monitoring queue length

[Jonathan Morton]

> On 1 Dec, 2018, at 12:05 am, Azin Neishaboori <azin.neishaboori at gmail.com> wrote:
> 
> I do not know if this backlog is indeed queue length or not. It seems strange to be queue length, because even when I flood the network with high UDP data rates over capacity, it still shows 0b of backlog. Am I looking at the wrong parameter? If so, could you please point out the tool that shows the instantaneous queue length?

There are two potential points of confusion here:

1: Linux throttles both TCP and UDP at the application socket level to prevent queues building up in the qdiscs and HW devices.  If it's your machine producing the packets, that's probably the effect you're seeing; there'll be a few packets queued in the HW (invisibly) and none in the qdisc.  That's approximately true regardless of which qdisc is in use, though with a shaping qdisc you might see a few packets collect there instead of in the HW.

2: If your traffic is coming from outside, it won't be queued upon receipt unless you introduce an artificial bottleneck.  There are ways of doing that.

For queuing experiments, we normally set up a "dumbbell" topology in which two different machines act as source and drain of traffic, and a third machine in the middle acts as a network emulator with artificial delays, losses and bandwidth limits.  That middlebox is where you would then observe the queuing.

 - Jonathan Morton