Hyper-v virtual processor usage

If I have a Win2012R2 host with 2 x 8 core processors and need it to host just a single large VM - what's the maximum virtual processors I should be assigning to make the most of the hardware?

I'm thinking no more than 28, leaving 2 x physical cores for the host.

Is this right?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
It's not really right, what performance is required, we normally start with a single vCPU, and add vCPU as and when required, to increase performance.
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
Why virtualize in the first place?

The workload running on that setup would run a lot better on physical hardware than virtual.

The two key areas that would hold a larger VM back would be CPU and memory (NUMA).
devon-ladAuthor Commented:
VM is used because hyper-v replica is in place for DR.

Have just added an additional host and VM because the existing VM was overloaded (high processor usage)

So have better performance now, but wanted to double check that had assigned the maximum VPUs.

Using static RAM allocation and NUMA nodes.
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Philip ElderTechnical Architect - HA/Compute/StorageCommented:
Our preference is the number of physical cores - 1 for vCPU assignment to a single VM. Hyper-Threading has never proven to make that much of a difference in our testing (think highway at 8 lanes up to 16 but then merge back to 8 at some point).

On the memory side that depends. There's no fixed formula as far as when a VM's memory gets tossed around between physical CPU memory banks (NUMA). Suffice it to say that when a host gets several large vRAM based VMs things can get a bit interesting.
devon-ladAuthor Commented:
Thanks for all input, but didn't really get a clear answer.

Any further input?
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
To best use all the resources of the server. you would allocate all the CPU and Memory, so you would create many virtual machines.

Many virtual machines with 2 vCPU, 4 vCPU and 8 VCPU where required, with the memory to match the application server or service.

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devon-ladAuthor Commented:
Ok, in the interest of everyone getting on with their lives we'll leave it at that.
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