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[Xen-users] Network performance problem

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Network performance problem
From: Fridtjof Busse <fbusse@xxxxxx>
Date: Tue, 18 Dec 2007 16:07:27 +0100
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I'm running Xen 3.1 on Kernel 2.6.22 (Ubuntu 7.10) and have an extremly
strange performance-problem.
The setup is using vif-route with ip_forward and proxy_arp, so every
domU has it's own vif-Interface.
The relevant line in the xen-config all read similiar to this:
"vif = [ 'ip=, mac=00:16:3e:11:22:33' ]"
The guest then have the default-gateway set to an ip in the same subnet
(/29) that is an alias for eth0 in the dom0.

Network-connectivity is fine, if I download something in the domU, I get
near wirespeed (up to 11MB/s). If I download something from the domU
(e.g. via apache from a remote site), I get maybe 40kB/s. 
So I started up tcpdump and I see many Retransmit, Out-of-order, Dup and
ICMP "fragmentation needed" packets. As every host on the way between
the domU and the testmachine is MTU 1500, I have no idea what causes the
I tried setting the domU as well as the dom0 to MTU 1300, without any
Is this a possible Xen-problem (configuration error on my end?), a
problem with the kernel or (rather unlikely as this is a big
colocation) a network problem? 
To be honest, I have no idea what to look for next, so I would really
appreciate any ideas. 

Fridtjof Busse

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