Windows Gurus -- optimising nr of V-Cpu's in a Windows VM

jimbo45 · Oct 27, 2025

Hi folks
Is there any sort of algorithm / tool to use that can determine the best nr of Virtual CPU's to use for a Windows VM. It's easy to "over-commit" resources especially these days as VM's only dynamically consume resources when actually running with activity. When idle not much is consumed.

I'm running on an i7 CPU with 8 virtual CPU's available and 32 GB RAM - Disk space not an issue. I'm not a gamer - basically just Office type stuff, some video editing - e.g Photoshop etc, file sharing and some streaming. I can't seem to see much user difference between allocating 8GB RAM with 4 virtual CPU's and 16 GB RAM with 8 virtual CPU's.

The Guest is Windows 11 PRO (standard 25H2) and I'm not running several VM's concurrently on that machine.

Cheers
jimbo

neemobeer · Oct 28, 2025

A VM will use "up to" what it is provisioned with. If your host system is struggling then just lower the available CPUs and memory to the VM. If you don't need performance in the VM then do like 2 vCPU and 4GB of RAM.

The fact that you have a very limited amount of CPU and RAM resources and not running multiple VMs means you have very little leeway in your configure vs your performance and little to nothing to consider for resource optimization.

PhilB · Oct 31, 2025

neemobeer said:
The fact that you have a very limited amount of CPU and RAM resources and not running multiple VMs means you have very little leeway in your configure vs your performance and little to nothing to consider for resource optimization.

Upgrade time?

jimbo45 · Nov 1, 2025

x509 said:
Upgrade time?

32GB RAM, 512GB fast Nvme (ssd type) plus i7 turbo boost with also 8 cores hardly is a "limited resource laptop. !!! Plus on this laptop I rarely would run more than 2 VM's concurrently --usually just one -- a Linux VM under Hyper-V for access to Linux files on a Linux NAS.

Cheers
jimbo

Hopachi · Nov 7, 2025

Indeed.
~~It was well mentioned that the host is a laptop.~~
Sorry, I got that from another one of your threads.
Anyway. That is still pretty decent specced for a laptop.

@jimbo45
Start with the provided defaults. But start minimal. No use to over-donate resources to the VMs.
Note virt-manager's (Linux) defaults for Windows VMs are pretty decent and work in Hyper-V too.
Then try Hyper-V's own defaults should also do fine but fine-tune accordingly.

Min specs Windows VMs:
Win8.1 and previous can run with 1 vCPU. As I said min specs.
Win10-11 needs min 2 vCPU for smooth performance.
* smooth with a grain of salt, it's just that with 1 vCPU on Win10+ I noticed very high CPU use and you can't get anything done like that.

So for a basic bare bone Windows VM use 2 vCPU.
For a more demanding VM go with 4 vCPU.
Recommended on most platforms, you usually can take up to the half of your vCPUs to the VM, leaving the other half for the host, if the host is Windows, having a heavy UI and such. For a light DE as XFCE and such on a Linux host, you may leave even less vCPUs to the host if resources allow it.

cereberus · Nov 7, 2025

I always just set it to maximum allowed. If item being run in vm needs the resources, it takes it. If host needs resources, then it shares OK.

It is little understood that if you have HyperV activated, the host OS is also running as a virtual machine being a type 1 supervisor.

You can set up what percentage of resources is used by a vm but I have never needed to do that - that is more for sysadmins on azure systems etc.

PhilB · Nov 7, 2025

cereberus said:
It is little understood that if you have HyperV activated, the host OS is also running as a virtual machine being a type 1 supervisor.

Who knew?

Why? Does the host run as a VM whenever Hyper-V is started, or only when a VM runs under Hyper-V? What happens if you exit the VM or exit Hyper-V

cereberus · Nov 7, 2025

x509 said:
Who knew?

Why? Does the host run as a VM whenever Hyper-V is started, or only when a VM runs under Hyper-V? What happens if you exit the VM or exit Hyper-V

Once you switch HyperV on, there is the hypervisor layer between the hardware and any operating systems.

