[Xen-users] sar incorrect network measurement in DomU

[Ricardo Jose Pfitscher <ricardo.pfitscher@xxxxxxxxx>]

Hello Everyone,

I'm using sar to monitor network throughput in a paravirtualized DomU and I'm seeing some odd values.

For example, if I run iperf on a 100 MBps network with another machine as the client and either dom0 or domU as the server, I get the same results:

DomU$ iperf -s
[  4] local 192.168.0.1 port 5001 connected with 10.20.129.175 port 37766
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.1 sec    114 MBytes  94.1 Mbits/sec

Dom0$ iperf -s
[  4] local 10.20.129.178 port 5001 connected with 10.20.129.175 port 58763
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.1 sec    113 MBytes  94.1 Mbits/sec

But when measure the throughput with sar while running iperf, the results are wrong in DomU:

DomU$ sar -n DEV 1 15

15:24:37        IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s rxcmp/s   txcmp/s  rxmcst/s
15:24:38         eth0   3177.30   1591.41   4651.67    102.58 0.00     0.00      0.00
15:24:39         eth0   5641.67   2822.22   8264.16    181.90 0.00     0.00      0.00
15:24:40         eth0   5684.62   2840.56   8327.07    183.08 0.00     0.00      0.00
15:24:41         eth0   5722.54   2856.34   8382.62    184.10 0.00     0.00      0.00
15:24:42         eth0   5684.62   2825.17   8327.07    182.09 0.00     0.00      0.00
15:24:43         eth0   5682.52   2843.36   8324.00    183.26 0.00     0.00      0.00
15:24:44         eth0   5721.83   2857.04   8381.59    184.15 0.00     0.00      0.00
15:24:45         eth0   5764.54   2878.01   8444.15    185.50 0.00     0.00      0.00
15:24:46         eth0   5641.67   2808.33   8264.16    181.01 0.00     0.00      0.00
15:24:47         eth0   5802.86   2892.14   8500.28    186.41 0.00     0.00      0.00
15:24:48         eth0   2343.27   1171.35   3430.90     75.50 0.00     0.00      0.00
15:24:49         eth0      0.00      0.00      0.00         0.00 0.00      0.00      0.00
15:24:50         eth0      0.00      0.00      0.00      0.00 0.00     0.00      0.00
15:24:51         eth0      0.00      0.00      0.00      0.00 0.00     0.00      0.00
15:24:52         eth0      0.50      0.50      0.01      0.02 0.00     0.00      0.00
Average:         eth0   3410.19   1702.32   4995.05    109.72 0.00     0.00      0.00

During the test, sar gives roughly 8,5MB/s in rxkB/s, while it should be 11,76MB/s (94Mbps), as measured by iperf.

When I run the same measurements in Dom0 I get far better results:

15:29:02        IFACE   rxpck/s   txpck/s    rxkB/s    txkB/s rxcmp/s  txcmp/s  rxmcst/s
15:29:02         eth0     20,00      0,00      2,38      0,00 0,00     0,00      0,00
15:29:03         eth0   5856,00   2938,00   8667,55    200,89     0,00     0,00      0,00
15:29:04         eth0   8125,00   4069,00  12029,87    278,15    0,00     0,00      0,00
15:29:05         eth0   8145,00   4073,00  12066,25    278,43    0,00     0,00      0,00
15:29:06         eth0   8121,00   4063,00  12027,94    277,74    0,00     0,00      0,00
15:29:07         eth0   8150,00   4055,00  12065,41    277,20    0,00     0,00      0,00
15:29:08         eth0   8122,00   4064,00  12029,42    277,81    0,00     0,00      0,00
15:29:09         eth0   8147,00   4079,00  12063,73    278,77    0,00     0,00      0,00
15:29:10         eth0   8123,00   4064,00  12029,34    277,88    0,00     0,00      0,00
15:29:11         eth0   8155,00   4096,00  12066,30    280,00    0,00     0,00      0,00
15:29:12         eth0   8129,00   4111,00  12029,02    281,03    0,00     0,00      0,00
15:29:13         eth0   3213,00   1631,00   4741,51    111,49     0,00     0,00      0,00
15:29:14         eth0     17,00      0,00      1,88      0,00             0,00     0,00      0,00
15:29:15         eth0     12,00      0,00      1,46      0,00            0,00     0,00      0,00
15:29:16         eth0     19,00      0,00      2,44      0,00            0,00     0,00      0,00
Average:         eth0   5490,27   2749,53   8121,63    187,96     0,00     0,00      0,00

Has anyone seen something like this?

DomU executes a Debian Squeeze 64 bits OS. Dom0 is provided by Xen 4.0.1.

Thanks,

Ricardo José Pfitscher

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