Re: [Xen-users] slow network with gplpv drivers in vlan setup

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Tue, Feb 09, 2010 at 09:03:49PM +1100, James Harper wrote:
> > > Large Send is probably what is causing you problems. Some network
> cards
> > > support large send offload only for untagged packets.
> >
> > Well that's a bit strange because on 2 identical dom0 servers
> configured
> > the same way I had this behavior only on one dom0 (the really loaded
> > server).
> 
> How identical is identical? Are you able to determine the exact chipset
> and firmware version of the network adapters? Even if you bought the two
> servers from the same supplier at exactly the same time, there is still
> a chance that there is some hardware difference, most likely firmware.
> 

ethtool -i <interface> in dom0 might show the firmware version.

like this:

# ethtool -i eth0
driver: e1000e
version: 1.0.2-k2
firmware-version: 5.11-8
bus-info: 0000:04:00.0


> 
> Large send means that the network card will accept TCP packets well in
> excess of the actual MTU, up to about 60K. The network card computes
> checksum, seq, etc for you. So if you want to send a lot of TCP data
> it's the difference between windows giving one 60K packet to the network
> card vs 40 packets.
> 
> It gets even better when you are talking about virtual machines because
> a Linux Dom0 can keep the packet 'large' as long as all the things it
> has to pass through can handle it, whether that's from the DomU to Dom0,
> DomU through the bridge to another DomU, or DomU through the bridge to
> the physical network card.
> 
> In the testing I've done it's been the difference between 2GBits/second
> iperf throughput and 3-4GBits/second. That's probably not representative
> of real-world workloads though.
> 
> So in turning it off you do lose out on performance, but only if it
> worked in the first place, which it doesn't for you.
> 
> If you could figure out exactly what is different between you're 2
> Dom0's I'd be grateful. I keep getting these reports of LSO causing
> problems for some people and have never been able to properly figure out
> exactly why...
> 

Please paste "ethtool ethX" and "ethtool -i ethX" output from both dom0s. 
If you're using the xen network-bridge script then the device is pethX instead 
of ethX.

-- Pasi


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