Change network qdisc

etc/kmod/whitelist.conf

@@ -154,6 +154,8 @@ whitelist realtek              # Realtek PHY driver
 whitelist rng_core             # H/W Random Number Generator (RNG) driver
 whitelist rpcsec_gss_krb5      # Sun RPC Kerberos 5 module
 whitelist sb_edac              # MC Driver for Intel Sandy Bridge and Ivy Bridge memory controllers
+whitelist sch_fq               # Fair Queue Packet Scheduler
+whitelist sch_fq_codel         # Fair Queue CoDel discipline
 whitelist sch_prio             # Simple 3-band priority qdisc
 whitelist sch_sfq              # Stochastic Fairness qdisc
 whitelist sha3_generic         # SHA-3 Secure Hash Algorithm

etc/sysctl.d/90-default-qdisc.conf

@@ -0,0 +1,19 @@
+# Use fq instead of pfifo_fast as the default qdisc.
+#
+# fq paces locally generated TCP traffic at the rate the TCP stack
+# computed for the socket. Data then leaves in an even stream instead of
+# line-rate bursts, which are likely to overflow the small buffers of
+# access points and switch ports in front of slow links (for example a
+# laptop on wifi).
+#
+# fq also keeps a separate queue per flow and serves them in turn. When
+# the local link is saturated, a single bulk transfer can no longer delay
+# all other flows on the interface, as it can with the shared FIFO of
+# pfifo_fast.
+#
+# Note: fq is meant for hosts that terminate TCP connections. On a router,
+# fq_codel may be the better choice: forwarded packets have no local
+# socket, so fq cannot pace them, while the CoDel part of fq_codel actively
+# keeps queueing delay low.
+
+net.core.default_qdisc = fq

mxrouter/mxrouterctl

@@ -423,10 +423,27 @@ sub radvd {
 our $want_if = {'lo' => 1}; # includes lo, veth and physical devices but not vlan devices
 our $want_veth;
 
+# Use fq_codel, which suits forwarded traffic, instead of the global default
+# qdisc. Keep mq on multiqueue devices and use fq_codel per tx queue.
+sub set_qdisc_fq_codel {
+    my ($dev) = @_;
+    my $tx_queues = grep /^tx-/, scandir("/sys/class/net/$dev/queues");
+    if ($tx_queues > 1) {
+        sys('tc', 'qdisc', 'replace', 'dev', $dev, 'root', 'handle', '1:', 'mq');
+        for my $i (1 .. $tx_queues) {
+            sys('tc', 'qdisc', 'replace', 'dev', $dev, 'parent', sprintf('1:%x', $i), 'fq_codel');
+        }
+    } else {
+        sys('tc', 'qdisc', 'replace', 'dev', $dev, 'root', 'fq_codel');
+    }
+}
+
 sub configure_if {
     my ($want_if) = @_;
     for my $if (keys %$want_if) {
-        netif_is_up($if) or sys('ip', 'link', 'set', $if, "up");
+        netif_is_up($if) and next;
+        sys('ip', 'link', 'set', $if, "up");
+        -d "/sys/class/net/$if/device" and set_qdisc_fq_codel($if);
     }
 }