Hi Simon,
On Jul 28, 2015, at 16:31 , Simon Barber <[email protected]> wrote:
> The issue is that Codel tries to keep the delay low, and will start dropping
> when sojourn time grows above 5ms for 100ms. For longer RTT links more delay
> is necessary to avoid underutilizing the link. This is due to the
> multiplicative decrease - it's worst with Reno, where the halving of cWind
> means that you need to have a full BDP of data in the buffer to avoid the
> link going idle when cWind is halved. With longer RTTs this means more delay
> than Codel allows is required to avoid a throughput hit. The worst case
> happens when a single flow is controlled, but that can be a common situation.
> My proposal is to sense and have the target value in Codel automatically
> adjust when this worst case scenario happens - which would mitigate most of
> the downside.
According to theory you should adapt interval if at all and the set
target between 5-10% of that interval. See
https://datatracker.ietf.org/doc/draft-ietf-aqm-codel/?include_text=1 sections
4.3 and 4.4. Now that could all be off.The upshot is increasing target as a
response for long RTTs will sacrifice latency again for bandwidth, pretty much
that the avoidance of is codel’s claim to fame ;)
Best Regards
Sebastian
>
> Simon
>
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> http://www.aqua-mail.com
>
>
> On July 28, 2015 7:21:56 AM Eric Dumazet <[email protected]> wrote:
>
>> On Tue, 2015-07-28 at 07:11 -0700, Simon Barber wrote:
>> > The main danger is the negative effects on performance of using Codel. You
>> > may experience low throughput on high RTT links.
>>
>> Really ? I've never seen this, unless you mess with codel qdisc
>> attributes maybe.
>>
>> (Some guys seem to not really understand whole concept of a queue, and
>> set very low values for 'limit' attribute for example)
>>
>>
>>
>>
>
>
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