Chapter 13. Allocating Node Resources

To provide more reliable scheduling and minimize node resource overcommitment, each node can reserve a portion of its resources for use by all underlying node components (e.g., kubelet, kube-proxy, Docker) and the remaining system components (e.g., sshd, NetworkManager) on the host. Once specified, the scheduler has more information about the resources (e.g., memory, CPU) a node has allocated for pods.

Resources reserved for node components are based on two node settings:

You can set these in the kubeletArguments section of the node configuration file (the /etc/origin/node/node-config.yaml file by default) using a set of <resource_type>=<resource_quantity> pairs (e.g., cpu=200m,memory=512Mi). Add the section if it does not already exist:

kubeletArguments:
  kube-reserved:
    - "cpu=200m,memory=512Mi"
  system-reserved:
    - "cpu=200m,memory=512Mi"

Currently, the cpu and memory resource types are supported. For cpu, the resource quantity is specified in units of cores (e.g., 200m, 0.5, 1). For memory, it is specified in units of bytes (e.g., 200Ki, 50Mi, 5Gi).

See Compute Resources for more details.

If a flag is not set, it defaults to 0. If none of the flags are set, the allocated resource is set to the node’s capacity as it was before the introduction of allocatable resources.

An allocated amount of a resource is computed based on the following formula:

[Allocatable] = [Node Capacity] - [kube-reserved] - [system-reserved]

If [Allocatable] is negative, it is set to 0.

To see a node’s current capacity and allocatable resources, you can run:

$ oc get node/<node_name> -o yaml
...
status:
...
  allocatable:
    cpu: "4"
    memory: 8010948Ki
    pods: "110"
  capacity:
    cpu: "4"
    memory: 8010948Ki
    pods: "110"
...

The scheduler now uses the value of node.Status.Allocatable instead of node.Status.Capacity to decide if a node will become a candidate for pod scheduling.

By default, the node will report its machine capacity as fully schedulable by the cluster.