[virt-tools-list] Info on allocating vCPUs on virt-manager

[Cole Robinson]

On 03/31/2015 07:12 AM, Satya Prasad Nemana wrote:
> Gurus,
> 
> I need clarity on what each item means and how it maps to the info we find on
> dmidecod or lscpu on the host when creating a guest from the virt-manager.
> 
> My question:
> 
> 1) Under CPUs , we have Logical Host CPUs: What number does it indicate? Is
> the max value that can be configured here the Cores* hyperthreads on the host?

Logical CPUs = Host Sockets * Host Cores * Host Threads

> 2) Under Topology : we have sockets, cores , threads  : do these correspond to
> the outputs from dmidecode?
>         Socket Designation: Proc 1
>         Core Count: 10
>         Core Enabled: 10
>         Thread Count: 20

libvirt/virt-manager gets the info from /proc/cpuinfo, but it should be the
same numbers.

> 3) Under Pinning, we have VCPU, On CPU, Pinning
>  : what do these values indicate

VCPU number corresponds to the emulated VM CPU. So if you tell virt-manager to
give your VM 8 CPUs, under VCPU pinning you will see VCPU number 0-7. Pinning
allows you to say 'VM CPU 2 only uses Host CPU 3-4' which may be useful for
specific performance scenarios.

FWIW in newer versions of virt-manager the VCPU pinning UI is removed, but you
can still do the same using the 'virsh' cli tool

> does each vcpu value correspond to each virtual cpu on the host (obtained by
> threads per core * number of cores)
> [root at sienna ~]# lscpu
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                20
> On-line CPU(s) list:   0-19
> Thread(s) per core:    2
> Core(s) per socket:    10
> Socket(s):             1
> 
> If there is any detailed documentation anywhere, could someone please point me
> to that ?
> 

Dan has an overview of some of the various options here:

https://www.berrange.com/posts/2010/02/12/controlling-guest-cpu-numa-affinity-in-libvirt-with-qemu-kvm-xen/

- Cole