hyperthreading enabled - does it hurt when with total of vCPU > total physical cores?

[thaink]

Hi,I'm new to Proxmox. I have a question now, wish your help
I'm about to build a dual processor computer with total 12 physical cores with hyper-threading (so it has 24 logical cores).
I want to use this computer to run 5VMs. So which option should I choose:
1. Enable Hyper-threading and run a VM with 4 vCPU. This will give a little more performance if
hyper-threading can help. But does it hurt when total vCPU > total physical cores?

2. Disable Hyper-threading and run a VM with 2 vCPU. This will guarantee that each VM will receive
2 physical cores.
What i concern about the first option is: I think Proxmox always try to run VM on physical core first.
I'm afraid that when i run first and second VM, Proxmox will try to assign them 8 physical core. So one
of the third, fourth for fifth VM will run only on hyper-threaded core and their performance will be bad.
Does my concern is a real problems? what option should I choose?Sorry for my bad English and thanks for your help
​

 

[udo]

Re: hyperthreading enabled - does it hurt when with total of vCPU > total physical co

Hi,I'm new to Proxmox. I have a question now, wish your help
I'm about to build a dual processor computer with total 12 physical cores with hyper-threading (so it has 24 logical cores).
I want to use this computer to run 5VMs. So which option should I choose:
1. Enable Hyper-threading and run a VM with 4 vCPU. This will give a little more performance if
hyper-threading can help. But does it hurt when total vCPU > total physical cores?


​

Hi,
with actual Intel CPUs it's recommended to enable HT.

2. Disable Hyper-threading and run a VM with 2 vCPU. This will guarantee that each VM will receive
2 physical cores.
What i concern about the first option is: I think Proxmox always try to run VM on physical core first.
I'm afraid that when i run first and second VM, Proxmox will try to assign them 8 physical core. So one
of the third, fourth for fifth VM will run only on hyper-threaded core and their performance will be bad.
Does my concern is a real problems? what option should I choose?Sorry for my bad English and thanks for your help

No, your assumtion are wrong.
PVE, or better kvm don't know/care about physical or HT processor. But this is in real life no problem.
With two CPUs you should enable NUMA in the VM-config, so that the kvm is aware about CPU-Memmory bindings.

You can also overcommit CPUs - this is the normal doing with virtualisation.
Just look on one server with an 6core HT CPU (12 cores): the running VMs have 32 active cores... but this depends of your workload, of course. If you VMs use allways 100% cpu power you should not overcommit your logical cpus.

Udo

Udo

 

[spirit]

Re: hyperthreading enabled - does it hurt when with total of vCPU > total physical co

just look on one server with an 6core HT CPU (12 cores): the running VMs have 32 active cores...
Udo

I'm curious to known how you do that, because in proxmox with don't start the vm if number of vcpus is bigger than host total cpus (sockets*cores*threads).

It's not a problem to oversubscribing with multiple vms.

But it really make no sense to have 1vm with more cpus than host. That will reduce performance because of context switches.

 

[udo]

Re: hyperthreading enabled - does it hurt when with total of vCPU > total physical co

I'm curious to known how you do that, because in proxmox with don't start the vm if number of vcpus is bigger than host total cpus (sockets*cores*threads).

It's not a problem to oversubscribing with multiple vms.

But it really make no sense to have 1vm with more cpus than host. That will reduce performance because of context switches.

Hi Spirit,
that was an misunderstanding... I wrote VMs ;-)

In this case, there are 18 running VMs with 32 vcpus in summary. Of course no VM have more vcpus like the host has cores (the biggest VM use 8 cores).

Udo

 

[thaink]

Re: hyperthreading enabled - does it hurt when with total of vCPU > total physical co

Hi,
Thanks a lot for clarifying.
It help me a lot