most modules can be evaluated for their documentation in a very
restricted environment that doesn't include all of nixpkgs. this
evaluation can then be cached and reused for subsequent builds, merging
only documentation that has changed into the cached set. since nixos
ships with a large number of modules of which only a few are used in any
given config this can save evaluation a huge percentage of nixos
options available in any given config.
in tests of this caching, despite having to copy most of nixos/, saves
about 80% of the time needed to build the system manual, or about two
second on the machine used for testing. build time for a full system
config shrank from 9.4s to 7.4s, while turning documentation off
entirely shortened the build to 7.1s.
some options have default that are best described in prose, such as
defaults that depend on the system stateVersion, defaults that are
derivations specific to the surrounding context, or those where the
expression is much longer and harder to understand than a simple text
snippet.
escape interpolations in descriptions where possible, replace them with
sufficiently descriptive text elsewhere. also expand cfg.* paths in
descriptions.
adds defaultText for all options that use `cfg.*` values in their
defaults, but only for interpolations with no extra processing (other
than toString where necessary)
Previously the extraComponents added to an overriden package would not
have been considered in hardening measures enforced by the module.
Home Assistant is warning the user about component definitions having
moved away from YAML, so using an override to include support for a
component might become the better way moving forward.
The nix.daemonNiceLevel options allows for setting the nice level of the
Nix daemon process. On a modern Linux kernel with group scheduling the
nice level only affects threads relative to other threads in the same
task group (see sched(7)). Therefore this option has not the effect one
might expect.
The options daemonCPUSchedPolicy and daemonIOSchedClass are introduced
and the daemonIONiceLevel option renamed to daemonIOSchedPrority for
consistency. These options allow for more effective control over CPU
and I/O scheduling.
Instead of setting daemonNiceLevel to a high value to increase the
responsiveness of an interactive system during builds -- which would not
have the desired effect, as described above -- one could set both
daemonCPUSchedPolicy and daemonIOSchedClass to idle.
