[Codel] GRO (generic receive offload) is not helpful with codel and fq_codel

[Eric Dumazet]

On Thu, 2012-11-22 at 08:22 +0100, Dave Taht wrote:
> qdisc fq_codel 120: parent 1:12 limit 10240p flows 16000 quantum 500
> target 20.0ms interval 100.0ms
>  Sent 2213780 bytes 4843 pkt (dropped 48, overlimits 0 requeues 0)
>  backlog 12160b 8p requeues 0
>   maxpacket 14546 drop_overlimit 0 new_flow_count 1237 ecn_mark 0
>                      *******
>   new_flows_len 1 old_flows_len 2

Yes, GRO/TSO are not good for speeds below 10Mb.

In your case, you probably have one Gigabit ethernet (allowing GRO
packets being constructed), forwarding to the WAN at 750 kbits ... 

Definitely GRO should be turned off in this case.

A GSO packet of 64KB would take about 600ms to be transmitted !

Ideally, we should segment GSO packets at these low forwarding rates,
and keep GRO turned on for local traffic (not forwarded)