Re: CPU Overcommitting question

[riccardo.brunetti@dnshosting.it]

Hi Paolo, thanks for the prompt reply.

On 2014-07-18 17:30, Paolo Bonzini wrote:
> Il 18/07/2014 15:06, Riccardo Brunetti ha scritto:
>> 
>> 
>> 1) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) = total number of 
>> physical
>> cores (12) (both VMs have < 12 vCPUs) (ie. 8+4)
>> 2) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) > total number of 
>> physical
>> cores (12) (both VMs have <= 12 vCPUs) (ie. 12+8)
>> 3) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) > total number of 
>> physical
>> cores (12) (one VM has > 12 vCPUs) (ie. 16+8)
>> 4) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) > total number of 
>> physical
>> cores (12) (both VMs have > 12 vCPUs) (ie. 16+16)
> 
> The last two are usually bad ideas.  The first two should be okay.
> Overcommitting works best if the VM are I/O bound (disk or network).
> 
> If you're overcommitting, hyperthreading should in general be enabled.
> 

So, it should be ok to have the first two options, but also considering 
that with hyperthreading instead of 12 cores we can consider 24? Thus we 
can go with:

1) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) = total number of cores 
(24) (both VMs have < 24 vCPUs) (ie. 18+6)
2) Assign vCPUs so that vCPU(VM1) + vCPU(VM2) > total number of cores 
(24) (both VMs have <= 24 vCPUs) (ie. 24+10)

> Answers for very specific scenarios are hard to give though, so you
> should also benchmark your deployment with realistic workloads.

Yes, I understand.

> 
> Paolo

Riccardo