Limits on mtu size ?

Limits on mtu size ?

[Hans de Bruin]

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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?

-- 
Hans

orion: 2 cpu, 1GB
darkstar: 2 cpu 512MB
...
dom0: 1 cpu's (dual core dual threaded), 1GB
kernels 3.1-rc7 and higher
xen 4.1.1
al pure pv.




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RE: Limits on mtu size ?

[James Harper]

> 
> 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

Re: Limits on mtu size ?

[Hans de Bruin]

On 10/08/2011 12:29 AM, James Harper wrote:
 >>
 >> Network performance on fast physical networks usually increase by
 >> switching to larger packet sizes. So I decided to increase the mtu on
...
 > Dom0 feels sluggish.

This was a wrong  dns setting.

  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.
 >

I should have read this http://lwn.net/Articles/188489/

-- 
Hans