Hannes Siebenhandl pushed to branch wip/fendor/external-unit-db-cache at Glasgow Haskell Compiler / GHC Commits: c9d5e531 by fendor at 2026-07-22T14:07:38+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. - - - - - 35 changed files: - + changelog.d/unit-index - compiler/GHC.hs - compiler/GHC/Driver/Backpack.hs - compiler/GHC/Driver/Env.hs - compiler/GHC/Driver/Main/Interactive.hs - compiler/GHC/Driver/Main/Passes.hs - compiler/GHC/Driver/Session/Units.hs - compiler/GHC/Types/Unique.hs - compiler/GHC/Unit/Env.hs - compiler/GHC/Unit/External/Database.hs - + compiler/GHC/Unit/External/Index.hs - + compiler/GHC/Unit/External/ModuleOrigin.hs - + compiler/GHC/Unit/External/Providers.hs - + compiler/GHC/Unit/External/Query.hs - + compiler/GHC/Unit/External/Substitution.hs - + compiler/GHC/Unit/External/Validate.hs - + compiler/GHC/Unit/External/Visibility.hs - + compiler/GHC/Unit/External/Wired.hs - compiler/GHC/Unit/Info.hs - compiler/GHC/Unit/State.hs - compiler/GHC/Unit/State.hs-boot - compiler/GHC/Unit/Types.hs - compiler/ghc.cabal.in - ghc/GHCi/UI.hs - hadrian/src/Rules/Generate.hs - testsuite/tests/count-deps/CountDepsParser.stdout - + testsuite/tests/driver/TUnitInfo/Foo.hs - + testsuite/tests/driver/TUnitInfo/Makefile - + testsuite/tests/driver/TUnitInfo/all.T - + testsuite/tests/driver/TUnitInfo/genMhu.sh - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space-mhu.stdout - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space-single.stdout - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space.hs - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space.stdout - utils/haddock/haddock-api/src/Haddock.hs The diff was not included because it is too large. View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c9d5e5318b4c9c1a2652b27fb079f4a7... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c9d5e5318b4c9c1a2652b27fb079f4a7... You're receiving this email because of your account on gitlab.haskell.org. Manage all notifications: https://gitlab.haskell.org/-/profile/notifications | Help: https://gitlab.haskell.org/help