[Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

[Eric Dumazet]

On Wed, 2015-04-15 at 18:23 +0100, George Dunlap wrote:

> Which means that max(2*skb->truesize, sk->sk_pacing_rate >>10) is
> *already* larger for Xen; that calculation mentioned in the comment is
> *already* doing the right thing.

Sigh.

1ms of traffic at 40Gbit is 5 MBytes

The reason for the cap to /proc/sys/net/ipv4/tcp_limit_output_bytes is
to provide the limitation of ~2 TSO packets, which _also_ is documented.

Without this limitation, 5 MBytes could translate to : Fill the queue,
do not limit.

If a particular driver needs to extend the limit, fine, document it and
take actions.