Simplest possible demo

To demonstrate the effect of the fq_codel scheduler, set up a fully saturated network and then observe fq_codel’s effect on ping time latency, as follows.

The effect can be clearly observed with a 10Mbps Ethernet connection:

sudo ethtool -s eth0 advertise 0x002   # restrict to 10Mbps for demo

In a terminal window, run a continuous ping to some other machine on the local network (the other machine does not need to have fq_codel) and leave it running:

ping othermachine

In another terminal window, start up a dd pipe to the other machine to fully saturate the network – you should see the adverse effect on the ping times:

dd if=/dev/urandom | ssh othermachine dd of=/dev/null

Now enable fq_codel – observe the beneficial effect on the ping times – about 30X reduction in latency on a saturated 10Mbps network!:

sudo tc qdisc add dev eth0 root fq_codel

Disable fq_codel and wait a moment for the buffers to get bloated again – the ping times go back up again:

sudo tc qdisc del dev eth0 root fq_codel

Remember to remove the demonstration 10Mbps restriction when you’re all done testing:

sudo ethtool -s eth0 advertise 0xFFF
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