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



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.

The management apps tend to avoid using cores as CPUs for the guests for performance reasons.

Any other ideas how to provide this kind of information to the mgmt apps?


For the guest there are two cases to consider. If there is no NUMA in the
guest there is no problem, because "total sockets" and "sockets per node"
are the same. In the case where there is NUMA set, we should just ignore
the guest 'sockets' attribute completely, and treat the 'cores' & 'threads'
attributes and <vcpu> and <numa> elements as providing canonical data.

Daniel



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