f**(**n**) is the size of the `n`th group.

       Setting the decay rate equal to **0** will cause all groups to be
       the same size. Setting the decay rate equal to **1** will cause
       the **n**`th **group to be** `1/n the size of the initial group.
       Higher values of the decay rate cause consecutive groups to
       shrink at an increasing rate. The default is **1**.

       If all groups are the same size, it is possible that groups
       containing newer commits will be able to be used less often
       than earlier groups, since it is more likely that the
       references pointing at newer commits will be updated more
       often than a reference pointing at an old commit.

   bitmapPseudoMerge.<name>.sampleRate
       Determines the proportion of non-bitmapped commits (among
       reference tips) which are selected for inclusion in an
       unstable pseudo-merge bitmap. Must be between **0** and **1**
       (inclusive). The default is **1**.

   bitmapPseudoMerge.<name>.threshold
       Determines the minimum age of non-bitmapped commits (among
       reference tips, as above) which are candidates for inclusion
       in an unstable pseudo-merge bitmap. The default is **1.week.ago**.

   bitmapPseudoMerge.<name>.maxMerges
       Determines the maximum number of pseudo-merge commits among
       which commits may be distributed.

       For pseudo-merge groups whose pattern does not contain any
       capture groups, this setting is applied for all commits
       matching the regular expression. For patterns that have one or
       more capture groups, this setting is applied for each distinct
       capture group.

       For example, if your capture group is **refs/tags/**, then this
       setting will distribute all tags into a maximum of **maxMerges**
       pseudo-merge commits. However, if your capture group is, say,
       **refs/remotes/**([**0-9**]**+**)**/tags/**, then this setting will be applied
       to each remote’s set of tags individually.

       Must be non-negative. The default value is 64.

   bitmapPseudoMerge.<name>.stableThreshold
       Determines the minimum age of commits (among reference tips,
       as above, however stable commits are still considered
       candidates even when they have been covered by a bitmap) which
       are candidates for a stable a pseudo-merge bitmap. The default
       is **1.month.ago**.

       Setting this threshold to a smaller value (e.g., 1.week.ago)
       will cause more stable groups to be generated (which impose a
       one-time generation cost) but those groups will likely become
       stale over time. Using a larger value incurs the opposite
       penalty (fewer stable groups which are more useful).

   bitmapPseudoMerge.<name>.stableSize
       Determines the size (in number of commits) of a stable
       psuedo-merge bitmap. The default is **512**.

Examples Suppose that you have a repository with a large number of references, and you want a bare-bones configuration of pseudo-merge bitmaps that will enhance bitmap coverage of the refs/ namespace. You may start with a configuration like so:

       [bitmapPseudoMerge "all"]
               pattern = "refs/"
               threshold = now
               stableThreshold = never
               sampleRate = 100
               maxMerges = 64

   This will create pseudo-merge bitmaps for all references,
   regardless of their age, and group them into 64 pseudo-merge
   commits.

   If you wanted to separate tags from branches when generating
   pseudo-merge commits, you would instead define the pattern with a
   capture group, like so:

       [bitmapPseudoMerge "all"]
               pattern = "refs/(heads/tags)/"

   Suppose instead that you are working in a fork-network repository,
   with each fork specified by some numeric ID, and whose refs reside
   in **refs/virtual/NNN/** (where **NNN** is the numeric ID corresponding to
   some fork) in the network. In this instance, you may instead write
   something like:

       [bitmapPseudoMerge "all"]
               pattern = "refs/virtual/([0-9]+)/(heads|tags)/"
               threshold = now
               stableThreshold = never
               sampleRate = 100
               maxMerges = 64

   Which would generate pseudo-merge group identifiers like
   "1234-heads", and "5678-tags" (for branches in fork "1234", and
   tags in remote "5678", respectively).

SEE ALSO top

   [git-pack-objects(1)](../man1/git-pack-objects.1.html) [git-repack(1)](../man1/git-repack.1.html)

GIT top

   Part of the [git(1)](../man1/git.1.html) suite

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Git 2.48.1.166.g58b580 2025-01-31 GITPACKING(7)

Pages that refer to this page:git-config(1), gitpacking(7)