RE: [Xen-users] High network latency on first packet

[Luca Corti <luca@xxxxxxxxxxxx>]

On Tue, 2010-04-27 at 08:34 -0400, Russ Purinton wrote:
> Try simultaneous tcpdumps on dom0 and domU while trying this test.  You
> can then see the ARP traffic and maybe be able to determine where the
> delay is introduced.  I guess it's possible that ARP resolution time
> could be incorporated into the first RTT result.  I just tested ARP
> response times on my physical and xen servers and saw resolution usually
> occurring between 0.02 ms and 0.2 ms.  
> 
> If ARP isn't your problem, tcpdumps should show this too.
> The command I used
> 
> tcpdump -w file.pcap -nei eth0

dump on domU:

ping remotehost -> domU

14:50:38.926785 IP remotehost > domU: ICMP echo request, id 22598, seq
1, length 64
14:50:38.927786 ARP, Request who-has remotehost tell domU, length 28
14:50:39.437493 ARP, Reply remotehost is-at fe:ff:ff:ff:ff:ff (oui
Unknown), length 28
14:50:39.437500 IP domU > remotehost: ICMP echo reply, id 22598, seq 1,
length 64

The problem is with ARP taking > 500ms to reply.

This seems very similar to 

http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=596

ciao

Luca


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