Understanding CPU Allocation in Hyper-V with Intel Xeon Silver 4214R

Question

I’m seeking insights into CPU allocation in Hyper-V, specifically related to my current hardware configuration and how best to optimize resource allocation for my virtual machines (VMs).

Hardware Overview:

• CPU Model: Intel Xeon Silver 4214R
• Cores: 24
• Logical Processors: 48

Current Configuration:

I have prepared two VMs with the following allocations:

1. VM1: Allocated: 8 CPU Cores
2. VM2: Allocated: 28 CPU Cores

Windows Server Hardware Overview screenshot in English

Windows Server Hardware Overview screenshot in Japanese

My Questions:

I've allocated more CPU Cores to my Hyper-V virtual machine than my physical server has available cores. while Hyper-V does support overcommitting CPU resources (assigning more virtual CPUs than there are physical cores) but how it works?

1. Overcommitment:

With a total of 48 logical processors, my vCPU allocation results in an overcommitment ratio of 75%. Is this generally acceptable, and what potential issues should I be aware of?

1. NUMA Considerations: How can I effectively align vCPU allocations with NUMA nodes to optimize performance? Given that I have two CPUs, should I allocate vCPUs in multiples of 12 to match the physical cores?
2. Best Practices: Are there specific best practices or recommendations for configuring vCPU allocations for varying workloads, especially for CPU-intensive applications?
3. Hyper-Threading: Should I consider disabling Hyper-Threading for specific VMs to improve performance, or is it beneficial to keep it enabled for most workloads?

I appreciate any insights, experiences, or resources you can share regarding managing CPU resources effectively in a Hyper-V environment.

Thank you in advance for your help!

Answer 1

Hi fti-samadhan,

Thanks for your post. Generally speaking, when dealing with hypervisors it is easier to think in cores and virtual processors than it is to think in cores, logical processors, and virtual processors.

In reality what really matters is cores and virtual processors.  Each core very safely support 8 virtual processors.  Logical processors are abstractions of time of the physical processors and really muddy the thinking as they apply to a physical machine one way and to a hypervisor in a different way.

Can you go beyond this number?  Oh, yes. Very easily - and as long as you don't have CPU intensive virtual machines you can do it safely as well.

The thing to plan for when moving to virtual machines is to plan hypervisor capacity in a scale-out type of way.  If one hypervisor get too crowded you add another, and / or pick a VM to move off to the other hypervisor.  Thus the overloaded hypervisor re-balances and your new one picks up some work.

So, you need to plan in the flexibility to move VMs (either with Live Migration or by powering them off) between hypervisors to balance the work.

Managing hypervisors and VM workloads is still an art, not a science - as there are so many variables that affect the workload, the VM OS, the hypervisor, and the hardware.

Some references:

• yper-V Concepts: vCPU (Virtual Processor) - Q&A
• Requirements and Limits for Virtual Machines and Hyper-V in Windows Server 2008 R2
• Virtual machine sizing guidance
• Hyper-V limits the maximum number of processors in the Hyper-V host OS to 64 - Windows Server | Microsoft Learn
• Microsoft community post vCPUs are actually the amounts of of time a virtual machine gets on a logical processor

Best Regards,

Ian Xue

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