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

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

> > 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.

> > Linux doesn't seem
> > to know this though so the result is just that it doesn't work. Try
> > turning off large send and see what happens.
> >
> >
> Ok so you suggest to change the value of 61440 to something smaller
(ie.
> 8192)

It may be worth a go, but I suspect that the difference would be in
turning it off or on, not the size. Unless the underlying chipset has
some limitation in size... I hadn't considered that.

> 
> > Turning off scatter gather will (almost completely) disable large
send
> > also, because windows is then limited to a total packet length of
4096
> > bytes which can't be broken up unless the MSS is really small.
> >
> The question that stay is what is drawback of not having  large send ?
> 

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...

Thanks

James

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