RE: [Xen-users] Slow outgoing IP traffic with PV drivers

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> James, I've received following from Øyvind at this list, which really
> helped me out:
> 
> "Try turning off Large Segment Offloading in the driver.
> Most likely your bridge is pushing packets out on the physical network
> with a larger size than the recipient can handle.
> You could test this by sniffing the recipient end the bridge. If you see
> large packets out from the bridge that doesn't get to the recipient, then
> you have found your problem.
> 
> Large Segment Offloading is enabled by default, and set to a high value
> (larger then MTU). Since it seems that the PV driver does not respect the
> MTU, and does not segment the TCP-data (for better domu-domu speeds?) this
> causes the bridge (this might be due to a bug in my setup which is gentoo
> with vanilla xen configuration) to transmit frames that are larger than
> the supported MTU of the recipients. I haven't yet tried enabling support
> for jumbo frames on my clients, but still."
> 
> James... Are there any more "tunings" possible?
> 

Not really. All of those offloads will make things go faster if they work, but 
much much slower if they don't. The idea of LSO is to give the physical network 
card 64k packets and let it break them into MTU sized chunks. If your linux 
kernel network card driver has bugs in that code path (many do) then it will 
either completely not work at all, or will seem to work for a few 
days/weeks/months and then start misbehaving.

James


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