Subject: Re: change MCLBYTES in kernel
To: Manuel Bouyer <>
From: Thor Lancelot Simon <>
List: tech-net
Date: 10/01/2005 10:18:48
On Sat, Oct 01, 2005 at 04:11:06PM +0200, Manuel Bouyer wrote:
> Hi,
> for Xen domain0 I'd like to change the mbuf cluster size to 4k (so that
> we have only one cluster per page, and it can be swapped with the guest
> rather than memcpy'd). It looks like I could use
> options MCLSHIFT=12
> on i386/conf/XEN0 for this. Any bad side-effect to expect ?
> /usr/include/machine/param.h wouldn't match the kernel's idea of MCLBYTES ...

I do this regularly on systems that use Ethernet jumbograms.  It works

You might consider adding the jumbogram capability bit to xennet, so
that when talking to other domains the domUs can use 4k - hdrlen
packets, which will save needless trips around the xennet ring/needless
traps to the hypervisor/etc.  Technically this would require some
changes to bridge(4) to support non-1500 MTUs but in practice you can
bring the bridge up, then change the interface MTUs out from underneath
it, so I think it'd just work.

 Thor Lancelot Simon	                            

"The inconsistency is startling, though admittedly, if consistency is to be
 abandoned or transcended, there is no problem."		- Noam Chomsky