{
  config,
  lib,
  pkgs,
  pkgs-unoptimized,
  ...
}:
{
  imports = [ ];

  config = lib.mkMerge [
    { }
    (lib.mkIf (!config.me.buildingIso) {
      nix.settings.system-features = lib.mkForce [
        "gccarch-znver4"
        "gccarch-skylake"
        # "gccarch-alderlake" missing WAITPKG
        "gccarch-x86-64-v3"
        "gccarch-x86-64-v4"
        "benchmark"
        "big-parallel"
        "kvm"
        "nixos-test"
      ];

      # Keep ALL dependencies so we can rebuild offline. This DRASTICALLY increase disk usage, but disk space is cheap.
      # system.includeBuildDependencies = true;

      # This also should enable building offline? TODO: test.
      # nix.extraOptions = ''
      #   keep-outputs = true
      #   keep-derivations = true
      # '';

      nixpkgs.hostPlatform = {
        gcc.arch = "znver4";
        gcc.tune = "znver4";
        system = "x86_64-linux";
      };

      nixpkgs.overlays = [
        (
          final: prev:
          let
            optimizeWithFlags =
              pkg: flags:
              pkg.overrideAttrs (old: {
                NIX_CFLAGS_COMPILE = [ (old.NIX_CFLAGS_COMPILE or "") ] ++ flags;
              });
            addConfig =
              additionalConfig: pkg:
              pkg.override (oldconfig: {
                structuredExtraConfig = pkg.structuredExtraConfig // additionalConfig;
              });
          in
          {
            linux_znver4 =
              addConfig
                {
                  # Full preemption
                  PREEMPT = lib.mkOverride 60 lib.kernel.yes;
                  PREEMPT_VOLUNTARY = lib.mkOverride 60 lib.kernel.no;

                  # Google's BBRv3 TCP congestion Control
                  TCP_CONG_BBR = lib.kernel.yes;
                  DEFAULT_BBR = lib.kernel.yes;

                  # Preemptive Full Tickless Kernel at 300Hz
                  HZ = lib.kernel.freeform "300";
                  HZ_300 = lib.kernel.yes;
                  HZ_1000 = lib.kernel.no;
                }
                (
                  optimizeWithFlags prev.linux_6_13 [
                    "-march=znver4"
                    "-mtune=znver4"
                  ]
                );
            # gsl = prev.gsl.overrideAttrs (old: {
            #   # gsl tests fails when optimizations are enabled.
            #   # > FAIL: cholesky_invert unscaled hilbert (  4,  4)[0,2]: 2.55795384873636067e-13                        0
            #   # >  (2.55795384873636067e-13 observed vs 0 expected) [28259614]
            #   doCheck = false;
            # });
          }
        )
        (final: prev: {
          haskellPackages = prev.haskellPackages.extend (
            final': prev': {
              crypton = pkgs-unoptimized.haskellPackages.crypton;
            }
          );
        })
        # (final: prev: {
        #   haskell = prev.haskell // {
        #     packageOverrides = final.lib.composeExtensions prev.haskell.packageOverrides (
        #       final': prev': {
        #         crypton = pkgs-unoptimized.haskellPackages.crypton;
        #       }
        #     );
        #   };
        # })
        (final: prev: {
          redis = pkgs-unoptimized.redis;
          gsl = pkgs-unoptimized.gsl;
        })
      ];

      boot.kernelPackages = pkgs.linuxPackagesFor pkgs.linux_znver4;
    })
    (lib.mkIf (config.me.buildingIso) {
      boot.kernelPackages = pkgs.linuxPackagesFor pkgs.linux_6_13;
      boot.supportedFilesystems.zfs = true;
    })
  ];
}