From 9ff95589907fd1c909db4597abb0e5caf2cb8bbd Mon Sep 17 00:00:00 2001
From: "Peter H. Hoeg" <peter@hoeg.com>
Date: Sat, 28 Jun 2025 10:43:58 +0200
Subject: [PATCH] nixos/nix-optimise: run with lower priority

It's painfully obvious to me on my anaemic system when `nix-optimise` kicks in,
so run it with much lower priority since it's not time critical anyway.

Additionally, do not run on battery power.

I can appreciate that these are oppinionated defaults but I am having a hard
imagining cases where this would be a show-stopper so there isn't much point in
making this directly configurable via the nixos module system as people can
always override the unit directly themselves via `systemd.services.nix-optimise`
---
 nixos/modules/services/misc/nix-optimise.nix | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/nixos/modules/services/misc/nix-optimise.nix b/nixos/modules/services/misc/nix-optimise.nix
index 3581b95ce18a..93d36839f3fd 100644
--- a/nixos/modules/services/misc/nix-optimise.nix
+++ b/nixos/modules/services/misc/nix-optimise.nix
@@ -65,9 +65,17 @@ in
     systemd = lib.mkIf config.nix.enable {
       services.nix-optimise = {
         description = "Nix Store Optimiser";
-        # No point this if the nix daemon (and thus the nix store) is outside
-        unitConfig.ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";
-        serviceConfig.ExecStart = "${config.nix.package}/bin/nix-store --optimise";
+        unitConfig = {
+          ConditionACPower = true;
+          # No point this if the nix daemon (and thus the nix store) is outside
+          ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";
+        };
+        serviceConfig = {
+          ExecStart = "${lib.getExe' config.nix.package "nix-store"} --optimise";
+          Nice = 19;
+          CPUSchedulingPolicy = "idle";
+          IOSchedulingClass = "idle";
+        };
         startAt = lib.optionals cfg.automatic cfg.dates;
       };