[libvirt] [RFC] Data in the <topology> element in the capabilities XML

[Daniel P. Berrange]

On Wed, Jan 16, 2013 at 07:31:02PM +0100, Peter Krempa wrote:
> On 01/16/13 19:11, Daniel P. Berrange wrote:
> >On Wed, Jan 16, 2013 at 05:28:57PM +0100, Peter Krempa wrote:
> >>Hi everybody,
> >>
> >>a while ago there was a discussion about changing the data that is
> >>returned in the <topology> sub-element:
> >>
> >><capabilities>
> >>   <host>
> >>     <cpu>
> >>       <arch>x86_64</arch>
> >>       <model>SandyBridge</model>
> >>       <vendor>Intel</vendor>
> >>       <topology sockets='1' cores='2' threads='2'/>
> >>
> >>
> >>The data provided here is as of today taken from the nodeinfo
> >>detection code and thus is really wrong when the fallback mechanisms
> >>are used.
> >>
> >>To get a useful count, the user has to multiply the data by the
> >>number of NUMA nodes in the host. With the fallback detection code
> >>used for nodeinfo the NUMA node count used to get the CPU count
> >>should be 1 instead of the actual number.
> >>
> >>As Jiri proposed, I think we should change this output to separate
> >>detection code that will not take into account NUMA nodes for this
> >>output and will rather provide data as the "lspci" command does.
> >>
> >>This change will make the data provided by the element standalone
> >>and also usable in guest XMLs to mirror host's topology.
> >
> >Well there are 2 parts which need to be considered here. What do we report
> >in the host capabilities, and how do you configure guest XML.
> >
> > From a historical compatibility pov I don't think we should be changing
> >the host capabilities at all. Simply document that 'sockets' is treated
> >as sockets-per-node everywhere, and that it is wrong in the case of
> >machines where an socket can internally have multiple NUMA nodes.
> 
> I'm too somewhat concerned about changing this output due to
> historic reasons.
> >
> >Apps should be using the separate NUMA <topology> data in the capabilities
> >instead of the CPU <topology> data, to get accurate CPU counts.
> 
> From the NUMA <topology> the management apps can't tell if the CPU
> is a core or a thread. For example oVirt/VDSM bases the decisions on
> this information.

Then, we should add information to the NUMA topology XML to indicate
which of the child <cpu> elements are sibling cores or threads.

Perhaps add a 'socket_id' + 'core_id' attribute to every <cpu>.

Regards,
Daniel
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