From e6a15db534d18d0eefb5916ee87b4951d51af2cf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Tue, 7 Apr 2020 08:21:02 +0100
Subject: [PATCH] nixos: default nix.maxJobs to auto

Instead of making the configuration less portable by hard coding the number of
jobs equal to the cores we can also let nix set the same number at runtime.
---
 .../modules/installer/tools/nixos-generate-config.pl  |  4 ----
 nixos/modules/services/misc/nix-daemon.nix            | 11 ++++++-----
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/nixos/modules/installer/tools/nixos-generate-config.pl b/nixos/modules/installer/tools/nixos-generate-config.pl
index 629c56814a16..a32c19a4eba9 100644
--- a/nixos/modules/installer/tools/nixos-generate-config.pl
+++ b/nixos/modules/installer/tools/nixos-generate-config.pl
@@ -91,9 +91,6 @@ sub hasCPUFeature {
 }
 
 
-# Detect the number of CPU cores.
-my $cpus = scalar (grep {/^processor\s*:/} (split '\n', $cpuinfo));
-
 
 # Determine CPU governor to use
 if (-e "/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors") {
@@ -562,7 +559,6 @@ my $hwConfig = <<EOF;
   boot.kernelModules = [$kernelModules ];
   boot.extraModulePackages = [$modulePackages ];
 $fsAndSwap
-  nix.maxJobs = lib.mkDefault $cpus;
 ${\join "", (map { "  $_\n" } (uniq @attrs))}}
 EOF
 
diff --git a/nixos/modules/services/misc/nix-daemon.nix b/nixos/modules/services/misc/nix-daemon.nix
index 8203cb13e506..e14df310ae87 100644
--- a/nixos/modules/services/misc/nix-daemon.nix
+++ b/nixos/modules/services/misc/nix-daemon.nix
@@ -92,13 +92,14 @@ in
 
       maxJobs = mkOption {
         type = types.either types.int (types.enum ["auto"]);
-        default = 1;
+        default = "auto";
         example = 64;
         description = ''
-          This option defines the maximum number of jobs that Nix will try
-          to build in parallel.  The default is 1.  You should generally
-          set it to the total number of logical cores in your system (e.g., 16
-          for two CPUs with 4 cores each and hyper-threading).
+          This option defines the maximum number of jobs that Nix will try to
+          build in parallel. The default is auto, which means it will use all
+          available logical cores. It is recommend to set it to the total
+          number of logical cores in your system (e.g., 16 for two CPUs with 4
+          cores each and hyper-threading).
         '';
       };