fq_codel.nix

{ pkgs, config, ...}: {

  boot.kernel.sysctl."net.core.rps_sock_flow_entries" = "32768";

  networking.usePredictableInterfaceNames = false; # we want eth0

  system.activationScripts.network_debloat = {
    text = ''
      interface=eth0
      ${pkgs.iproute}/bin/tc qdisc replace dev "$interface" root fq_codel
      ${pkgs.iproute}/bin/tc qdisc replace dev "$interface" handle 1 root mq
      I=0
      for q in $(ls -d /sys/class/net/"$interface"/queues/rx-*); do
        echo $((I++)) > /dev/null
        ${pkgs.iproute}/bin/tc qdisc add dev "$interface" parent 1:$(printf "%x" $I) fq_codel
      done
      
      (
      ${pkgs.ethtool}/bin/ethtool -K "$interface" tso off
      ${pkgs.ethtool}/bin/ethtool -K "$interface" gso off
      ${pkgs.ethtool}/bin/ethtool -K "$interface" ufo off
      # Presently unknown if gro/lro affect latency much
      ${pkgs.ethtool}/bin/ethtool -K "$interface" gro off
      ${pkgs.ethtool}/bin/ethtool -K "$interface" lro off
      ) 2>/dev/null
      npr=$(getconf _NPROCESSORS_ONLN)
      for q in $(ls -d /sys/class/net/"$interface"/queues/rx-*); do
        echo ${config.boot.kernel.sysctl."net.core.rps_sock_flow_entries"} > $q/rps_flow_cnt
        if [[ "$npr" == "2" ]]; then
          echo 3 > $q/rps_cpus
        elif [[ "$npr" -gt 3 ]]; then
          echo 15 > $q/rps_cpus
        fi
      done
    '';
    deps = [];
  };
}