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

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

> 
> Hello James,
> 
> I faced quite a long time ago a problem on some of my servers:
> network interface with gplpv driver has an horrible throughput
(~6minute
> to copy ~15MBytes).
> I supposed it's partially linked to my setup as I have a vlan
interface
> in the dom0 in the bridge (because I want to put some PV/HV servers in
> different vlans.
> 
> The problem occurs only with servers/workstation not on the same
server
> host (ie. communication domU/domU on the same host are ok and
dom0/domU
> also).
> 
> * If i put the plain interface (ie. eth0) in the bridge and do the
vlan
> * Without GPLPV the throughput is OK
> 
> Today I remade some test:
> 
> * I tried the 0.11.0.188
> * I tried to desactivate checksum offload
> 
> As a last test I deactivated scatter/gather and it turns out that it
> make it.
> 
> I have two captures:
> http://www.matws.net/mat/misc/xen_slow.gz
> http://www.matws.net/mat/misc/xen_notslow_part.gz
> 
> Any chance for an explanation ? what scatter/gather is supposed to
bring ?

Large Send is probably what is causing you problems. Some network cards
support large send offload only for untagged packets. 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.

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.

James

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