How to implement automatic memory allocation with pure QEMU commands?

[sacarias]

At office we use Proxmox for production use. But for particular desktop user I wanted to give pure qemu-system-<arch> a try.

After a big while of studying I think I got almost everything needed, except the detail of "automatic ballooning"; i.e., automatic memory allocation:
https://pve.proxmox.com/pve-docs/pve-admin-guide.html#qm_memory

But as far as I could read in QEMU's docs and manuals, there's no info at all regarding how to do this; only manually with the monitor.

Is it possible to do this with pure QEMU/KVM? Or is this actually a Proxmox-only thing?

Thanks beforehand.

 

[fabian]

you can check how PVE configures a particular VM with qm showcmd XXX --pretty (where XXX is the VMID).

 

[sacarias]

Thanks very much sir.

I set up a VM with RAM ballooning via the web GUI, with total RAM 8 Gb and minimum RAM 4 Gb.
However, even with "qm showcmd" the *only* balloon thing I could see was the virtio-balloon-pci device, with options "addr" and "free-page-reporting", which are not documented in the QEMU docs by the way...

Not even with "qm config" I was able to see anything related to this "minimum 4 Gb RAM" thing I tried setting up.

Is this "minimum" ballooning thing really working? Or am I failing to understand how ballooning works?

Thanks again.

 

[Neobin]

Not even with "qm config" I was able to see anything related to this "minimum 4 Gb RAM" thing I tried setting up.

In a VM-config the minimum memory is defined with e.g.: balloon: 4096 for 4 GiB and the maximum memory is defined with e.g.: memory: 8192 for 8 GiB.

If it is not there, double check your VM-config over the GUI. There it will also show it as: 4.00 GiB/8.00 GiB in the "Hardware"-tab.

 

[fabian]

virtio-ballon-pci is (sparsely ) documented in qemu's help output:

$ kvm -device virtio-pci-balloon,help
virtio-balloon-pci options:
  acpi-index=<uint32>    -  (default: 0)
  addr=<int32>           - Slot and optional function number, example: 06.0 or 06 (default: -1)
  aer=<bool>             - on/off (default: false)
  any_layout=<bool>      - on/off (default: true)
  ats=<bool>             - on/off (default: false)
  deflate-on-oom=<bool>  - on/off (default: false)
  disable-legacy=<OnOffAuto> - on/off/auto (default: "auto")
  disable-modern=<bool>  -  (default: false)
  event_idx=<bool>       - on/off (default: true)
  failover_pair_id=<str>
  free-page-hint=<bool>  - on/off (default: false)
  free-page-reporting=<bool> - on/off (default: false)
  guest-stats-polling-interval=<int>
  guest-stats=<guest statistics>
  indirect_desc=<bool>   - on/off (default: true)
  iommu_platform=<bool>  - on/off (default: false)
  iothread=<link<iothread>>
  migrate-extra=<bool>   - on/off (default: true)
  modern-pio-notify=<bool> - on/off (default: false)
  multifunction=<bool>   - on/off (default: false)
  notify_on_empty=<bool> - on/off (default: true)
  packed=<bool>          - on/off (default: false)
  page-per-vq=<bool>     - on/off (default: false)
  page-poison=<bool>     - on/off (default: true)
  qemu-4-0-config-size=<bool> -  (default: false)
  rombar=<uint32>        -  (default: 1)
  romfile=<str>
  romsize=<uint32>       -  (default: 4294967295)
  use-disabled-flag=<bool> -  (default: true)
  use-started=<bool>     -  (default: true)
  virtio-backend=<child<virtio-balloon-device>>
  virtio-pci-bus-master-bug-migration=<bool> - on/off (default: false)
  x-ats-page-aligned=<bool> - on/off (default: true)
  x-disable-legacy-check=<bool> -  (default: false)
  x-disable-pcie=<bool>  - on/off (default: false)
  x-ignore-backend-features=<bool> -  (default: false)
  x-pcie-deverr-init=<bool> - on/off (default: true)
  x-pcie-extcap-init=<bool> - on/off (default: true)
  x-pcie-flr-init=<bool> - on/off (default: true)
  x-pcie-lnkctl-init=<bool> - on/off (default: true)
  x-pcie-lnksta-dllla=<bool> - on/off (default: true)
  x-pcie-pm-init=<bool>  - on/off (default: true)

when starting the VM, PVE will set 'guest-stats-polling-interval' to '2', the actual ballooning is then done by pvestatd:
https://git.proxmox.com/?p=pve-mana...2d153a07f557af2881c;hb=refs/heads/master#l180
https://git.proxmox.com/?p=pve-mana...2bb70c8f2b1a3e234e895558;hb=refs/heads/master

 

[sacarias]

Oohhh, so straight answer to OP is indeed a Proxmox original thing. Yes, I think I meant "auto-ballooning" since the beginning.

As much as I initially read, this was not possible with pure QEMU/KVM:
https://www.linux-kvm.org/page/Projects/auto-ballooning
And only possible manually by setting "balloon" value in QEMU monitor.

I expected to be able to find a way to set the "balloon" value directly with qemu-system-<arch>, i.e., right at VM start; but probably this is not possible...

Thanks for everything sir.