Windows 2012 R2 -- HyperV Gen1, Virtual Processors ?

I have two 8-core hyperTread processors,
therefore the attached give me "32" as
the MAX number of Virtual Processors
I can dedicate to one of my VMs

Can I lower this number on
a Gen1 VM without rebooting/etc
after finding out the machine does
not need that much based on ONE
month of Perf_Mon results ?
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finance_teacherAsked:
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George SimosIT Pro Consultant - IT Systems AdministratorCommented:
Hello,

CPU hot add/remove is not supported in Hyper-V and it's not going to be for a long time :-)
The best practice though is to always use 1 vCPU and then monitor the VM's load and act accordingly.
Now I understand that you don't want to reboot this VM until there is a convenient time to do so, but really you have to shut the machine down to change it.
Keep in mind that assigning all those vCPUs  doesn't mean that you take all the processing power from your Hyper-V Host, things are more smartly handled from the hypervisor, please consult the following article from Eric Siron:
Hyper-V Virtual CPUs Explained
It will help you a lot!

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Philip ElderTechnical Architect - HA/Compute/StorageCommented:
I have an EE article that should help: Some Hyper-V Hardware and Software Best Practices.

Our rule of thumb is # physical cores on one CPU - 1 = Maximum vCPUs for one VM.
Muhammad BurhanManager I.T.Commented:
calculate from the formula given in the link
http://blogs.technet.com/b/virtualization/archive/2011/04/25/hyper-v-vm-density-vp-lp-ratio-cores-and-threads.aspx

if its support Hot add cpu so you can easily do it without restarting, but when adding cpu its better to turn off VM and then add. Because sometimes the existing driver doesn't respond correctly to the newly added cpu, and with rebooting it prompts you if any driver needed.
https://charbelnemnom.com/2014/01/hotadd-additional-cpu-resources-in-hyper-v-2012-r2/
George SimosIT Pro Consultant - IT Systems AdministratorCommented:
@Philip Elder: This formula of yours is quite interesting, could you please give us more info how you end up in this ratio?

@Muhammad: There is not support for hot-add of vCPUs as I stated above, the ones you are referring to are about giving more priority to the VM(s) or constraining the already assigned vCPUs, however the blog article you are also referring is not the clearest one I have read (and I have read a lot of books, articles and training materials before stating that) and wouldn't recommend it if the person having questions doesn't have a strong  background in Hyper-V, to me this article is just making you wonder instead of helping you out. I say this because I've read other posts from this guy and his instructions are not clear enough to make you feel that you have understood what he wants to communicate to the public.
He may be a great engineer (which I strongly believe is) but the ability to communicate correctly the technical details is something else.
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
One Virtual CPU = 1 Thread to the physical CPU.
1 Thread needs a core to be processed.
vCPU threads must be processed in parallel through the CPU pipeline.

There are a number of reasons:
 + vCPU threads should be processed on one Physical CPU
 + More vCPUs than physical cores = threads bouncing across bus between physical CPUs
 + Physical CPU pipeline juggling: Too many high vCPU count VMs cause latency

There are a number of factors along with how virtual memory is allocated that can impact a VM's performance. Too many VMs with high vCPU counts can slow the physical CPU thread juggling right down.

Our rule of thumb is to bench all aspects of our solution set before it goes out the door.
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