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

----- Original Message ----- From: Daniel P. Berrange <berrange(a)redhat.com&gt; To: Peter Krempa <pkrempa(a)redhat.com&gt; Cc: Jiri Denemark <jdenemar(a)redhat.com&gt;, Amador Pahim <apahim(a)redhat.com&gt;, libvirt-list(a)redhat.com, dougsland(a)redhat.com Sent: Wed, 16 Jan 2013 13:39:28 -0500 (EST) Subject: Re: [libvirt] [RFC] Data in the <topology> element in the capabilities XML 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>.

In this case, we will also need to add the thread siblings and perhaps even core siblings information to allow reliable detection.