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

[Peter Privat <peter.privat@xxxxxxxxxxxx>]

On Sun, 2009-03-01 at 15:56 +1100, James Harper wrote:
> > Hi,
> > 
> > I'm running Ubuntu 8.04 XEN with latest kernel as a dom0. I'm running
> > Windows 2003 SP1 as domU, where I also have installed the 0.9.11 PV
> > Drivers. Everything works fine except for the outgoing IP traffic from
> the
> > domU, which seems to be limited to a maximum of around 0.5 mbit. The
> > incoming IP traffic works fine. No limitations. If I start the dumU
> > without the PV drivers activated, the outgoing IP traffic is working
> like
> > normal. What am I doing wrong here?
> > 
>
> Where is your IP traffic going? (to Dom0, to another DomU, or to a
> physical network adapter)
>
> If a physical network adapter, what sort?

Hi James,

I'm using Xen on two different servers. On one server, the IP traffic is going through the onboard 100 mbit ethernet port IP adaptor (nVidia Corporation MCP61 Ethernet).

vif = [ 'type=ioemu, bridge=eth0' ]

The other server is using both the onboard 1 Gbit ( Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller) port and an PCI slot 1 Gbit (Realtek Semiconductor Co., Ltd. RTL-8169 Gigabit Ethernet) port.

Either:
vif = [ 'type=ioemu, bridge=eth0' ]
or
vif = [ 'type=ioemu, bridge=eth1' ]


> I'd start by turning off the available offloads in the advanced
> properties of the gplpv xennet adapter.

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?

Regards,
/Peter



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