Kube-Scheduler is responsible for scheduling pods on nodes. The scheduler is only responsible for deciding which pod and which node. It does not actually place the pods on the nodes. That is the job of the Kubelet.
Why do we need a Kube-Scheduler?
When there are many containers, you want to make sure that the right container ends up on the nodes. There could be different resources in the nodes. You want to make sure the node has sufficient capacity to accommodates those containers.
In Kubernetes, the Kube-scheduler decides which nodes the pods are placed on depending on certain criteria. You may have Pods with different resource requirements, you can have nodes in the cluster dedicated to certain applications.
How does the Kube-scheduler assign these pods?
The scheduler looks at each pod and tries to find the best node for it.
- Filter Nodes
- The Kube-scheduler tries to filter out the nodes that do not fit the profile for the pod. (the nodes that do not have sufficient CPU and memory resources requested by the pod).
- Rank Nodes
- The Kube-scheduler ranks the node to identify the best fit for the pod. It uses a priority function to assign a score to the nodes on a scale of 0 to 10.
- More
- More resource requirements and limits
- Taints and tolerations
- Node Selectors/Affinity
Kube-Scheduler with Kubeadm
$ kubectl get pods -n kube-system
NAME READY STATUS RESTARTS AGE
coredns-f9fd979d6-wtk5k 1/1 Running 1 7d
coredns-f9fd979d6-x5zxv 1/1 Running 1 7d
etcd-kubemaster 1/1 Running 1 7d
kube-apiserver-kubemaster 1/1 Running 1 7d
kube-controller-manager-kubemaster 1/1 Running 1 7d
kube-proxy-jnf5q 1/1 Running 1 6d23h
kube-proxy-m9krm 1/1 Running 1 6d23h
kube-proxy-zfbsh 1/1 Running 1 7d
kube-scheduler-kubemaster 1/1 Running 1 7d
weave-net-g4l7r 2/2 Running 3 7d
weave-net-skdlq 2/2 Running 4 6d23h
weave-net-xg67h 2/2 Running 4 6d23h
- kube-scheduler-kubemaster
Kube-Scheduler pod definition file information with Kubeadm
$ cat /etc/kubernetes/manifests/kube-scheduler.yaml apiVersion: v1
kind: Pod
metadata:
creationTimestamp: null
labels:
component: kube-scheduler
tier: control-plane
name: kube-scheduler
namespace: kube-system
spec:
containers:
- command:
- kube-scheduler
- --authentication-kubeconfig=/etc/kubernetes/scheduler.conf
- --authorization-kubeconfig=/etc/kubernetes/scheduler.conf
- --bind-address=127.0.0.1
- --kubeconfig=/etc/kubernetes/scheduler.conf
- --leader-elect=true
- --port=0
image: k8s.gcr.io/kube-scheduler:v1.19.4
imagePullPolicy: IfNotPresent
livenessProbe:
failureThreshold: 8
httpGet:
host: 127.0.0.1
path: /healthz
port: 10259
scheme: HTTPS
initialDelaySeconds: 10
periodSeconds: 10
timeoutSeconds: 15
name: kube-scheduler
resources:
requests:
cpu: 100m
startupProbe:
failureThreshold: 24
httpGet:
host: 127.0.0.1
path: /healthz
port: 10259
scheme: HTTPS
initialDelaySeconds: 10
periodSeconds: 10
timeoutSeconds: 15
volumeMounts:
- mountPath: /etc/kubernetes/scheduler.conf
name: kubeconfig
readOnly: true
hostNetwork: true
priorityClassName: system-node-critical
volumes:
- hostPath:
path: /etc/kubernetes/scheduler.conf
type: FileOrCreate
name: kubeconfig
status: {}