Thus rhe host OS runs alongside any other virtual machines.

It does not matter if you have any VMs running or not.

If you uninstall HyperV, hypervisor layer is removed, and the host OS runs directly on the hardware layer.

The hypervisor layer is very effecient, and you will barely detect the performance difference on host OS if running with or without the hypervisor layout.

Of course, cpu speed etc. may have a slight impact. You would have to benchmark with and without the hypervisor layer to see if there is any performance reduction.

Compare this with type 2 hypervisors (e.g. virtualbox) where the hypervisor layer sits on top of the host OS (only when vms are running) and the vms sit on top of the hypervisor.

From a host OS perspective, nothing changes but the type 2 hypervisor have a bigger impact on vm performance.

There are pluses and minuses running HyperV vms, but in terms of raw performance, vms usually run much more efficiently with HyperV.

PhilB · Nov 8, 2025

cereberus said:
Once you switch HyperV on, there is the hypervisor layer between the hardware and any operating systems.

If you uninstall HyperV, hypervisor layer is removed, and the host OS runs directly on the hardware layer.

@cereberus Uninstall Hyper-V or just terminate the application?

Thanks for the explanation.

I realize that there may be negatives to Hyper-V but I am a VM newbie and it seems straightforward to use Hyper-V.

cereberus · Nov 8, 2025

x509 said:
@cereberus Uninstall Hyper-V or just terminate the application?

Thanks for the explanation.

I realize that there may be negatives to Hyper-V but I am a VM newbie and it seems straightforward to use Hyper-V.

You do not need to take any special action. Windows sorts it all out for you.

PhilB · Nov 8, 2025

cereberus said:
You do not need to take any special action. Windows sorts it all out for you.

So far it has been very easy run HyperV and create (and delete) VMs.

jimbo45 · Nov 13, 2025

HYPER-V is fine unless you need to dynamically attach and remove disk / cd rom devices. Installing other devices too is a problem with VM's on Hyper-V but within its limitations -- no complaints with the ease of creating VM's or the performance.

Cheers
jimbo.

cereberus · Nov 13, 2025

jimbo45 said:
HYPER-V is fine unless you need to dynamically attach and remove disk / cd rom devices. Installing other devices too is a problem with VM's on Hyper-V but within its limitations -- no complaints with the ease of creating VM's or the performance.

Cheers
jimbo.

What do you mean. If the guest is running in enhanced mode, sure you can dynamically attach usb drives.

Click on more, then tick drives that I plug in later. You can even save the settings - see 3rd picture

It also works for dvd drives see 4th picture. However, without messing around, not sure how to play music\video dvds.
Certainly works for data. I rip all my owned cds to mp3 on usb drives anyway.

jimbo45 · Nov 15, 2025

cereberus said:
What do you mean. If the guest is running in enhanced mode, sure you can dynamically attach usb drives.

Click on more, then tick drives that I plug in later. You can even save the settings - see 3rd picture

It also works for dvd drives see 4th picture. However, without messing around, not sure how to play music\video dvds.
Certainly works for data. I rip all my owned cds to mp3 on usb drives anyway.

View attachment 152752

View attachment 152751

View attachment 152753

View attachment 152760

OK for Windows VM's running under HYPER-V but doesn't work for Linux VM's using HYPER-V -- mainly because (and it's not Windows's fault BTW ) the default desktop GUI if using windowed GUI's is Wayland and not X11 - and Wayland is incompatible (currently) with Ms's RDP protocol. Some distros are OK e.g older Debian and Ubuntu which still use X11 or some 32 bit distros. Modern one's -- you'll just have to wait until either Ms releases RDP into the public domain (or at least programmable API's) or Linux developers "reverse engineer" linux modules" such as freerdp to provide dynamic usb redirection.

