Re: [Cerowrt-devel] [Bloat] BBR congestion control algorithm for TCP in net-next

[Alan Jenkins <alan.christopher.jenkins@gmail.com>]

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On 21/09/16 13:40, Mikael Abrahamsson wrote:
> On Wed, 21 Sep 2016, Dave Taht wrote:
>
>> * It seriously outcompetes cubic, particularly on the single queue 
>> aqms. fq_codel is fine. I need to take apart the captures to see how 
>> well it is behaving in this case. My general hope was that with fq in 
>> place, anything that was delay based worked better as it was only 
>> competing against itself.
>
> I'm looking at 4up-sqwave-fq_bfifo-256k. Is this really fq_bfifo, or 
> just bfifo? Looks like there is no fq.

the queue before the (simulated) _bottleneck_ is simple bfifo.

sch_fq is used on the _endpoint_, because it's a requirement of tcp-bbr

It's disambiguated by knowing you don't add child qdiscs to sch_fq, and 
no-one has written a linux qdisc called fq_bfifo.

>
> If someone doesn't have the correct Flent available, I posted two 
> screenshots here: http://imgur.com/a/cFtMd
>
> What I think I see:
>
> The flows are started in order: "BBR1, CUBIC2, BBR4, CUBIC3" (a bit 
> confusing, but according to your description).
>
> So it looks like BBR1 fills the pipe within half a second or so, nice 
> steady state. Then CUBIC2 starts, and slowly over a few seconds, 
> starts to starve BBR1 of BW, it looks like steady state here would be 
> that CUBIC2 would end up with around 65-70% of the BW, and BBR1 
> getting 30-35%. Then BBR4 comes along (10 seconds in), and just KILLS 
> them both, smacks them over the head with a hammer, taking 90% of the 
> BW, wildly oscillating in BW way above 20 megabit/s down to 10. The 
> ping here goes up to around 150-160ms. CUBIC3 starts at 15 seconds and 
> get basically no bw at all.
>
> Then at around 22 seconds in, I guess pretty close to 12-13 seconds 
> after BBR4 was started, BBR4 starts to calm down, slowly letting the 
> other streams come back to life. At around 30 seconds, they all seem 
> to get at least a bit of the bw each and nobody is completely starved, 
> but BBR1 seems to not get much BW at all (very dotted line).
>
> When at the end there is only CUBIC3 and BBR4 left, it looks like BBR4 
> has a 2/3 to 1/3 advantage.
>
> Looking at cake_flowblind_noecn, BBR1 and BBR4 just kills both CUBIC 
> flows. Same with PIE.
>
> So it seems my intuition was wrong, at least for these scenarios. It 
> wasn't CUBIC that would kill BBR, it's the other way around. Great to 
> have testing tools! Thanks Flent!
>

wow.  I was wondering how BBR ever relinquished bandwidth.  "With some 
difficulty".

Though not in the second screenshot where there's a strict AQM (cake 
flowblind).  Then it shares with other BBRs while never letting CUBIC 
get established.  (I think if you look at drops, they'd be higher than 
what 4xCUBIC  converges to on that link.  See 
https://groups.google.com/forum/#!msg/bbr-dev/EXgiWxHvaEU/wl3Q39svAAAJ ).

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