[Bloat] AQM creeping into L2 equipment

[Eric Dumazet]

On Fri, 2014-03-21 at 09:15 +0100, renaud sallantin wrote:

> 
> FQ/pacing enables to do a lot of things, 
> and as I already said, it could be used to easily implement the
> Initial Spreading.
> (we did it and it' s just a few lines to add, and a couple of
> parameters to change)
> 
> 
> But for the moment, FQ/Pacing sends the IW in one burst (up to 10
> segments).
> 

This is not true. This depends on RTT and your qdisc parameters.

Whole point of TSO autosizing is to make all this stuff automatic.

Here is the tcpdump output for a 10ms RTT, which is quite standard.

You can see 5 packets are sent, with a delay of more than 1 ms.

07:58:52.616379 IP 10.246.11.51.39905 > 10.246.11.52.41276: S 2187811646:2187811646(0) win 29200 <mss 1460,nop,nop,sackOK,nop,wscale 6>
07:58:52.626575 IP 10.246.11.52.41276 > 10.246.11.51.39905: S 81785763:81785763(0) ack 2187811647 win 29200 <mss 1460,nop,nop,sackOK,nop,wscale 7>
07:58:52.626642 IP 10.246.11.51.39905 > 10.246.11.52.41276: . ack 1 win 457
07:58:52.626671 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 1:2921(2920) ack 1 win 457
07:58:52.627740 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 2921:5841(2920) ack 1 win 457
07:58:52.628815 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 5841:8761(2920) ack 1 win 457
07:58:52.629946 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 8761:11681(2920) ack 1 win 457
07:58:52.631054 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 11681:14601(2920) ack 1 win 457
07:58:52.637147 IP 10.246.11.52.41276 > 10.246.11.51.39905: . ack 2921 win 274
07:58:52.637207 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 14601:17521(2920) ack 1 win 457
07:58:52.638117 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 17521:20441(2920) ack 1 win 457
07:58:52.638114 IP 10.246.11.52.41276 > 10.246.11.51.39905: . ack 5841 win 320
07:58:52.639011 IP 10.246.11.51.39905 > 10.246.11.52.41276: . 20441:23361(2920) ack 1 win 457


You also can tune /proc/sys/net/ipv4/tcp_min_tso_segs from 2 to 1 if you
really want...

No kernel patches needed...