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
> 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 127.0.0.1)
> 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 127.0.0.1)
> 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 188.8.131.52-xenU 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.
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