Passthrough of Cores and Threads

[andy77]

Hi all,

can I pass all cores and threads to a Virtual Machine?
So that the machine has for example all 8 cores and all 16 threads of the node.
Purpose for this is licensing.

Thanks in advance for answering.


Regards,
Andreas

 

[wolfgang]

can I pass all cores and threads to a Virtual Machine?

Yes, you can but this is not smart.
You should never use more than n -1 cores for a VM.
n is the total count of logical cores.

The point is the PVE OS need one core for system operation like webserver, logging, .... .
So if you give the VM all cores the host has to schedule the process what ends in less performance.

 

[andy77]

Hi,

sorry, I didn't express myself properly.

The goal is to give one VM max. 8 virtual cores, but still have 16 threads in this VM. Reason for this is SQL Licensing because it is licensed per Core, so that on a physical machine with 8 hardware cores, you would only have to license 8 cores and would have 16 threads, but in a VM for having 16 threads you would need 16 vCores what means double licensing costs.


Regards,
Andreas

 

[wolfgang]

Proxmox VE has no setting for Pinning down Cores to CPUs.
But you can do this manually with taskset and write this in a post start hook script.

 

[andy77]

Hmm... but as I understand "taskset" it would not help in my example.
I still would have to assign 8 vcors only to the VM what would result in running only 8 threads and the usage of 4 hardware cores????

 

[wolfgang]

This is not supported, because in the past has this feature significant performance problems.
But you can try to add args in the config.

args: -smp '16,sockets=1,cores=8,threads=2,maxcpus=16'

 

[andy77]

Unfortunately this config is resulting in Windows VM showing 16 vCores instead of 8.

 

[andy77]

Any other idea?