**** questions (hopefully simple answers) about Fusion on Mac M2

[Daniel Britt]

Always appreciate the help here. I'm running Windows 11 64-bit ARM Virtual Machine on MacBook Air M2. 

My Mac specs are: 24GB memory, Cores: 8 (4 performance and 4 efficiency); GPU 10 cores

Question 1: I know the Fusion settings for my VM warns me if I exceed # of cores. I've run in at the safe level of 4 (out of 8) and I've run it higher. The **** question I have is: will my performance on the Windows VM be faster (processing audio effects for instance) with more than 4 cores assigned? I figure I risk taxing my system by using too many cores, but wondering if it really makes a difference on the process performance of rendering and saving files if I assign it more cores.  (Note: I have turned off "Accelerate 3D Graphics" as well as "Use full resolution for Retina Displays" in hopes of maximizing process power and being ok with less-than-crisp graphics as a tradeoff).

Question 2: When switching from an active VM session to another app or window on Mac (such as when using CMD+Tab for application switching), I will frequently get this error blinking back over on the VM: 

What does that mean? What did it think I tried to do? And I've let it blink for a long time while doing other work on MacOS - only to come back to VM and it had seemingly paused any processes or progress being done until I clicked OK. I'm wondering if there is a conflicting keyboard shortcut, something close to CMD+Tab... 

Question 3: Is there anything "dangerous" or unwise about never running the "Clean Up Viritual Machine" option. I don't need to reclaim the space (I have plenty), so I figured not running it might present fewer opportunities for corruption or something going wrong during that process. But maybe I'm not understanding it properly. 

Hopefully these are simple "****" questions easily solved. 

Thanks!

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[Daniel Britt]

ha! It censored my use of the word d ...umb :) 

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[Technogeezer]

Here are my thoughts:

Re: 1. Assigning more than 4 virtual cores to the VM on a machine with only 4 performance cores risks the possibility that some of the virtual cores may run on the lower performing efficiency cores. That may be more pronounced if you are saturating CPU performance in the VM. More cores may give you a performance boost, but not as much as you may expect.

The only real way to know if the additional cores benefit you (and impact your performance) is to measure performance in the VM and getting a feel for how well your VM performs.

I also think that since the 3D acceleration code is a bridge to Apple's GPU assisted Metal, I'd leave that enabled so that graphical displays get a boost. Again. try it on and off and measure what you're seeing. If your code is CPU intensive vs graphics intensive, I woudn't think that 3D support would be an issue.

Re: 3:

If you're fine with never freeing up space on the host for space that the VM releases, then never running the cleanup is OK. Especially if your VM is running on an SSD. My Linux VMs never see a "virtual machine cleanup" and they have run just fine. I do run the cleanup on my Windows VMs occasionally (but not frequently or scheduled). 

Cleaning up a Windows virtual machine has never given me a problem. But there are some theoretical possibilities (such as running out of disk space or a machine crash) that could become a problem when performing maintenance on the virtual disks. 

I'd be a lot more concerned with having a good backup routine for my virtual machine than to be wary of running the virtual machine cleanup. If you have a good backup (and that does NOT mean Time Machine or VM snapshots), then you don't have to worry about the possibility of problems being introduced by a disk cleanup. 

Re: 2

That's an interesting dialog. I wonder if there's something up with keyboard mappings. I'll have to see if I can reproduce what you're seeing.

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- Paul (technogeezer)
vExpert 2025
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[Daniel Britt]

Thank  you -- very well explained! On the backup: I agree and I learned the hard way, (misunderstanding snapshots). Probably overkill, but I manually keep two recent copies of my VM file on the same hard drive (different folder) and will once a month, place a backup on an external hard drive - all for "insurance."

re: 2 -- I will keep an eye open for any other clues. I don't seem to have the issue when I use the CTRL+CMD keyboard short cut to pop the mouse out of the VM window. 

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[Technogeezer]

And if your VM is on an APFS- formatted disk, making a copy (actually a clone) via the Finder is a very space efficient way of copying a VM to a different folder on the same volume. It’s like a Fusion snapshot, but Apple’s file system based cloning (that the Finder uses in this case) is a lot less brittle, more reliable and IMHO more flexible than Fusion snapshots. I wish Fusion would support APFS clones directly in its GUI.

[Daniel Britt]

True! Hey, back on the first question about the Graphics options ... I'll give the 3D Acceleration a try again. What's your opinion of the other option regarding "full resolution for retina" (which I do have)?

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[slgarwood]

1. Apple Silicon cores aren't all equal

• The M2 Max has a mix of performance cores (P-cores) and efficiency cores (E-cores).

• VMware on macOS doesn't necessarily bind vCPUs to just the P-cores, so you can get unpredictable scheduling if you over-allocate.

2. vCPU coordination overhead

• A VM with multiple vCPUs must wait for all of them to be ready to complete certain tasks (known as "co-stop" in VMware).

• If one vCPU gets delayed by the macOS scheduler, the others sit idle. That delay grows with more vCPUs.

3. Diminishing returns past 2 vCPUs

• Unless your guest OS and workload are heavily parallel (like compiling large projects or running multiple busy services), more than 2 vCPUs can actually slow things down.

• This is especially true if you're also using the host for other apps - macOS will compete for CPU time.

4. Memory and I/O might be the real bottleneck

• On Apple Silicon, virtualization has to do extra translation work for x86 VMs, so CPU cores aren't always the limit - disk and emulated hardware can also bottleneck first.

Rule of thumb:

• Start with 2 vCPUs and see if your CPU usage in the VM regularly maxes out.

• If it does and the workload is truly multi-threaded, bump to 4 vCPUs - but don't jump straight to 8 or more.

• Keep in mind that each extra vCPU adds scheduling overhead that can outweigh raw core count benefits.

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[Technogeezer]

• On Apple Silicon, virtualization has to do extra translation work for x86 VMs, so CPU cores aren't always the limit - disk and emulated hardware can also bottleneck first.
  
  

just to be clear, Fusion on Apple Silicon does not run x86 VMs. Only ARM (aarch64/arm64) architecture operating systems. So there's no "extra translation" that comes into play - with the possible exception of what x64 translators such as Microsoft's Prism on Windows 11 ARM needs to do  to run Intel compiled applications. 

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- Paul (technogeezer)
vExpert 2025
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