Poor guest (windows10) performance since host updated to Windows 10 20H2, 2 or more CPU cores

[Krieger]

Hello everybody,

I've been struggling with this issue ever since I have updated the host to 20H2 version of Windows 10 a few days ago. Previously, there was no problem.
Essenially, what happens is the following: When the guest is having 2 or more CPU cores assigned in VBOX, the performance is extremely poor. Most of the times it does not even finish booting when it freezes.
When reducing the number of available cores to one, everything works fine, at of course the cost of having less cores to work with.

This is happening regardless if the image is a clone or not. I have done a full clean VirtualBox reinstallation on the host and I have recreated the guest(s). The behavior is the same.
I am attaching the logs of a run cycle from boot, to shut down. In this particular case, the guest booted, but the performance problem manifested itself. This was the first time it booted into the OS.

Host details:

Device name cvmfc
Processor Intel(R) Core(TM) i5-4670K CPU @ 3.40GHz 3.40 GHz
Installed RAM 16,0 GB
System type 64-bit operating system, x64-based processor

Edition Windows 10 Pro
Version 20H2
Installed on ‎29.‎10.‎2020
OS build 19042.572
Experience Windows Feature Experience Pack 120.2212.31.0

Guest details:
Same OS and build version as the host. Windows 10 version 20H2, created with media creation tool.


Is there anything I am missing? Can anybody suggest what I could try to solve this?

I'd also like to let you know that I am not a professional with VMs, I am doing this as a hobby to try things out and learn stuff.
Thank you very much.

[arQon]

Probably this:
00:00:03.160985 HM: HMR3Init: Attempting fall back to NEM: VT-x is not available

You need to disable Hyper-V.

Search the forums for any of the last 50 times someone's had this problem , and follow the advice / instructions / etc given there.

Full marks for providing zipped logs on your very first post without having to be asked!

[Krieger]

Probably this:
00:00:03.160985 HM: HMR3Init: Attempting fall back to NEM: VT-x is not available

You need to disable Hyper-V.

Search the forums for any of the last 50 times someone's had this problem , and follow the advice / instructions / etc given there.

Full marks for providing zipped logs on your very first post without having to be asked!

Hello arQon,

Thank you very much for the reply.
The problem is that Hyper-V is already disabled on my system.
All I had enabled were Windows Hypervisor Platform and Virtual Machine Platform.
I have retried my situation after having disabled Virtual Machine Platform and left Hypervisor Platform on. As far as I know, it is impossible to start any VBOX guest without this enabled on the host.

Even under these circumstances, I am having the same problem.
I have already searched the forum, but I couldn't find anything resembling my scenario .

At this point, I am willing to try different options, but I couldn't find too many conclusive technical resources to go by.
--------------
LE:
- I have disabled the Windows Hypervisor Platform feature of the host, disabled PAE/NX in virtual box client and it seems that it works again now. I am unaware whether the update to Windows 10 20H2 has changed anything without me knowing, I don't remember whether this was a thing before.
Just to let everybody know my conclusion. Very strange.

[arQon]

The problem is that Hyper-V is already disabled on my system.

No, it isn't. That's what the line from your logs means. (well, that or you don't have VT-x support at all, but since things were good in the past it seemed likely you do have it - I didn't actually check the ARK).

- I have disabled the Windows Hypervisor Platform feature of the host, disabled PAE/NX in virtual box client and it seems that it works again now. I am unaware whether the update to Windows 10 20H2 has changed anything without me knowing, I don't remember whether this was a thing before.
Just to let everybody know my conclusion. Very strange.

Not strange at all. As I said, you needed to disable Hyper-V, and you now have, therefore everything works properly again. Not your fault, but you need to know to watch out for it after any W10 update (especially a major one), and now you do.

Yes, it's possible/likely that the W10 update enabled Hyper-V without telling you. Silently changing settings is W10's thing.

Somewhere - probably in one of the sticky threads (the "I have a 64-bit OS but can't run 64-bit guests" one, I think) - there's a list of all the W10 pieces that use Hyper-V. If any of those are still running, so is Hyper-V.