[Cake] Emulating Bufferbloat

Dave Taht dave at taht.net
Thu Nov 22 16:30:07 EST 2018

Toke Høiland-Jørgensen <toke at toke.dk> writes:

> 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

I have not upgraded to this branch. And in general I do recomend using a
separate box. "Nuke it from orbit, it's the only way to be *sure*".

However, recently I was able to get bufferbloat by using network
namespaces for my test, and perhaps that "still works"?

In this extremely long thread there's a test setup of network namespaces
you can try:


>> 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
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