How much Number of virtual processors for VMs

Question

Hi,
there is lots more information about logical processors, cores and CPU and their definitions, on the internet. also here in this forum.
In order to make sure i am not lost on internet, it is better to take experts advice like you.
I have simple question read on:
I have a Hyper-v cluster host with 4 sockets (4 CPU's) with 6 cores each. so total are 24 cores.
I can see in hyper-v host Logical processors are 48. (which may be 1 proc = 12 logical processors, so 4 proc x 12 = 48 logical processors)
now how much virtual processors I can assign to each VM. I know I can assign more logical processor depending on application requirement. and it affects application performance. For each VM, it is showing maximum i can select 48 virtual
processors.
is it true I can select any number of virtual processors in each VM? is there any limitation?
is it that if i have 8 VMs if i set virtual processors to 6 then it will occupy all logical processors (48 in my case) and i  will not be able to allocate logical processor to new VMs?
is it that if i have 10 VMs and all set to 10 virtual processors? 
thank you

Answer 1

Quite honestly - this math really does not matter that much with modern processors. (a combination of speed and core count, and advances in CPU scheduling in the hypervisor)
You have to have a VM with many vCPU to get into any point of contention. 
In practice, It is very difficult to over-subscribe your CPU resources without hitting some other bottleneck first.
Set the vCPU count where it is appropriate for your workload.  That means testing your workload to see where adding additional vCPU to a VM provides no benefit in throughput or speed or capacity for your application.
A select few applications are written in a way that there is a linear correlation between CPU count and throughput.
There is an old point about not assigning so many vCPU to a VM that the VM could end up with more than one execution thread on the same processor (core).
You have 24 cores.  and each has 12 logical processors (execution threads).  So, with zero CPU over-subscription you could have 288 vCPU assigned to any number of VMs.  To expand my point in the previous paragraph - you can easily / safely go
over this count of 288 - what I am referring to above is an old guideline that would guide you to not assigning more than 288 vCPU to a single VM - ensuring that no single execution thread is trying to service more than one vCPU of the same VM.
Brian Ehlert
http://ITProctology.blogspot.com
Learn. Apply. Repeat.