Multiple Schedulers

Concept and Usage of Multiple Schedulers

Multiple schedulers are used to schedule pods on different nodes based on the requirements. By default, Kubernetes uses the default scheduler to schedule pods on nodes using an algorithm to distribute the pods across the nodes evenly. But in some cases, you may want to setup your own scheduling algorithm or any custom conditions to place pods on nodes.

Therefore, Kubernetes allows you to write and deploy your own scheduler as default scheduler or as an additional scheduler. In this case, you can use your own custom scheduler to schedule some specific pods (applications) on specific nodes based on your requirements, while other pods can still be scheduled by the default scheduler.

This is the default scheduler, the scheduler name is default-scheduler and it must be unique in the cluster. You can find your default scheduler configuration on the master node at /etc/kubernetes/manifests/kube-scheduler.yaml.

scheduler-config.yaml

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
profiles:
  - schedulerName: default-scheduler # unique name

Steps to setup and use Multiple Schedulers

Step 1: Create a new scheduler configuration file

/etc/kubernetes.my-new-scheduler.yaml

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
profiles:
  - schedulerName: my-new-scheduler
leaderElection:
  leaderElect: true
  resourceNamespace: kube-system
  resourceName: lock-object-my-scheduler

leaderElect - ensures that only one instance of the scheduler is active at a time. Assume that there are multiple instances of the scheduler running on different master nodes as a high-availability setup, only one instance will be elected (selected) as a leader to schedule the pods.
resourceName - assume you have multiple masters, you will need to specify the name of the resource object used for leader election. This is used to ensure that only one instance of the scheduler is active at a time and to avoid conflicts between multiple instances of the scheduler.

Step 2: Deploy Additional Scheduler

You may choose to deploy the scheduler as a Pod or Deployment.

Deploy as a Pod

my-new-scheduler.yaml

apiVersion: v1
kind: Pod
metadata:
  name: my-new-scheduler
  namespace: kube-system
spec:   
  containers:
    - name: my-new-scheduler
      image: k8s.gcr.io/kube-scheduler:v1.22.0
      command:
        - kube-scheduler
        # make sure these files are exists on the host
        - --kubeconfig=/etc/kubernetes/scheduler.conf  # this file has the authentication information to access the API server
        - --config=/etc/kubernetes.my-new-scheduler.yaml
      volumeMounts:
        - name: kubeconfig
          mountPath: /etc/kubernetes
          readOnly: true
  volumes:
    - name: kubeconfig
      hostPath:
        path: /etc/kubernetes

kubectl apply -f my-new-scheduler.yaml

Deploy as a Deployment

Package the Scheduler

git clone https://github.com/kubernetes/kubernetes.git
cd kubernetes
make

Create a new container image containing the kube-scheduler binary

Dockerfile

FROM busybox
ADD ./_output/local/bin/linux/amd64/kube-scheduler /usr/local/bin/kube-scheduler

Build the dockerfile

docker build -t gcr.io/my-gcp-project/my-kube-scheduler:1.0 .     # The image name and the repository
gcloud docker -- push gcr.io/my-gcp-project/my-kube-scheduler:1.0 # used in here is just an example

Define a Kubernetes Deployment for the scheduler

my-new-scheduler.yaml

apiVersion: v1
kind: ServiceAccount
metadata:
  name: my-scheduler
  namespace: kube-system
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: my-scheduler-as-kube-scheduler
subjects:
- kind: ServiceAccount
  name: my-scheduler
  namespace: kube-system
roleRef:
  kind: ClusterRole
  name: system:kube-scheduler
  apiGroup: rbac.authorization.k8s.io
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: my-scheduler-as-volume-scheduler
subjects:
- kind: ServiceAccount
  name: my-scheduler
  namespace: kube-system
roleRef:
  kind: ClusterRole
  name: system:volume-scheduler
  apiGroup: rbac.authorization.k8s.io
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: my-scheduler-extension-apiserver-authentication-reader
  namespace: kube-system
roleRef:
  kind: Role
  name: extension-apiserver-authentication-reader
  apiGroup: rbac.authorization.k8s.io
subjects:
- kind: ServiceAccount
  name: my-scheduler
  namespace: kube-system
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: my-scheduler-config
  namespace: kube-system
data:
  my-scheduler-config.yaml: |
    apiVersion: kubescheduler.config.k8s.io/v1beta2
    kind: KubeSchedulerConfiguration
    profiles:
      - schedulerName: my-scheduler
    leaderElection:
      leaderElect: false    
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    component: scheduler
    tier: control-plane
  name: my-scheduler
  namespace: kube-system
spec:
  selector:
    matchLabels:
      component: scheduler
      tier: control-plane
  replicas: 1
  template:
    metadata:
      labels:
        component: scheduler
        tier: control-plane
        version: second
    spec:
      serviceAccountName: my-scheduler
      containers:
      - command:
        - /usr/local/bin/kube-scheduler
        - --config=/etc/kubernetes/my-scheduler/my-scheduler-config.yaml
        image: gcr.io/my-gcp-project/my-kube-scheduler:1.0
        livenessProbe:
          httpGet:
            path: /healthz
            port: 10259
            scheme: HTTPS
          initialDelaySeconds: 15
        name: kube-second-scheduler
        readinessProbe:
          httpGet:
            path: /healthz
            port: 10259
            scheme: HTTPS
        resources:
          requests:
            cpu: '0.1'
        securityContext:
          privileged: false
        volumeMounts:
          - name: config-volume
            mountPath: /etc/kubernetes/my-scheduler
      hostNetwork: false
      hostPID: false
      volumes:
        - name: config-volume
          configMap:
            name: my-scheduler-config

