vCPU

Pre-requisites

Hypervisor

• software used to create and run virtual machines (VMs)
• allows one host computer to support multiple guest VMs by virtually sharing its resources

Socket

• array of pins that hold a processor in place and connect the motherboard to the available processing power.

Thread

• “a lightweight process”
• a path of execution within a process.
• Threads within same process run in shared memory space
• As an example, having multiple tabs open in a browser represents different threads.

Physical core

• an independent single processing unit present on the CPU chip

Logical core

• A logical core makes it possible for a single physical core to perform two or more actions simultaneously
• Logical cores made the concept of Hyper-threading Technology possible, CPU can work on 2 tasks simultaneously
• There are limitations to hyper-threading versus the physical capabilities of the core.

How does vCPU work?

vCPU represents a portion or share of the underlying, physical CPU that is assigned to a particular virtual machine (VM).

• Concept behind vCPUs : share of the time spent on the processor’s core
• Resource allocation is controlled by Hypervisor

Calculations of available vCPU

$$\begin{gathered} (Threads\text{*} \\ Cores)\ast Physical\_CPU=Number\_of\_vCPU \end{gathered}$$

e.g. Intel Xeon E-2288G chipset has  8 cores / 16 threads
Assuming we have 1 chipset in the motherboard
Thus,
$(16\ast 8)\ast 1=128vCPU$
You can decide how many $vCPU$ s you want to allocate per Virtual Machine.
Depending on the workload, you may get more or less VMs from the same compute. For example:
4 vCPUs per VM
128 vCPUs/4 vCPUs per VM = 32 VMs

2 vCPUs per VM
128 vCPUs/2 vCPUs per VM = 64 VMs

References

https://www.datacenters.com/news/what-is-a-vcpu-and-how-do-you-calculate-vcpu-to-cpu