fc027554
by fendor at 2026-07-20T17:02:42+02:00
Introduce global unit database cache
As a first step for better sharing of `UnitInfo` across `UnitEnv`,
we introduce a new datatype called `ExternalUnitDatabases`.
It primarily serves as an in-memory representation of *all*
`UnitDatabase`s across `UnitEnv`. This means, if multiple `HomeUnitEnv`s
depend on the same database, one way or another, we make sure that we
don't parse from disk every time.
Instead, we store the in-memory representation in `ExternalUnitDatabases`.
`ExternalUnitDatabaseCache` is the equivalent of `ExternalUnitState` in
the `UnitEnv`. It is a mutable variable wrapping `ExternalUnitDatabases`.
The mutable `ExternalUnitDatabaseCache` is used in `initUnits` to make
sure we don't parse the same unit database multiple times.
Almost by accident, we change the semantics of `initUnits` to honour
modifications to `packageDBFlags`.
The inability to change `packageDBFlags` while also reusing the already
parsed `UnitDatabase`s was reported in #26423 as a bug.
Hence, we think this behaviour change is warranted and acceptable,
especially since it comes with a breaking change to the `initUnits` API.
Add regression test for #26423
Closes #26423
44561225
by fendor at 2026-07-20T17:03:02+02:00
Introduce UnitIndex for global external unit caching
`UnitInfo`s have been observed to cause a lot of memory usage in #27500.
Especially with multiple home units, as the same (external) units are
processed from scratch, even though most of the time we end up with
exactly the same `UnitInfo`.
We introduce a `UnitEnv` global cache that allows us to store external
unit information that is used across all `HomeUnitEnv`s.
The most important change in this commit is the introduction of the `UnitIndex`.
It stores a global mapping of `UnitId` -> `UnitInfo`, and `initUnits`
always uses the cached `UnitInfo` entry to populate each
`HomeUnitEnv`'s `UnitState`.
This allows us to ensure the following property:
> Each `UnitInfo` should be alive exactly once in GHC.
In addition, the `ExternalUnitDatabaseCache` may also hold a reference
to each on-disk representation of `UnitInfo`.
This means, we impose an hard upper bound on the number of `UnitInfo`s
alive in the GHC session:
> The number of alive `UnitInfo`s closure objects must be the
> sum of all loaded unit database times two.
We add performance regression tests that make sure the number of live
`UnitInfo` cannot exceed this threshold.
Closes #27500
-------------------------
Metric Decrease:
MultiComponentModules
MultiComponentModulesRecomp
MultiComponentModulesRecomp100
mhu-perf
-------------------------
These metrics increases are especially notable, as we are not even
sharing anything big but merely the global package database with 50
entries.
It shows how careful sharing of `UnitInfo` can improve memory usage.
We expect this to be much more notable when the whole cabal package
database is shared across multiple home units.
---
Technical details
To share the `UnitInfo`s correctly, it is important that we extract
the `WireMap` into the `UnitIndex`. At the moment of writing, `WireMap`
must be globally the same for all `HomeUnitEnv`s.
This is important, as we could otherwise not cache the "fully-resolved"
`UnitInfo`.
We consider a `UnitInfo` fully-resolved, if wired-in units have been
updated, the `UnitInfo` has been validated and variables in the unit
config, such as `${pkgroot}` have been resolved.
Updating the wired-in units requires the `WireMap` to be globally the
same.