Proxmox vCPUs - behaviour

[renegade]

Hi all,

I am interested in how the assigned cores and sockets is used by Proxmox (and in general any default hypervisor behaviour).

I read here in pve-docs/chapter-qm.html#qm_cpu that:

It is perfectly safe if the overall number of cores of all your VMs is greater than the number of cores on the server
(e.g., 4 VMs with each 4 cores on a machine with only 8 cores). In that case the host system will balance the Qemu execution
threads between your server cores, just like if you were running a standard multi-threaded application.
However, Proxmox VE will prevent you from starting VMs with more virtual CPU cores than physically available,
as this will only bring the performance down due to the cost of context switches.

To me this means the following:

If I 16 cores, across 2 sockets on the physical machine:

I can have 10 virtual machines each with 4 cores : 40 cores - Qemu balances execution threads between server cores.
Proxmox will prevent me from starting VMs with more virtual CPU cores than physically available --- this is the bit which I need clarity on.

Let's say I have the following started in order 1 - 10:

1. VM1 - 4 CORES - STARTS
2. VM2 - 4 CORES - STARTS
3. VM3 - 4 CORES - STARTS
4. VM4 - 4 CORES - STARTS
5. VM5 - 4 CORES - REFUSES TO START?
6. VM6 - 4 CORES - REFUSES TO START?
7. VM7 - 4 CORES - REFUSES TO START?
8. VM8 - 4 CORES- REFUSES TO START?
9. VM9 - 4 CORES- REFUSES TO START?
10. VM10 - 4 CORES- REFUSES TO START?

Is this the correct expected behaviour? What If I wanted 10 VMs but I only have 16 cores? Would I have to limit some VMS to just one core?

OR, is this rule saying:

All of the 10 VMs would start, but if I assigned VM11 as having 17 cores - THAT would fail to start, as it is more than physically available?

 

[dietmar]

OR, is this rule saying:

All of the 10 VMs would start, but if I assigned VM11 as having 17 cores - THAT would fail to start, as it is more than physically available?

exactly.