[uanog] switch's port buffers

[Volodymyr Litovka]

Привет,

для любителей лонгридов, есть прекрасная статья про стратегии 
буферизации и обработки очередей по результатам воркшопа в Стэнфордском 
университете в 2019 году - "*It hosted 98 attendees from 12 economies 
with 26 from academia and 72 from industry.*" -- 
https://blog.apnic.net/2019/12/12/sizing-the-buffer/

Для совсем уж любителей копать - материалы этого воркшопа доступны тут: 
http://buffer-workshop.stanford.edu/program/

> [ ... ] buffers also add additional lag to a packet’s transit through 
> the network. If you want to implement a low jitter service, then deep 
> buffers are decidedly unfriendly! The result is the rather enigmatic 
> observation that network buffers have to be as big as they need to be, 
> but no bigger!

Так какими же? :) Главные выводы из статьи:

=== 1. утверждение "буфер 40MB - отстой" не выдерживает проверки 
академическими исследованиями ===

> A study by a Stanford TCP research group in 2004 
> <http://doi.acm.org/10.1145/1030194.1015499> used the central limit 
> theorem to point to a radically smaller model of buffer size. Link 
> efficiency can be maintained for N desynchronized flows with a buffer 
> that is dimensioned to the size of:
>
> *Size* = (*/BW/* ∙ */RTT/*) / √*/N/*
>
> This is a radical result for high-speed extended latency links in a 
> busy network. The consequences on router design are enormous:
>
>     “For example, a 1 Tb/s ISP router carrying one TCP flow with an
>     RTTmin of 100ms would require 12.5 GB of buffer and off-chip
>     buffering. *If it carries 100,000 flows, then the buffer can be
>     safely reduced to less than 40MB, reducing the buffering and
>     worst-case latency by 99.7%*. With small buffers, the buffer would
>     comfortably fit on a single chip switch ASIC.”
>
>     Nick McKeown et. al. Sizing Router Buffers (Redux)
>     <https://ccronline.sigcomm.org/2019/ccr-october-2019/sizing-router-buffers-redux/>
>
иными словами, совершенно не случайно большинство коммутаторов из списка 
https://people.ucsc.edu/~warner/buffer.html в портовом диапазоне 
48x10/25 + 6-8/100 укомплектованы размером буфера 32-40MB - этого 
достаточно. А если учесть, что операторские реалии - это 1/10G агрегация 
(даже не 25), то можно предположить, что оператор is absolutely safe с 
буфером такого размера в своих коммутаторах.

=== 2. использование inline notification улучшает качество передачи 
трафика ===

> The advantage of ECN is that the sender is not placed in the position 
> of being informed of a congestion condition well after the condition 
> has occurred. Explicit notification allows the sender to be informed 
> of a condition as it is forming so that it can take action while there 
> is still a coherent ack pacing signal coming back from the receiver 
> (that is before packet loss occurs).
>
> However, ECN is only a single bit marking. Is this enough? [ ... ] The 
> conclusion from one presentation is that the single-bit marking, while 
> coarse and non-specific is probably sufficient to moderate 
> self-clocking TCP flows such that they do not place pressure on 
> network buffers, leaving the buffers to deal with short term bursts 
> from unconstrained sources.
>
[ ... ]
>
> And if we want to reduce buffer size and maintain efficient and fair 
> performance how can we achieve it? One view is that sender pacing can 
> remove much of the pressure on buffers, and self-clocking flows can 
> stabilise without emitting transient bursts that need to be absorbed 
> by buffers. Another view, and one that does not necessarily contradict 
> the first, is that the self-clocking algorithm can operate with higher 
> precision if there was some form of feedback from the network on the 
> state of the network path. This can be as simple as a single bit (ECN) 
> or a complete trace of path element queue state (HPCC).
>

всё же имеет смысл хотеть поддержки ECN

=== 3. flow-aware traffic management не является однозначной заявкой на 
успех ===

> If the network was in a position to be able to classify all currently 
> active flows into either elephants or mice, then the network could be 
> able to use different queuing regimes for each traffic class. This 
> sorting adds to the cost and complexity of packet switches, and if 
> scaling pressures are a factor in switch design then it’s not clear 
> that the additional cost of switch complexity would be offset by a far 
> superior efficiency outcome in the switching function.

=== 4. и если вдруг вендора начинают говорить про гигантские (напр 
100ms) буфера ===

> Further analysis reveals an estimate of packet drop rates if the 
> network’s buffers were reduced in size, and for this particular case, 
> the analysis revealed that an 18msec buffer would be able to sustain a 
> packet drop rate of less than 0.005%.

Спасибо.


On 15.10.2021 14:48, Volodymyr Litovka wrote:
>
> Привіт,
>
> а ось тут мене запитали, а я не знаю як відповісти :) Я розумію, що 
> питання може звучати трішки дивно, але тим не менш -
>
> за вашим досвідом, скільки буферу (на порт або shared) достатньо для 
> уникнення або зниження до непомітного рівня втрат пакетів при появі 
> traffic bursts? Ну тобто цікавлять не теоретичні розрахунки після 
> відповідей на питання на кшталт "а який час барста?", "а скільки 
> складає барст у відсотках до нормального" та таке інше, а по 
> робітничо-селянському - поточний досвід в живій мережі в поточних 
> умовах поточного Інтернету? Ну, наприклад, на будинковому комутаторі 
> 24x1G + 2x10G з підключеними середньостатистичними абонентами?
>
> Хтось взагалі цим питанням париться? :)
>
> Дякую.
>
> -- 
> Volodymyr Litovka
>    "Vision without Execution is Hallucination." -- Thomas Edison

-- 
Volodymyr Litovka
   "Vision without Execution is Hallucination." -- Thomas Edison
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