Re: [Xen-devel] Strange network behaviour

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

What's the granularity of your measurement?

If, for example, the actual delta between was 1.5ms, but you're only
measuring at a 1ms granularity, you'd expect to see something like
this:

Actual time, measured time
1 , 1
2.5, 3
4 , 4
5.5 , 6
7 , 7

&c

 -George

On Tue, Jul 28, 2009 at 9:21 AM, Marco Tizzoni<marco.tizzoni@xxxxxxxxx> wrote:
> 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
> environments.
> 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
> resources?
>
> Thaks,
> Marco
> On Xen the packet are sent, and then receive
>
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