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

[Stefano Stabellini]

On Wed, 15 Apr 2015, Eric Dumazet wrote:
> 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.

What actions do you have in mind exactly?  It would be great if you
could suggest how to move forward from here, beside documentation.

I don't think we can really expect every user that spawns a new VM in
the cloud to manually echo blah >
/proc/sys/net/ipv4/tcp_limit_output_bytes to an init script.  I cannot
imagine that would work well.