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[Xen-devel] Strange network behaviour

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Strange network behaviour
From: Marco Tizzoni <marco.tizzoni@xxxxxxxxx>
Date: Tue, 28 Jul 2009 10:21:19 +0200
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Hi guys,
I'm doing some tests to understand how credit and sedf schedulers
perform on real-time systems in mixed io and cpu intesive
During my first test I saw a quite strange behaviour I'm going to describe.

The test has been setup as follow: 1 domain running Linux with default
scheduler setup and one bridged network. Here iperf run as a client
and sends udp packet to an host on the same net with 1Mb/s rate. Dom0
runs nothing except XenMon. Iperf (server mode) and a tcpdump run on a
non-virtualized host.

Measuring packets inter occurences, I'd expect to find out a costant
value because of the constant rate at which the packet are sent. It's
not so.
Let say the first packet arrives after 1ms, the second arrives after
3, the third after 4, the fourth after 6 and so on. The interoccurence
times are:
Packet#         Absolute Time        Interoccurence Time
 1                           1                            1
 2                           3                            2
 3                           4                            1
 4                           6                            2
 5                           7                            1
 6                           9                            2
...                          ....

I tried to shutdown domU and run iperf on dom0, same result.
The last test I made was run iperf on a non-virtualized environment.
And here I got the results I expect: constant interoccurence time.

Do  you have any idea of the reasong why the packet are sent with this
strange rate even if there are no other domains competiting for

On Xen the packet are sent, and then receive

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