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

[Dave Taht <dave@taht.net>]

"Bless, Roland (TM)" <roland.bless@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 "

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
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

>
>> link fully utilized is defined as Q>0 unless you don't include the
>> packet currently being transmitted. I do,
>> so the TXtteer is never idle. But that's a detail.
>
> I wouldn't define link fully utilized as Q>0, but if Q>0 then
> the link is fully utilized (that's what I meant by the direction
> of implication).
>
> Rgards,
>  Roland
>
>> 
>> 
>> On Tue, Nov 27, 2018 at 11:35 AM Bless, Roland (TM)
>> <roland.bless@kit.edu <mailto:roland.bless@kit.edu>> wrote:
>> 
>>     Hi,
>> 
>>     Am 27.11.18 um 11:29 schrieb Luca Muscariello:
>>     > I have never said that you need to fill the buffer to the max size to
>>     > get full capacity, which is an absurdity.
>> 
>>     Yes, it's absurd, but that's what today's loss-based CC algorithms do.
>> 
>>     > I said you need at least the BDP so that the queue never empties out.
>>     > The link is fully utilized IFF the queue is never emptied.
>> 
>>     I was also a bit imprecise: you'll need a BDP in flight, but
>>     you don't need to fill the buffer at all. The latter sentence
>>     is valid only in the direction: queue not empty -> link fully utilized.
>> 
>>     Regards,
>>      Roland
>> 
>>     >
>>     >
>>     >
>>     > On Tue 27 Nov 2018 at 11:26, Bless, Roland (TM)
>>     <roland.bless@kit.edu <mailto:roland.bless@kit.edu>
>>     > <mailto:roland.bless@kit.edu <mailto:roland.bless@kit.edu>>> wrote:
>>     >
>>     >     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
>>     >     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
>>     >
>>     >     > 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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