sysctl.d: default to fq_codel, fight bufferbloat

Quoting from Jon Corbet's report of Stephen Hemminger's talk at Linux
Plumbers Conference 2014 (https://lwn.net/Articles/616241/):

    [...] So Stephen encouraged everybody to run a command like:

    sysctl -w net.core.default_qdisc=fq_codel

    That will cause fq_codel to be used for all future connections
    [Qdiscs apply to interfaces, not connections. Pointed out by TomH
    in the article comments. -- mschmidt] (up to the next reboot).
    Unfortunately, the default queuing discipline cannot be changed,
    since it will certainly disturb some user's workload somewhere.

Let's have the recommended default in systemd.

Thanks to Dave Täht for advice and the summary at
https://lists.bufferbloat.net/pipermail/cerowrt-devel/2014-October/003701.html

NEWS

@@ -74,6 +74,18 @@ CHANGES WITH 217:
           systemd-ask-password gained a new --echo option to turn
           that on.
 
+        * The default sysctl.d/ snippets will now set:
+
+                net.core.default_qdisc = fq_codel
+
+          This selects Fair Queueing Controlled Delay as the default
+          queueing discipline for network interfaces. fq_codel helps
+          fight the network bufferbloat problem. It is believed to be
+          a good default with no tuning required for most workloads.
+          Downstream distributions may override this choice. On 10Gbit
+          servers that do not do forwarding, "fq" may perform better.
+          Systems without a good clocksource should use "pfifo_fast".
+
 CHANGES WITH 216:
 
         * timedated no longer reads NTP implementation unit names from

sysctl.d/50-default.conf

@@ -25,6 +25,9 @@ net.ipv4.conf.all.accept_source_route = 0
 net.ipv4.conf.default.promote_secondaries = 1
 net.ipv4.conf.all.promote_secondaries = 1
 
+# Fair Queue CoDel packet scheduler to fight bufferbloat
+net.core.default_qdisc = fq_codel
+
 # Enable hard and soft link protection
 fs.protected_hardlinks = 1
 fs.protected_symlinks = 1