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vCPU vs Physical CPU

How does OpenStack/KVM set the number of vCPUs based on the physical CPU on the physical machine?

In my case, I have two different model of Intel CPU, Xeon E5645 and Xeon E5-2690 for my compute-node servers:

I checked Intel website, there is 6 cores in Xeon E5645 while Xeon E5-2690 has 8 cores.

In the 'compute_nodes' table, it shows that there are 12 vCPU for the machine with Xeon E5645; whereas there are 16 vCPU in Xeon E5-2690.

--> It seems that the ratio is 1:2

How does it calculate?

Can I adjust it?