nix-daemon: improve open files for NixOS VM tests and thundering effects for GC

modules/nix-daemon.nix

@@ -24,11 +24,19 @@ in
 
   config = {
     warnings = lib.optionals (config.simd.arch == null) [ "Please set simd.arch for ${config.networking.hostName}" ];
+    # Allow more open files for non-root users to run NixOS VM tests.
+    security.pam.loginLimits = [
+      { domain = "*"; item = "nofile"; type = "-"; value = "20480"; }
+    ];
 
     nix = {
+      # Garbage-collect often
       gc.automatic = true;
-      gc.dates = "03:15";
-      gc.options = "--delete-older-than 30d";
+      gc.dates = "*:45";
+      gc.options = ''--max-freed "$((128 * 1024**3 - 1024 * $(df -P -k /nix/store | tail -n 1 | ${pkgs.gawk}/bin/awk '{ print $4 }')))"'';
+
+      # Randomize GC to avoid thundering herd effects.
+      gc.randomizedDelaySec = "1800";
 
       # 2.11, 2.12 suffers from a bug with remote builders…
       package = pkgs.nixVersions.nix_2_13;