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

[Luca Corti <luca@xxxxxxxxxxxx>]

On Tue, 2010-04-27 at 15:06 +0200, Luca Corti wrote:
> The problem is with ARP taking > 500ms to reply.
> 
> This seems very similar to 
> 
> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=596


A workaround is to add a gateway host in addition to the device route.
This way arp lookups are skipped.

$ netstat -rn 

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt
Iface
0.0.0.0         0.0.0.0         0.0.0.0         U         0 0          0
eth0

$ netstat -rn

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt
Iface
0.0.0.0         1.2.3.4         0.0.0.0         UG        0 0          0
eth0
0.0.0.0         0.0.0.0         0.0.0.0         U         0 0          0
eth0


Note that this works despite the gateway being on a subnet which is not
configured on any interface, the IP address could also be fake (1.2.3.4
can be actually configured and solving the issue without harm, since it
is a bogon). ARP lookups are skipped in the presence of any route with a
gateway host because the gateway ip is itself resolved via ARP and then
cached:

$ arp -a
? (1.2.3.4) at fe:ff:ff:ff:ff:ff [ether] on eth0

This is ugly anyway, a cleaner solution would be great.

ciao

Luca


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