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[Xen-users] Slow TCP performance between Windows Vista and Xen PV-on-HVM

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Subject: [Xen-users] Slow TCP performance between Windows Vista and Xen PV-on-HVM guest
From: "Fischer, Anna" <anna.fischer@xxxxxx>
Date: Fri, 11 Jun 2010 19:01:15 +0000
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Thread-topic: Slow TCP performance between Windows Vista and Xen PV-on-HVM guest
I am running a Xen HVM guest with netfront PV drivers. This is running SLES10 
SP3 inside the guest. The Dom0 is also SLES10 SP3.

Now I am trying to communicate from that HVM guest to a Windows Visa or also 
Windows 7 machine and I am getting really poor TCP performance. When tracing on 
the network traffic, I can see that no packets are dropped or missing or 
anything, but what happens is that the Vista machine does not seem to send an 
ACK back to the Xen guest, and then the Xen guest is doing a retransmit after a 
while. But this is causing huge delays and SCP, for example, mostly fails 
completely because of the poor performance. I am thinking that perhaps the 
problem here could be that the Windows machine is using the delayed ACK 
mechanism but the Xen guest does not actually send further data so things 
deadlock until the retransmit timer kicks in. But I don't really understand why 
this is happening and what I can do to fix this problem.

Now you could think that perhaps this has nothing to do with Xen. However, 
strangely enough we had exactly the same setup but a VMWare SLES10 SP3 guest 
instead of a Xen guest. And with this we did not see this problem at all. So 
now I am wondering if this might be to do with the Xen network drivers? Has 
anyone else ever experienced this? TCP works absolutely fine between any Linux 


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