Increasing the VirtualMachineInstance Density on Nodes

KubeVirt does not yet support classical Memory Overcommit Management or Memory Ballooning. In other words VirtualMachineInstances can’t give back memory they have allocated. However, a few other things can be tweaked to reduce the memory footprint and overcommit the per-VMI memory overhead.

Remove the Graphical Devices

First the safest option to reduce the memory footprint, is removing the graphical device from the VMI by setting spec.domain.devices.autottachGraphicsDevice to false. See the video and graphics device documentation for further details and examples.

This will save a constant amount of 16MB per VirtualMachineInstance but also disable VNC access.

Overcommit the Guest Overhead

Before you continue, make sure you make yourself comfortable with the Out of Resource Managment of Kubernetes.

Every VirtualMachineInstance requests slightly more memory from Kubernetes than what was requested by the user for the Operating System. The additional memory is used for the per-VMI overhead consisting of our infrastructure which is wrapping the actual VirtualMachineInstance process.

In order to increase the VMI density on the node, it is possible to not request the additional overhead by setting spec.domain.resources.overcommitGuestOverhead to true:

apiVersion: kubevirt.io/v1alpha3
kind: VirtualMachineInstance
metadata:
  name: testvmi-nocloud
spec:
  terminationGracePeriodSeconds: 30
  domain:
    resources:
      overcommitGuestOverhead: true
      requests:
        memory: 1024M
[...]

This will work fine for as long as most of the VirtualMachineInstances will not request the whole memory. That is especially the case if you have short-lived VMIs. But if you have long-lived VirtualMachineInstances or do extremely memory intensive tasks inside the VirtualMachineInstance, your VMIs will use all memory they are granted sooner or later.

Overcommit Guest Memory

The third option is real memory overcommit on the VMI. In this scenario the VMI is explicitly told that it has more memory available than what is requested from the cluster by setting spec.domain.memory.guest to a value higher than spec.domain.resources.requests.memory.

The following definition requests 1024MB from the cluster but tells the VMI that it has 2048MB of memory available:

apiVersion: kubevirt.io/v1alpha3
kind: VirtualMachineInstance
metadata:
  name: testvmi-nocloud
spec:
  terminationGracePeriodSeconds: 30
  domain:
    resources:
      overcommitGuestOverhead: true
      requests:
        memory: 1024M
    memory:
      guest: 2048M
[...]

For as long as there is enough free memory available on the node, the VMI can happily consume up to 2048MB. This VMI will get the Burstable resource class assigned by Kubernetes (See QoS classes in Kubernetes for more details). The same eviction rules like for Pods apply to the VMI in case the node gets under memory pressure.

Implicit memory overcommit is disabled by default. This means that when memory request is not specified, it is set to match spec.domain.memory.guest. However, it can be enabled using memory-overcommit in the kubevirt-config. For example, by setting memory-overcommit: "150" we define that when memory request is not explicitly set, it will be implicitly set to achieve memory overcommit of 150%. For instance, when spec.domain.memory.guest: 3072M, memory request is set to 2048M, if omitted. Note that the actual memory request depends on additional configuration options like OvercommitGuestOverhead.

Configuring the memory pressure behaviour of nodes

If the node gets under memory pressure, depending on the kubelet configuration the virtual machines may get killed by the OOM handler or by the kubelet itself. It is possible to tweak that behaviour based on the requirements of your VirtualMachineInstances by:

Configuring Soft Eviction Thresholds

Note: Soft Eviction will effectively shutdown VirtualMachineInstances. They are not paused, hibernated or migrated. Further, Soft Eviction is disabled by default.

If configured, VirtualMachineInstances get evicted once the available memory falls below the threshold specified via --eviction-soft and the VirtualmachineInstance is given the chance to perform a shutdown of the VMI within a timespan specified via --eviction-max-pod-grace-period. The flag --eviction-soft-grace-period specifies for how long a soft eviction condition must be held before soft evictions are triggered.

If set properly according to the demands of the VMIs, overcommitting should only lead to soft evictions in rare cases for some VMIs. They may even get re-scheduled to the same node with less initial memory demand. For some workload types, this can be perfectly fine and lead to better overall memory-utilization.

Configuring Hard Eviction Thresholds

Note: If unspecified, the kubelet will do hard evictions for Pods once memory.available falls below 100Mi.

Limits set via --eviction-hard will lead to immediate eviction of VirtualMachineInstances or Pods. This stops VMIs without a grace period and is comparable with power-loss on a real computer.

If the hard limit is hit, VMIs may from time to time simply be killed. They may be re-scheduled to the same node immediately again, since they start with less memory consumption again. This can be a simple option, if the memory threshold is only very seldom hit and the work performed by the VMIs is reproducible or it can be resumed from some checkpoints.

Requesting the right QoS Class for VirtualMachineInstances

Different QoS classes get assigned to Pods and VirtualMachineInstances based on the requests.memory and limits.memory. KubeVirt right now supports the QoS classes Burstable and Guaranteed. Burstable VMIs are evicted before Guaranteed VMIs.

This allows creating two classes of VMIs:

  • One type can have equal requests.memory and limits.memory set and therefore gets the Guaranteed class assigned. This one will not get evicted and should never run into memory issues, but is more demanding.

  • One type can have no limits.memory or a limits.memory which is greater than requests.memory and therefore gets the Burstable class assigned. These VMIs will be evicted first.

Setting --system-reserved and --kubelet-reserved

It may be important to reserve some memory for other daemons (not DaemonSets) which are running on the same node (e.g. ssh, dhcp servers, …). The reservation can be done with the --system-reserved switch. Further for the Kubelet and Docker a special flag called --kubelet-reserved exists.

Enabling KSM

The KSM (Kernel same-page merging) daemon can be started on the node. Depending on its tuning parameters it can more or less aggressively try to merge identical pages between applications and VirtualMachineInstances. The more aggressive it is configured the more CPU it will use itself, so the memory overcommit advantages comes with a slight CPU performance hit.

Config file tuning allows changes to scanning frequency (how often will KSM activate) and aggressiveness (how many pages per second will it scan).

Enabling Swap

Note: This will definitely make sure that your VirtualMachines can’t crash or get evicted from the node but it comes with the cost of pretty unpredictable performance once the node runs out of memory and the kubelet may not detect that it should evict Pods to increase the performance again.

Enabling swap is in general not recommended on Kubernetes right now. However, it can be useful in combination with KSM, since KSM merges identical pages over time. Swap allows the VMIs to successfuly allocate memory which will then effectively never be used because of the later de-duplication done by KSM.