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Re: [Xen-users] Very slow domU network performance

To: Winston Chang <winston@xxxxxxxxxx>
Subject: Re: [Xen-users] Very slow domU network performance
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Tue, 4 Apr 2006 11:51:17 +0300
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On Tue, Apr 04, 2006 at 03:26:14AM -0400, Winston Chang wrote:
> I set up a domU as a backup server, but it has very, very poor  
> network performance with external computers.  I ran some tests with  
> iperf and found some very weird results.

I also see (UDP) packet loss from external box to domU, and from domU to 
box. TCP performance is poor because of this packet loss (TCP automatically
re-transmits the broken packets - this causes slow tcp speeds). 

I haven't tried latest unstable version of xen.. so I don't know if it's
already fixed. 

- Pasi
> Using iperf, I get these approximate numbers  (the left column is the  
> iperf client and the right column is the iperf server):
> domU --> domU  1.77 Gbits/sec (using
> domU --> domU  1.85 Gbits/sec (using domU eth0 IP address)
> dom0 --> domU  91.5 Mbits/sec
> domU --> dom0  85.2 Mbits/sec
> So far, so good.  The relatively slow dom0<->domU communication may  
> indicate a problem, but it's fine for my purposes.  The real problem  
> is when I use my iBook (running Mac OS X) to run some iperf tests.   
> The computers are connected via a crossover cable.  They were  
> originally connected with a hub, but I changed to a crossover cable  
> connection to reduce variables (it turns out this had no effect).
> dom0 --> iBook  89.0 Mbits/sec
> iBook --> dom0  86.9 Mbits/sec
> domU --> iBook 87.1 Mbits/sec
> iBook --> domU 1.87 Mbits/sec
> The last entry has me baffled.  Why would it be so incredibly slow in  
> one direction but not the other?
> I decided to run some UDP tests as well.
> server: iperf -s -u -i 1
> client: iperf -c server_ip -u -b 90M -t 5
> The packet loss is as follows:
> domU --> domU  0% (using
> domU --> domU  0% (using domU eth0 IP address)
> dom0 --> domU  ~100% (only 7 of 38464 made it!)
> domU --> dom0  0.09%
> dom0 --> iBook  4.7%
> iBook --> dom0  0.33%
> domU --> iBook  11%
> iBook --> domU  1.6%
> There are some odd things here.  First, dom0->domU with UDP loses  
> almost everything, but the reverse direction is fine.  Somehow, dom0- 
> >domU TCP was OK (if you consider ~90Mbps OK).
> The second weird thing is that in contrast with TCP, UDP works fine  
> in both directions between the iBook and domU.  There's 11% packet  
> loss in one case, but that's not a lot -- it's probably just a little  
> more than poor little iBook can handle.
> My dom0 is Fedora Core 5, with the included xen0 kernel.  The domU is  
> a very basic install of Centos 4.3, based on a jailtime.org image,  
> running the xensource kernel.  The domU has bridged  
> networking and 64MB of RAM (I ran the iBook->domU TCP test with 196MB  
> of RAM but it was still ~2Mb/s).  Firewalling is off in domU and  
> dom0; the only iptables rules are the ones created by the xen  
> bridging script.
> Has anyone else seen anything like this, or have any idea what's  
> going on?  This seems bizarre to me.
> Thanks for any help.
> --Winston

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