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I'm having some serious issues with packetloss with one of my servers. This server is a somewhat old (P4-era) machine running Debian Squeeze and Xen 4.0. There are two domUs running on it (both also Debian Squeeze), one gateway and a fileserver. Unfortunatly the processor has no virtualization extensions, therefore only PV can be used.

While investigating why our network seems to be slower than it should I found some pretty bad packet loss (~25%). After further investigation and several experiments I did a measurment between the dom0 and one of the domUs:

Server listening on UDP port 5001
Receiving 1470 byte datagrams
UDP buffer size:   110 KByte (default)
------------------------------------------------------------
------------------------------------------------------------
Client connecting to dom0, UDP port 5001
Sending 1470 byte datagrams
UDP buffer size:   110 KByte (default)
------------------------------------------------------------
[  3] local 192.168.1.2(domU) port 33817 connected with 192.168.1.100(dom0) port 5001
[  4] local 192.168.1.2(domU) port 5001 connected with 192.168.1.100(dom0) port 48606
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  46.3 MBytes  38.7 Mbits/sec
[  3] Sent 33020 datagrams
[  3] Server Report:
[  3]  0.0-10.0 sec  46.2 MBytes  38.6 Mbits/sec  0.030 ms   89/33019 (0.27%)
[  3]  0.0-10.0 sec  1 datagrams received out-of-order
[  4]  0.0-10.2 sec  43.0 MBytes  35.3 Mbits/sec  13.074 ms 11575/42256 (27%)

tl;dr: 27% packet loss from dom0 to domU with 50Mbit UDP packets.

Same thing happens from anywhere in the network. The problem gets better for smaller bandwidths (0.047% for 5Mbit) and worse for higher (59% for 200Mbit) ones.

I did increase the CPU-weight of the dom0, there is no swapping going on, and actual networking-hardware is not involved. I never expected Xen (or anything related) to drop packets, and I'm completly clueless what to try next.

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closed as off topic by joeqwerty, EEAA, rnxrx, Michael Hampton, Scott Pack Oct 27 '12 at 17:51

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1 Answer 1

Xen used to have problems with TCP checksum offload, but I'm not sure whether this is also used for UDP and would affect your test. You can try disabling this by running the following in the domU:

ethtool -K eth0 tx off
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