How to configure cpu core usage

[fontajos]

We have a cluster of 5 dual socket cpu servers with each running some vms each. We have the following questions regarding the max. usage of cpu-cores that we can allocate.

E.g. if we look at one server that has two sockets, 32 cpu cores (16 real cores, and 16 hyperthreading cores) and two VMs using about the same amount of cpu.

Via Hardware - Processors, should we both give:
- Sockets: 2
- Cores: 32
- VCPUs 16

(we would like to use cpu hotplug, so thats why we use VCPUs)

Or should we keep some cores reserved for the proxmox hypervisor?

 

[leesteken]

Proxmox does need some CPU because it has things to do (logging, emulation of devices, running backups, virtual disk and network. etc.).
It's fine to over-commit on cores (but not on memory!) as longs as all VMs together don't use them all at the same time (for long).
Please don't count hyperthreads as whole cores, as they same so much silicon with a core. Hyperthreads can help hide memory latency but should be counted more like 10%-30%.
I tend to keep the hyperthreads for Proxmox (keep average CPU usage below 50%) and everything is really smooth.