[openstack-dev] [nova][neutron] PCI pass-through SRIOV

Ian Wells ijw.ubuntu at cack.org.uk
Mon Jan 27 21:31:46 UTC 2014

On 27 January 2014 15:58, Robert Li (baoli) <baoli at cisco.com> wrote:

>  Hi Folks,
>  In today's meeting, we discussed a scheduler issue for SRIOV. The basic
> requirement is for coexistence of the following compute nodes in a cloud:
>       -- SRIOV only compute nodes
      -- non-SRIOV only compute nodes
>       -- Compute nodes that can support both SRIOV and non-SRIOV ports.
> Lack of a proper name, let's call them compute nodes with hybrid NICs
> support, or simply hybrid compute nodes.
>  I'm not sure if it's practical in having hybrid compute nodes in a real
> cloud. But it may be useful in the lab to bench mark the performance
> differences between SRIOV, non-SRIOV, and coexistence of both.

I think in fact hybrid nodes would be the common case  - there's nothing
wrong with mixing virtual and physical NICs in a VM and it's been the
general case we've been discussing till now.  VMs that *only* support SRIOV
and have no soft switch sound like a complete outlier to me.  I'm assuming
that passthrough devices are a scarce resource and you wouldn't want to
waste them on a low traffic control connection, so you would always have a
softswitch on the host to take care of such cases.

I believe there *is* a use case here when  you have some, but not all,
machines that have SRIOV devices.  They will also have a softswitch of some
sort and are therefore not only 'SRIOV only' in that sense.  But the point
is that if you have a limited SRIOV resource you may want to preserve these
machines for VMs that have SRIOV requirements, and avoid mapping general
VMs with no SRIOV requirements onto them.

You can expand the problem further and avoid loading up machines with
specific PCI devices of any sort if you have a VM that doesn't need a
device of that sort, which comes down to prioritising your machines at
schedule time based on whether they're a good fit for the VM you intend to

In any case, as discussed in the meeting, this is an optimisation and not
something we have to solve in the initial release, because:

> Irena brought up the idea of using host aggregate. This requires creation
> of a non-SRIOV host aggregate, and use that in the above 'nova boot'
> command. It should work.
So, while it's not the greatest solution, there's at least a way of
achieving it right now.
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