On Tue, Feb 09, 2010 at 12:22:09PM +0200, Pasi Kärkkäinen wrote:
> 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.
>
Oh, and also "ethtool -k ethX" to get the dom0 offloading settings.
-- Pasi
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