CPU oversubscription for virtual machines

Oversubscription lets you give the virtual machines (VMs) on a node more virtual cores than the node physically has. This makes sense because VMs rarely load the CPU at the same time and at full capacity.

The degree of oversubscription is controlled by the coreFraction parameter of a virtual machine, and you define its allowed values in the sizing policy of the class. The parameter defines the share of a core’s capacity guaranteed to a VM. For example, with coreFraction: 20%, a VM always gets a fifth of a core, and it can take a whole core when the node has spare resources.

When placing a VM on a node, Deckhouse Platform (DP) sums the guaranteed shares of all VMs on that node using the Σ(cores × coreFraction / 100) formula. If the sum exceeds the number of physical cores, the VM doesn’t start on that node.

Consider a node with 4 physical cores and 5 VMs, each with 2 cores and coreFraction: 20%. The guaranteed load is 5 × 2 × 0.2 = 2 cores, with 10 virtual cores on 4 physical ones, which is an oversubscription of 2.5 to 1. All five VMs fit on the node, because 2 cores is less than the available 4.

Fixed oversubscription

A list with a single value leaves the project owner no choice, and you define the degree of oversubscription for all VMs of the class:

apiVersion: virtualization.deckhouse.io/v1alpha2
kind: VirtualMachineClass
metadata:
  name: oversubscribed
spec:
  sizingPolicies:
    - cores:
        min: 1
        max: 8
      memory:
        perCore:
          min: 1Gi
          max: 8Gi
      coreFractions: [20] # The only allowed value.
      defaultCoreFraction: 20

All VMs of this class get 20% of a core each, which gives an oversubscription of 5 to 1.

Oversubscription chosen by the project owner

A list with several values leaves the choice to the project owner:

apiVersion: virtualization.deckhouse.io/v1alpha2
kind: VirtualMachineClass
metadata:
  name: standard
spec:
  sizingPolicies:
    - cores:
        min: 1
        max: 4
      memory:
        perCore:
          min: 1Gi
          max: 8Gi
      coreFractions: [5, 10, 20, 50, 100]
      defaultCoreFraction: 20

The project owner selects coreFraction from the list, and if they don’t, the VM gets 20%.