CPU Overcommitting question

[Riccardo Brunetti <riccardo.brunetti@dnshosting.it>]

Dear KVM community people,

we are running on a kvm hypervisor with the following configuration:

CPU : Dual socket 6 cores: Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz
RAM : 48 GB
OS: CentOS release 6.5

2 very cpu intensive virtual machines and we are trying to find the best 
way to optimize the available resources.
In particular what we would like to understand is how many vCPUs can be 
assigned to each VM, in order to possibly exploit the overcommitting 
features of the virtualization without degrading the performances.

The first question is about hyperthreading: should it be enabled and the 
virtual cores be considered?

The second quesion is about the way to assign vCPUs between the two VMs.
Let's suppose we do not enable the hyperthreading, in our understanding 
we have the following options:

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)

According to your experience which would be the best one?

Thank you very much.
Kind Regards.

B. Riccardo