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.
mkDerivedConfig : Option a -> (a -> Definition b) -> Definition b
Create config definitions with the same priority as the definition of another option.
This should be used for option definitions where one option sets the value of another as a convenience.
For instance a config file could be set with a `text` or `source` option, where text translates to a `source`
value using `mkDerivedConfig options.text (pkgs.writeText "filename.conf")`.
It takes care of setting the right priority using `mkOverride`.
Before this change, one could set environment.etc.*.text and .source.
.source would always take precedence, regardless of the priorities set.
This change means that if, for instance, .text is set with mkForce but
.source is set normally, the .text content will be the one to take
effect. If they are set with the same priority they will conflict.
Since e791519f0f ("nixos/qemu-vm: use qemu_kvm"), VMs generated with
nixos-rebuild build-vm use the qemu_kvm package instead of the qemu
package. (The difference between them is that qemu_kvm is only built
with support for the host architecture, not all architectures.)
But with this change, nixos-rebuild build-vm would now depend on
_both_ QEMUs, because the guest agent module was still using the one
from the full QEMU package. There's no need for it to use this
instead of the lighter qemu_kvm, because the guest agent shouldn't be
affected by which platforms QEMU can emulate.
