Hotplug CPU Not Working on Win10 Pro

[Allister]

Hey there, I've been playing around with PMX 8 trying to get CPU Hotplug working on Windows 10 Pro. I'm installing the OS from the 22H2 image which looks like the last one they've released. I've made sure that NEMA and CPU hot plug are enabled, I'm using the q35 chipset, have all of the drivers installed, and have guest tools installed. I have 1 socket and 4 cores set, with 1 vCPU to start.

Why am I doing this? Because I'd like to see the feature work - so when I need it, I know it works. It should work based on what I'm reading but I'm unable to get it to work.

When I put a single threaded load on the CPU, Task Manager is at 100%. Then I add another vCPU, I see the additional CPU get added to device manager, but in Task Manager, the CPU doesn't go down to 50% - even if I close and reopen it.

What am I missing? I know this was a bug in Windows for some time, but it looks like all that was fixed in the past based on what I see in the registry. Anyone have any ideas what's going on? Thanks for any tips anyone may have for me.

 

[bbgeek17]

When I put a single threaded load on the CPU, Task Manager is at 100%. Then I add another vCPU, I see the additional CPU get added to device manager, but in Task Manager, the CPU doesn't go down to 50% - even if I close and reopen it.

Can you describe what is "single threaded load"? Is it your believe that this "load" should automatically scale to multiple CPUs, ie its actually designed to do so? Very few application are smart enough to take advantage of hot-plugged CPUs on the fly.
What happens when you start a second single-threaded load? Does the CPU utilization go to 200%?

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Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox

 

[Allister]

Can you describe what is "single threaded load"? Is it your believe that this "load" should automatically scale to multiple CPUs, ie its actually designed to do so? Very few application are smart enough to take advantage of hot-plugged CPUs on the fly.
What happens when you start a second single-threaded load? Does the CPU utilization go to 200%?

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Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox

Hey there - thanks for the reply. The single threaded load is just a .vbs with while true wend in it. I know it's single threaded because if I reboot the machine and do the same thing, it only uses 50% of the available vCPUs (1 of 2) that I've assigned to the VM.

I just tried it, with running the .vbs file, it went to 100% cpu in task manager. Running it again it stayed at 100%. Looking in the processes, you can see that each instance of the .vbs file is roughly getting about 50% of the available CPU. Does the type of CPU I have set matter at all? It's the default x86-64-v2-AES. I tried KVM64 as well but it didn't change anything. If you have any tips on what to try to get this to work, thanks for any info you may be willing to share with me.

 

[Allister]

I tried again with the latest stable version of the virtio drivers ISO and it didn't have any change. The additional CPU in task manager is only reflected after you reboot. It also didn't reflect in the system and it wasn't usable until I rebooted.

 

[bbgeek17]

The mechanism of recognizing a hot-plugged CPU is embedded into Kernel of the OS. One cant "add" it via a 3rd party driver.
I assume you read this article fully: https://blogs.vmware.com/apps/2021/06/cpu-hotadd-for-windows-vms-how-badly-do-you-want-it.html

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Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox

 

[Allister]

Just wanted to follow up on this.. The only one out of Win10, 11, and Server 22 that actually worked with hot plug CPU was server 2022.