Question about dual CPU motherboard setups

[mstow]

I am debating using Proxmox for my next homelab upgrade and I was wondering about how it handles passthrough for CPUs.

The main question I have is whether I can pass through an entire CPU in a dual CPU motherboard and have it view it as not being emulated and have access to the whole CPU pinning and not share it with any other processes. The other CPU would handle the rest. Is this possible?

Thanks in advance

 

[LnxBil]

The main question I have is whether I can pass through an entire CPU in a dual CPU motherboard and have it view it as not being emulated and have access to the whole CPU pinning and not share it with any other processes. The other CPU would handle the rest. Is this possible?

Technically, sure! Yet you will run into performance problems if you access RAM that is located on the other CPU. In multi-socket CPU (or multi-package CPUs), you will have to enable non uniform memory access (NUMA) to speed things up and NOT use RAM that is handled by the other CPU.

Such a pinning seldomly yields significant performance improvements. If you need raw performance, just don't use a hypervisor. For me, the hassle of deconfiguring virtualization and hard-pinning resources is more work than just use the hardware directly without a hypervisor.

 

[leesteken]

There will always be some emulated and para-virtualized devices, which also require a thread (or more if you enable IO Thread), which either run on the other CPU (with distant memory) or prevent you from using the whole CPU for the virtual cores of the VM. It's also not uncommon that all PCI(e) devices/lanes are physically connected to the first CPU (maybe to allow running without a second CPU?), which you might want to account for.
If you want a smoothly running VM then make sure that it can never use all of the performance (of any subsystem), otherwise it might stutter or have interference from other VMs or background processing of Proxmox. But this also prevents you from getting maximum performance...

Virtualization is usually about efficiency (consolidation, power conservation) or convenience (management. backups, etc.) and this usually comes with a price on (raw) performance.