[Bloat] AQM creeping into L2 equipment

[Eric Dumazet]

On Fri, 2014-03-21 at 16:53 +0100, renaud sallantin wrote:
> For our tests,  we needed to adjust the "tcp_initial_quantum" in the
> FQ, 
> but as you said, it is just a FQ parameter. 
> 

Yep, default ones are a compromise between performance and pacing
accuracy. At 40Gbps speeds, it is a bit challenging.

The consensus is that IW10 is adopted, meaning that we can send 10 MSS
at whatever speed we want without knowing anything of the network
conditions.

If people want to play with other values, they have to change the route
settings of their linux box, and fq parameters if they want.


ip ro change default via 192.168.1.254 dev eth0 initcwnd 20

(As a matter of fact it seems some providers use higher values than
IW10)

> The "patch" we added, and once again, it was just a  few lines,
> enabled to set, via a sysctl parameter, the initial pacing value,
> regardless of the RTT.
> This can be valuable for different reasons:
>      o  In case of long RTT, not set the pacing value is going to
> introduce an un-necessary delay
> (we aims to use this mechanism for satcom, so the delay could be
> greater than 500ms)

If you have a 500ms rtt, then you also want a bigger IW. Sending 10 MSS
in the first RTT is going to be slow, no matter how you pace them.
The first ACK wont come before 500 ms.

>      o  In case of a wrong RTT measurement, i.e. an RTT measurement
> that is higher that the real RTT (because of congestion for example),
>  you are going to have a wrong pacing evaluation... 

Well, if you have big rtt because of congestion, you exactly want to
reduce the rate... 

rate = cwnd * mss / srtt

And fq/pacing uses srtt, not rtt, so a single wrong rtt doesn't have a
big impact (unless it is the first sample, as it will serve as the ewma
initial value)

You can not predict the network conditions just by studying the
SYN/SYNACK/ACK initial messages. It gives a guess, but it is hard to
send everything you want in a single RTT at 'optimal speed'

Thats why it was so hard to decide the IW if you want an universal
value.
It depends on the state of the Internet, and it changes every day or
so...