Node Allocatable in AKS

Kubernetes’ Node Allocatable feature allows the cluster to reserve the resources of node for system daemons of OS and Kubernetes itself. For example, when I ran kubectl describe node for a node in my AKS cluster, I got the following capacity related output. The size of this node was Standard DS2 v2 which had 2 CPU cores and 7GB memory.

Capacity:
  attachable-volumes-azure-disk:  8
  cpu:                            2
  ephemeral-storage:              101445900Ki
  hugepages-1Gi:                  0
  hugepages-2Mi:                  0
  memory:                         7113660Ki
  pods:                           110
Allocatable:
  attachable-volumes-azure-disk:  8
  cpu:                            1900m
  ephemeral-storage:              93492541286
  hugepages-1Gi:                  0
  hugepages-2Mi:                  0
  memory:                         4668348Ki
  pods:                           110

From the output, out of 2 cores and 7GB memory, there were 1900 millicores and 66% memory (4668348/7113660) allocatable to pods. The details about how these numbers were calculated are described in this document. In short, the configuration of node allocatable in AKS is as follows:

CPU

CPU cores on host	1	2	4	8	16	32	64
Kube-reserved (millicores)	60	100	140	180	260	420	740

Memory

eviction-hard: 750Mi, which is the default configuration of the upstream aks-engine.
kube-reserved: regressive rate
- 25% of the first 4 GB of memory
- 20% of the next 4 GB of memory (up to 8 GB)
- 10% of the next 8 GB of memory (up to 16 GB)
- 6% of the next 112 GB of memory (up to 128 GB)
- 2% of any memory above 128 GB

However, the above memory reservation doesn’t seem to applicable to Windows nodes. The following was the output when I ran the kubectl describe node for a Windows node. More memory was reserved for Windows nodes.

Capacity:
  attachable-volumes-azure-disk:  8
  cpu:                            2
  ephemeral-storage:              133703676Ki
  memory:                         7339572Ki
  pods:                           30
Allocatable:
  attachable-volumes-azure-disk:  8
  cpu:                            1900m
  ephemeral-storage:              133703676Ki
  memory:                         3565108Ki
  pods:                           30

The document doesn’t have any information regarding Windows node. GKE reserves approximately 1.5 times more resources on Windows Server nodes. Not sure if it’s the same for AKS. I’ve opened an issue to ask for further information.

This post provides a good comparison of reserved resources for 3 major cloud offerings: AKS, GKE and EKS.