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[Xen-devel] Xen and tc problems

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Subject: [Xen-devel] Xen and tc problems
From: "Padala, Pradeep" <pradeep.padala@xxxxxx>
Date: Thu, 1 Mar 2007 23:25:27 -0000
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Thread-topic: Xen and tc problems

I am trying to setup traffic shaping with tc on Xen. I have already
posted a message to the tc mailing list regarding my problems with tc.
http://mailman.ds9a.nl/pipermail/lartc/2007q1/020395.html (message
pasted below for convenience). One of my colleagues has a similar setup
but the traffic shaping works. The only difference is that he has a
slower (100mbit) interface. Any ideas on why this is happening would be

Leaving the tc issue at that, I found from the mailing lists that there
is a parameter 'rate' that can be specified while creating vifs. There
seems to be very little documentation about it. I am using Xen
3.0.3-unstable and I am wondering whether I can use this option. Does it
work ? Can I change the rate dynamically ? If yes, how ?


-- Message posted to lartc mailing list


I am trying to shape traffic to two VMs hosted in Xen. There seems to be
very little information regarding this. I found this web page
ge-on-xen-linux-setup/ and followed the instructions. But, the real
bandwidth experienced from clients always seems to exceed the set rate.
Part of the problem may be because of the way Xen bridging is setup.
There are many interfaces that the packets go through. So, I switched to
the Xen routed networking, in which dom0 simply sees two virtual
interfaces for the VM, which are kind of PPP connections to the eth0
interfaces in VM.

eth0 +---- vif1.0  -- eth0 in VM1
     +---- vif2.0  -- eth1 in VM2

Say, I want to limit the bandwidth to VM1 to 100mbit and VM2 to 500mbit
(eth0 is a 1gbit interface), I used to following commands.

iptables -t mangle -F POSTROUTING
tc qdisc add dev eth0 root handle 1: htb r2q 1000
iptables -t mangle -A POSTROUTING -s $vm1_ip -j CLASSIFY --set-class 1:1
iptables -t mangle -A POSTROUTING -d $vm1_ip -j CLASSIFY --set-class 1:1
tc class add dev eth0 parent 1: classid 1:1 htb rate 512mbit
iptables -t mangle -A POSTROUTING -s $vm2_ip -j CLASSIFY --set-class 1:2
iptables -t mangle -A POSTROUTING -d $vm2_ip -j CLASSIFY --set-class 1:2
tc class add dev eth0 parent 1: classid 1:2 htb rate 512mbit

I setup a web server in VM1 and download a 1GB file from another machine
that is on the same network (actually on the same enclosure). I always
see wire speeds on the client side. I have tried many configurations
including adding a sfq, pfifo, tbf class under the leaf classes, but
either the rate becomes too low (because packets are dropped at the
leaves) or too high.

Part of the problem lies in the fact the vif1.0 has already received the
traffice, so it has to be overlimited at eth0, instead of dropping. So,
I tried a simple tbf within the VM. That doesn't work either with very
low speeds. Xen VMs don't have very precise clocks, so that might be one
reason why the reliable tbf is also not performing well.

I also set the burst sizes manually and the speed again becomes
exceptionally low.

Please let me know if you have any ideas on why this is happening. I can
paste the stats as well, if required.


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