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

[Dave Taht]

"Bless, Roland (TM)" <roland.bless at kit.edu> writes:

> Hi Luca,
>
> Am 27.11.18 um 10:24 schrieb Luca Muscariello:
>> A congestion controlled protocol such as TCP or others, including QUIC,
>> LEDBAT and so on
>> need at least the BDP in the transmission queue to get full link
>> efficiency, i.e. the queue never empties out.
>
> This is not true. There are congestion control algorithms
> (e.g., TCP LoLa [1] or BBRv2) that can fully utilize the bottleneck link
> capacity without filling the buffer to its maximum capacity. The BDP

Just to stay cynical, I would rather like the BBR and Lola folk to look
closely at asymmetric networks, ack path delay, and lower rates than
1Gbit. And what the heck... wifi. :)

BBRv1, for example, is hard coded to reduce cwnd to 4, not lower - because
that works in the data center. Lola, so far as I know, achieves its
tested results at 1-10Gbits. My world and much of the rest of the world,
barely gets to a gbit, on a good day, with a tail-wind.

If either of these TCPs could be tuned to work well and not saturate
5Mbit links I would be a happier person. RRUL benchmarks anyone?

I did, honestly, want to run lola, (codebase was broken), and I am
patiently waiting for BBRv2 to escape (while hoping that the googlers
actually run some flent tests at edge bandwidths before I tear into it)

Personally, I'd settle for SFQ on the CMTSes, fq_codel on the home
routers, and then let the tcp-ers decide how much delay and loss they
can tolerate.

Another thought... I mean... can't we all just agree to make cubic
more gentle and go fix that, and not a have a flag day? "From linux 5.0
forward cubic shall:

Stop increasing its window at 250ms of delay greater than
the initial RTT? 

Have it occasionally rtt probe a bit, more like BBR?


> rule of thumb basically stems from the older loss-based congestion
> control variants that profit from the standing queue that they built
> over time when they detect a loss:
> while they back-off and stop sending, the queue keeps the bottleneck
> output busy and you'll not see underutilization of the link. Moreover,
> once you get good loss de-synchronization, the buffer size requirement
> for multiple long-lived flows decreases.
>
>> This gives rule of thumbs to size buffers which is also very practical
>> and thanks to flow isolation becomes very accurate.
>
> The positive effect of buffers is merely their role to absorb
> short-term bursts (i.e., mismatch in arrival and departure rates)
> instead of dropping packets. One does not need a big buffer to
> fully utilize a link (with perfect knowledge you can keep the link
> saturated even without a single packet waiting in the buffer).
> Furthermore, large buffers (e.g., using the BDP rule of thumb)
> are not useful/practical anymore at very high speed such as 100 Gbit/s:
> memory is also quite costly at such high speeds...
>
> Regards,
>  Roland
>
> [1] M. Hock, F. Neumeister, M. Zitterbart, R. Bless.
> TCP LoLa: Congestion Control for Low Latencies and High Throughput.
> Local Computer Networks (LCN), 2017 IEEE 42nd Conference on, pp.
> 215-218, Singapore, Singapore, October 2017
> http://doc.tm.kit.edu/2017-LCN-lola-paper-authors-copy.pdf


This whole thread, although diversive... well, I'd really like everybody
to get together and try to write a joint paper on the best stuff to do,
worldwide, to make bufferbloat go away.

>> Which is: 
>> 
>> 1) find a way to keep the number of backlogged flows at a reasonable value. 
>> This largely depends on the minimum fair rate an application may need in
>> the long term.
>> We discussed a little bit of available mechanisms to achieve that in the
>> literature.
>> 
>> 2) fix the largest RTT you want to serve at full utilization and size
>> the buffer using BDP * N_backlogged.  
>> Or the other way round: check how much memory you can use 
>> in the router/line card/device and for a fixed N, compute the largest
>> RTT you can serve at full utilization. 
>> 
>> 3) there is still some memory to dimension for sparse flows in addition
>> to that, but this is not based on BDP. 
>> It is just enough to compute the total utilization of sparse flows and
>> use the same simple model Toke has used 
>> to compute the (de)prioritization probability.
>> 
>> This procedure would allow to size FQ_codel but also SFQ.
>> It would be interesting to compare the two under this buffer sizing. 
>> It would also be interesting to compare another mechanism that we have
>> mentioned during the defense
>> which is AFD + a sparse flow queue. Which is, BTW, already available in
>> Cisco nexus switches for data centres.
>> 
>> I think that the the codel part would still provide the ECN feature,
>> that all the others cannot have.
>> However the others, the last one especially can be implemented in
>> silicon with reasonable cost.
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