GITPACKING(7) Git Manual GITPACKING(7)

NAME

gitpacking - Advanced concepts related to packing in Git

SYNOPSIS

gitpacking

DESCRIPTION

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), git-
repack(1), and others, as well as gitformat-pack(5), 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
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).

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

git-pack-objects(1) git-repack(1)

GIT

Part of the git(1) suite

Git 2.48.1 2025-01-13 GITPACKING(7)