RE: [Xen-devel] Limits on mtu size ?

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> Network performance on fast physical networks usually increase by
> switching to larger packet sizes. So I decided to increase the mtu on
my
> virtual networks to see if that has a positive effect. Anoyingly the
bridge code
> does not allow you to increase the mtu above 1500 if no interface is
attached
> to the bridge. Google tels me admins attach a nic to the bridge,
increase the
> mtu and then remove the nic from the bridge as a work around. Then I
> noticed that the loopback has a whopping 16k mtu. For some reason
adding
> lo to a bridge is not allowed. To shortcut workarounds and have fun
with the
> mtu size I modified linux-2.6/net/bridge/br_if.c. There is a line: mtu
=
> ETH_DATA_LEN; which I changed in to mtu = 16436; If lo can handle that
size
> why not a bridge. So my setup now has two brides with oversized mtu's
and
> one normal sized bidge to talk to my lan. This setup has run for two
day's and
> now it suddenly broke while ssh/rsync-ing data between the to internal
> networks. A server called darkstar ran out of memory, as far as i can
see with
> an empty swapfile, a server called orion, which was reseaving the
data,
> stopped responding. Dom0 feels sluggish. So 16436 is to big and 9000
is not?
> 
> I have attached the complaints of darkstar. Its running out of kernel
> memory?
> 

If all the hosts are Linux and GSO is enabled, the machines should be
exchanging 64K packets anyway, regardless of MTU.

James

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