nixos/hardware: Add option to enable KSM.

This is essentially what's been done for the official NixOS build slaves
and I'm using it as well for a few of my machines and my own Hydra
slaves.

Here's the same implementation from the Delft server configurations:

https://github.com/NixOS/nixos-org-configurations/blob/f47c2fc7f82cc8aa4ea8047487cbb819d6829fe2/delft/common.nix#L91-L101

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/nixos/modules/hardware/ksm.nix b/nixos/modules/hardware/ksm.nix
new file mode 100644
index 000000000000..d6ac69b5d65e
--- /dev/null
+++ b/nixos/modules/hardware/ksm.nix
@@ -0,0 +1,18 @@
+{ config, lib, ... }:
+
+{
+  options.hardware.enableKSM = lib.mkEnableOption "Kernel Same-Page Merging";
+
+  config = lib.mkIf config.hardware.enableKSM {
+    systemd.services.enable-ksm = {
+      description = "Enable Kernel Same-Page Merging";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "systemd-udev-settle.service" ];
+      script = ''
+        if [ -e /sys/kernel/mm/ksm ]; then
+          echo 1 > /sys/kernel/mm/ksm/run
+        fi
+      '';
+    };
+  };
+}