[libvirt] [PATCH v2] qemu: bulk stats: add pcpu placement information

[Francesco Romani]

----- Original Message -----
> From: "Peter Krempa" <pkrempa at redhat.com>
> To: "Francesco Romani" <fromani at redhat.com>, libvir-list at redhat.com
> Sent: Thursday, December 11, 2014 10:04:24 AM
> Subject: Re: [libvirt] [PATCH v2] qemu: bulk stats: add pcpu placement	information
> 
> On 12/11/14 08:43, Francesco Romani wrote:
> > This patch adds the information about the physical cpu
> > placement of virtual cpus for bulk stats.
> > 
> > This is the only difference in output with the
> > virDomainGetVcpus() API.
> > Management software, like oVirt, needs this information
> > to properly manage NUMA configurations.
> 
> Are you sure that you are getting what you expect? When this stats group
> was first implemented I asked not to include this stat as it only shows
> the actual host cpu id where the guest cpu is running at that precise
> moment. The problem with that is that usual configurations don't map the
> cpus in a 1:1 fashion, but rather allow a specific guest CPU to be run
> on a subset of host cpus according to it's scheduling decisions.
> 
> That means that the stat might oscillate in the given set where the
> guest vcpu is pinned at. Could you please share your use case for this
> one? I'm curious to see whether you have some real use of such data.

There is one use case on oVirt on which this very data is used to build
a what is claimed to be a Vcpu runtime pinning map. It is used on NUMA flow.

I'm not really familiar with that code, and by inspecting it after
your answer above, I'm not 100% convinced everything's right in oVirt.

I'll need to check more deeply, and I'll reply as soon as I have trustworthy information.

Thanks for the insight,

-- 
Francesco Romani
RedHat Engineering Virtualization R & D
Phone: 8261328
IRC: fromani