Xen users - Jun 2008 - Xen with Multiple NIC as a logical router

Hi *,

My sincere apologies for cross posting in the mailing lists.

I am evaluating the case of using XEN as a logical router.

http://www.cs.ucl.ac.uk/staff/M.Handley/papers/xen-vrouters.pdf

In the conclusions section it is mentioned

"  CPU saturation is a main feature of PC-based virtual routers, and
that to avoid context switching overheads, the virtualization platform
(e.g. Xen) so all forwarding is handled in the privileged domain. In
other words, domUs should only host the control (slow) path of its
associated virtual router, while the corresponding forwarding
should be “migrated” to dom0. "


I am considering the case to provide every DOMU a
separate physical port. I know that this solution does not scale but do
you believe that such a choice would limit the CPU overhead caused by
the copying functionality between the DOMU and DOM0 ?

Kind regards,
Dimitris




-- 
--
Dimitrios K. Kalogeras

Electrical Engineer, Ph.D.
Network Engineer
NTUA/GR-Net Network Management Center
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pub   1024D/0E421B50 2007-01-17 [expires: 2008-01-17] Dimitrios
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> -----Original Message-----
> From: xen-research-bounces@lists.xensource.com [mailto:xen-research-
> bounces@lists.xensource.com] On Behalf Of Dimitrios Kalogeras
> Sent: 20 June 2008 12:39
> To: xen-users@lists.xensource.com
> Cc: xen-research@lists.xensource.com
> Subject: [Xen-research] Xen with Multiple NIC as a logical router
>
> Hi *,
>
> My sincere apologies for cross posting in the mailing lists.
>
> I am evaluating the case of using XEN as a logical router.
>
> http://www.cs.ucl.ac.uk/staff/M.Handley/papers/xen-vrouters.pdf
>
> In the conclusions section it is mentioned
>
> "  CPU saturation is a main feature of PC-based virtual routers, and
> that to avoid context switching overheads, the virtualization platform
> (e.g. Xen) so all forwarding is handled in the privileged domain. In
> other words, domUs should only host the control (slow) path of its
> associated virtual router, while the corresponding forwarding
> should be “migrated” to dom0. "
>
>
> I am considering the case to provide every DOMU a
> separate physical port. I know that this solution does not scale but do
> you believe that such a choice would limit the CPU overhead caused by
> the copying functionality between the DOMU and DOM0 ?
Sure, there are various ways to improve network performance and limit CPU
overhead for DomUs. One would be to assign dedicated physical NICs to guest
domains via pass-through as you suggest but the big problem is that you need a
physical NIC per guest which is as you say not scaling very well. The next
approach is to use virtualization-aware NICs that actually provide VNICs in
hardware (PCI-SIG IOV). With that you can concurrently and directly access a
single card from multiple guests. A third approach is to improve the performance
of the inter-domain communication channels between DomU and Dom0 in Xen to
reduce the CPU overhead for guest domain networking. There's currently work
going on in all of those areas.

Regarding the virtual router approach there have been other reasons to leave the
forwarding paths within a single domain instead of distributing them across
multiple guests - those are also application-specific and not just performance
related.


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