monitoring queue length

Azin Neishaboori azin.neishaboori at
Sat Dec 1 02:37:33 EST 2018

Hi Jonathan
Thank you for your response. I think did not describe my setup well in the
first email. So here it goes:

I have an ubuntu virtual machine on my laptop that sends/receives data from
remote iperf servers. My laptop is connected by a 100Mbps Ethernet cable to
a router, and the router has two cellular antennas and an LTE SIM. The
router routes all the incoming and outgoing traffic to/from the VM from/to
remote iperf servers using the cellular link. The cellular link is thus my
bottleneck link, and I am looking for queue buildup at the router’s egress
interface, i.e., the cellular interface. I am looking into uplink mostly,
and the uplink rate of LTE is both limited and has errors. The uplink rate
I see is around 10 Mbps on a good received signal strength condition.

So based on the dumbbell topology you described, I should see queue buildup
at the egress cellular interface of the router, right?
 But when I periodically (once every second) run tc -s qdisc ls, the
backlog is consistently zero. Am I looking at the wrong information? Or am
I missing something even bigger?

Thanks a lot

On Fri, Nov 30, 2018 at 10:14 PM Jonathan Morton <chromatix99 at>

> > On 1 Dec, 2018, at 12:05 am, Azin Neishaboori <
> azin.neishaboori at> 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
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