Trimmed module lists

NixOS evaluates about 1,900 base modules for every host, used or not. A host with trimmedModules = true in the reference fleet evaluates only the ones it needs: about 400.

The list is the closure of a learned core and the options the host defines. It filters module-list.nix itself, so the order and the inline entries stay. The host passes it to nixosSystem as baseModules.

Reference fleet: web-1, app-2 toplevel	CPU	Thunks	GC total
full list	11.4 s	14.57 M	1.03 GB
trimmed list	10.5 s	13.21 M	0.94 GB

Three oracles

• modlist-same-drv: each trimmed toplevel has the same drvPath as the full build.
• modlist-negative: the list without nginx fails for a host that runs nginx, and the same list with nginx builds.
• cost.sh: the trimmed eval uses at most 95% of the thunks.

One nixpkgs revision

A list belongs to one nixpkgs revision. After an update, modlist-same-drv fails until you compute the lists again. Use this only with the oracle in CI.

platforms/nixos.nix The host sink: trimmed, full and own-pkgs builds.

modlist research How the lists are computed.