[Cake] Emulating Bufferbloat

[Toke Høiland-Jørgensen]

Fabian Ruffy <fruffy at cs.ubc.ca> writes:

> Hello,
>
> this is a somewhat esoteric question. I am trying to actually force bufferbloat
> in an emulation setup I am using. I set up a dumbbell topology and push traffic
> through it, causing congestion at the central link. I use this setup to compare
> congestion avoidance algorithms such as DCTCP to other solutions.
> This has worked nicely with the 4.18 kernel. However, after upgrading to 4.19 I
> cannot reproduce bufferbloat anymore. The traffic (even UDP packets) is
> perfectly rate limited and I never see any congestion happening. This is great,
> but in practice it prevents me from prototyping algorithms.

Ha, that's awesome! :D

> My interface configuration for bottlenecked links is:
>
>     qdisc tbf 5: dev OBcbnsw1-eth2 root refcnt 2 rate 10Mbit burst 15000b lat
>     12.0ms
>      Sent 6042 bytes 51 pkt (dropped 0, overlimits 0 requeues 0)
>      backlog 0b 0p requeues 0
>     qdisc netem 10: dev OBcbnsw1-eth2 parent 5:1 limit 500
>      Sent 6042 bytes 51 pkt (dropped 0, overlimits 0 requeues 0)
>      backlog 0b 0p requeues 0
>
>
> I have the suspicion that it is related to the CAKE changes in the 4.19 kernel,
> but I am not exactly sure. I am not using tc cake at all. Do you maybe know
> what could cause this behavior? Apologies if this is the wrong mailing
> list.

My guess would be changes in the TCP stack; can't point you to anything
specific off the top of my head, though...

-Toke