Cheers
jimbo

cereberus · Nov 15, 2025

jimbo45 said:
OK for Windows VM's running under HYPER-V but doesn't work for Linux VM's using HYPER-V -- mainly because (and it's not Windows's fault BTW ) the default desktop GUI if using windowed GUI's is Wayland and not X11 - and Wayland is incompatible (currently) with Ms's RDP protocol. Some distros are OK e.g older Debian and Ubuntu which still use X11 or some 32 bit distros. Modern one's -- you'll just have to wait until either Ms releases RDP into the public domain (or at least programmable API's) or Linux developers "reverse engineer" linux modules" such as freerdp to provide dynamic usb redirection.

Cheers
jimbo

Well Hyper-V was never really designed as an all purpose vm tool. It is really geared up for corporate use (Azure nased tools) coming from server editions of HyperV. In fact, the version on consumer editions is less flexible.

By nature of its roots, it was never really designed for non Windows guests.

As you say, changes to Linux have removed the ability of some Linux guests to run in enhanced mode by dint of not allowing guests to act as RDP servers.

I doubt MS will do anything to HyperV to resolve the above as it is the guests not complying with HyperV enhanced mode requirements.

In the end, there are workarounds to handle drives but the biggest issue is lack of sound if guest OS cannot act as an RDP server.

DJNelson · Nov 17, 2025

It's unfortunate that we can't put the OS in the sandbox. That would solve quite a few of the problems found on the other threads in this forum.

cereberus · Nov 17, 2025

DJNelson said:
It's unfortunate that we can't put the OS in the sandbox. That would solve quite a few of the problems found on the other threads in this forum.

and create lots of other problems. Type 1 hyperviaor guests are segregated installation much like a sandbox apart from shared/network drives.

Frankly sandboxes are more trouble than they are worth.

DJNelson · Nov 17, 2025

cereberus said:
and create lots of other problems. Type 1 hyperviaor guests are segregated installation much like a sandbox apart from shared/network drives.

Frankly sandboxes are more trouble than they are worth.

Couldn't agree more..
The fact windows gets angry when it's isolated and unable to 'repair' itself by changing back all the settings and reinstalling unwanted "features" every month, in utter disregard of the ownership rights to your own hardware, makes an excellent case that windows should be sandboxed.

jimbo45 · Nov 18, 2025

DJNelson said:
Couldn't agree more..
The fact windows gets angry when it's isolated and unable to 'repair' itself by changing back all the settings and reinstalling unwanted "features" every month, in utter disregard of the ownership rights to your own hardware, makes an excellent case that windows should be sandboxed.

Actually not a very good legal argument --it might be your hardware but you've agreed the terms to your license by allowing Windows to run on it.

If you have a car -- it's your car but you have to comply with the law when using it on public highways.
Plenty of problems with Windows etc - but at least if you use the legal stuff / arguments then make them sensible, !!!

The only possible advantage (and far outweighed by the disadvantages) of using a Sandbox is that you can try out a limited number of things without needing another copy of Windows - but since loads of changes require a re-boot to test / implement properly and sandboxing loses persistence you might just as well go for a proper VM or another version of Windows. If you have a licensed copy of windows you can back that up, create another build version e.g Canary - test on that and then restore your original.

Actually if you are reasonably skilled you can create physical vhdx files so you can have the option of booting a whole slew of Windows systems - all activated -- not against the EULA which states only 1 version of a licensed copy of Windows can run on the hardware it's licensed on - so you can't run more than one copy of Windows concurrently if it has the original license -- same applies to VM's as well.

Cheers
jimbo

DJNelson · Nov 18, 2025

The legality of any EULA stops where my privacy and ownership rights start, and is invalidated by the fact the terms are demanded AFTER the purchase is complete. Deleting or moving files off of my computer without my knowledge or consent is THEFT. I do not have to care how MS feels about me choosing not to use some of the product they sold me. End of that line of discussion.

Windows Gurus -- optimising nr of V-Cpu's in a Windows VM

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