CPU/RAM hot plugging with KVM

[ohmer]

Hello,

I'm experimenting CPU and RAM hot plugging with KVM.

I can add or remove RAM from a running VM. It works great! I have read that reducing RAM is not supposed to work, but all worked at the first try. Maybe I read outdated articles? (tested with proxmox 4.3, ubuntu 16.04 guest)

I can also set vCPU on a running VM. Seem to work too. But I can't change the number of cores.

I'm confused about the difference between cores and vCpu settings.

Right now, my VMs are configured with a specific number of cores. This is with that settings I'm limiting the number of cpu the guest can see/use.

Host is running dual-cpu quadcore with HT (2x4x2 = 16 cpu seen by the OS). What is the difference between theses two settings?

- 1 socket, 4 cores, undef vCpu
- 1 socket, 16 cores, 4 vCPUs

Thanks!

 

[spirit]

Great that ram unplug is working for you. I'll retest with last ubuntu. Maybe some improvements has been done.

About cpus:

socket-cores define the topology and sockets*cores = the max vcpus the can have.
By default the vms has always all vcpus.

Vcpus= the current number of cores "online". You can add hotplug them or unplug them (unplug is since qemu 2.7, you need to do last updates from last week)

 

[ohmer]

So my two examples are equivalents? 1 socket, 4 cores, undef vCpu == 1 socket, 16 cores, 4 vCPUs

 

[spirit]

So my two examples are equivalents? 1 socket, 4 cores, undef vCpu == 1 socket, 16 cores, 4 vCPUs

yes.

but with second example, you can hotplug up to 16 vcpus

 

[ohmer]

I have read that reducing RAM is not supposed to work, but all worked at the first try. Maybe I read outdated articles? (tested with proxmox 4.3, ubuntu 16.04 guest)

Doesn't work anymore.. strange!

 

[spirit]

Doesn't work anymore.. strange!

as I said, memory hot-unplug is half working. sometimes it's works, sometimes not. Depends of memory regions which are free or not.
(This is a linux guest kernel limitation, not qemu)

 

[BXGrzesiek]

Do I understand correctly that running NUMA should help me add and remove resources more efficiently?

My host - 1 socket / 8 cores.

I have 4 VM's with Debian 10
- socket 1
- cores 8
- vCPU 1
- CPU limit 8
- NUMA enabled
- CPU units (what does it mean?) 1024
Cores summary: 8

I don't know how it is possible that with 8 cores I can have 4 (vm) X8 (cores).

The question is that I can manage RAM but core changes won't go through despite:
- qemu-guest-agent running
- HotPlug activated

None of the attempts to change the number of cores for the VM's work.

What could be wrong?

 

[spirit]

you need some udev rules for cpu hotplug

https://pve.proxmox.com/wiki/Hotplug_(qemu_disk,nic,cpu,memory)


about cpuunit, it's not related to hotplug, it's a priority, when your host is at 100% cpu usage, to give more ressources to a specific vm.