tcp offloading

[Colin Percival]

On 05/23/13 07:02, Jay West wrote:
> Environment is Citrix Xenserver Advanced, with (mostly) freebsd 9.1 64bit
> HVM with PV drivers.
> 
> I've heard much mention around the net that in pretty much all cases, TCP
> offloading should be disabled inside the VM. I have also seen other posts
> that TCP offloading should be disabled on the Dom0.
> 
> So my question is; for best performance should TCP offloading be disabled in
> the VM, or the Xenserver (dom0), or should it be disabled in BOTH?

There's an incompatibility between FreeBSD's TCP stack, TSO, and the Linux
Xen netback driver: With TSO enabled on netfront, we can generate longer
mbuf chains than the Linux netback is able to handle.  I have an ugly patch
for this on EC2, but andre will be fixing this better in the near future.

For now I recommend
# ifconfig xn0 -tso
unless you need multi-Gbps networking performance.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid