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.

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

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).

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.

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:

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:

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:

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:

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