[Bloat] when does the CoDel part of fq_codel help in the real world?

[Bless, Roland (TM)]

Hi Dave,

Am 29.11.18 um 08:39 schrieb Dave Taht:
> "Bless, Roland (TM)" <roland.bless at kit.edu> writes:
> 
>> Hi Luca,
>>
>> Am 27.11.18 um 11:40 schrieb Luca Muscariello:
>>> OK. We agree.
>>> That's correct, you need *at least* the BDP in flight so that the
>>> bottleneck queue never empties out.
>>
>> No, that's not what I meant, but it's quite simple.
>> You need: data min_inflight=2 * RTTmin * bottleneck_rate to filly
>> utilize the bottleneck link.
>> If this is true, the bottleneck queue will be empty. If your amount
>> of inflight data is larger, the bottleneck queue buffer will store
>> the excess packets. With just min_inflight there will be no
>> bottleneck queue, the packets are "on the wire".
>>
>>> This can be easily proven using fluid models for any congestion
>>> controlled source no matter if it is 
>>> loss-based, delay-based, rate-based, formula-based etc.
>>>
>>> A highly paced source gives you the ability to get as close as
>>> theoretically possible to the BDP+epsilon
>>> as possible.
>>
>> Yep, but that BDP is "on the wire" and epsilon will be in the bottleneck
>> buffer.
> 
> I'm hoping I made my point effectively earlier, that
> 
> " data min_inflight=2 * RTTmin * bottleneck_rate "

That factor of 2 was a mistake in my first mail (sorry for that...).
I corrected that three minutes after. I should have written:
data min_inflight=RTTmin * bottleneck_rate

> when it is nearly certain that more than one flow exists, means aiming
> for the BDP in a single flow is generally foolish. Liked the stanford

I think one should not confuse the buffer sizing rule with the
calcluation for inflight data...

> result, I think it's pretty general. I see hundreds of flows active
> every minute. There was another paper that looked into some magic
> 200-ish number as simultaneous flows active, normally

So for buffer sizing, the BDP dependent rule is foolish in general,
because it is optimized for older loss-based TCP congestion controls
so that they can keep the utilization high. It's correct that in
presence of multiple flows and good loss desynchronization, you
still get high utilization with a smaller buffer (Appenzeller et. al,
SIGCOMM 2004).

However, when it comes to CWnd sizing, that inflight rule would convert
to:
data min_inflight=RTTmin * bottleneck_rate_share
because other flows are present at the bottleneck.

Interestingly enough: flows with a different RTT_min should
use different CWnds, but their amount of queued data at the bottleneck
should be nearly equal if you want to have flow rate fairness.

Regards
 Roland