kubectl apply -f my-new-scheduler.yaml

Step 3: Verify the new scheduler is running

kubectl get pods -n kube-system
 
# output
NAME                                           READY     STATUS    RESTARTS   AGE
....
my-scheduler-lnf4s-4744f                       1/1       Running   0          2m
...

Step 4: Create a Pod with the new scheduler

pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  containers:
    - name: sample-pod
      image: ubuntu
  schedulerName: my-new-scheduler # the name of the scheduler

When you create a pod with the schedulerName field, the pod will be scheduled by the specified scheduler. You can see the pod assignment events by running the following command:

# method 1 ---> view the events of the pod
kubectl get events -o wide
 
# output
LAST SEEN   TYPE     REASON      OBJECT       SUBOBJECT                 SOURCE                                                            MESSAGE                                                              FIRST SEEN   COUNT   NAME
10s         Normal   Scheduled   pod/ubuntu                             custom-scheduler, custom-scheduler-kind-cluster-control-plane     Successfully assigned default/ubuntu to kind-cluster-control-plane   10s          1       ubuntu.18159790928c8e97
10s         Normal   Pulling     pod/ubuntu   spec.containers{ubuntu}   kubelet, kind-cluster-control-plane                               Pulling image "ubuntu"                                               10s          1       ubuntu.18159790b760375c
 
# method 2 ---> view the logs of the scheduler
kubectl logs <your-new-scheduler-pod-name> -n kube-system
kubectl logs my-new-scheduler -n kube-system
 
# output
I1229 05:00:45.515663       1 serving.go:380] Generated self-signed cert in-memory
I1229 05:00:47.196685       1 server.go:154] "Starting Kubernetes Scheduler" version="v1.30.0"
I1229 05:00:47.196790       1 server.go:156] "Golang settings" GOGC="" GOMAXPROCS="" GOTRACEBACK=""
I1229 05:00:47.208976       1 secure_serving.go:213] Serving securely on 127.0.0.1:10259
I1229 05:00:47.209676       1 tlsconfig.go:240] "Starting DynamicServingCertificateController"
I1229 05:00:47.212183       1 configmap_cafile_content.go:202] "Starting controller" name="client-ca::kube-system::extension-apiserver-authentication::requestheader-client-ca-file"
I1229 05:00:47.212327       1 shared_informer.go:313] Waiting for caches to sync for client-ca::kube-system::extension-apiserver-authentication::requestheader-client-ca-file
I1229 05:00:47.212336       1 requestheader_controller.go:169] Starting RequestHeaderAuthRequestController
I1229 05:00:47.212350       1 shared_informer.go:313] Waiting for caches to sync for RequestHeaderAuthRequestController
I1229 05:00:47.212578       1 configmap_cafile_content.go:202] "Starting controller" name="client-ca::kube-system::extension-apiserver-authentication::client-ca-file"
I1229 05:00:47.212627       1 shared_informer.go:313] Waiting for caches to sync for client-ca::kube-system::extension-apiserver-authentication::client-ca-file
I1229 05:00:47.311593       1 leaderelection.go:250] attempting to acquire leader lease kube-system/my-new-scheduler...
I1229 05:00:47.312578       1 shared_informer.go:320] Caches are synced for RequestHeaderAuthRequestController
I1229 05:00:47.312641       1 shared_informer.go:320] Caches are synced for client-ca::kube-system::extension-apiserver-authentication::requestheader-client-ca-file
I1229 05:00:47.312773       1 shared_informer.go:320] Caches are synced for client-ca::kube-system::extension-apiserver-authentication::client-ca-file
I1229 05:01:05.150116       1 leaderelection.go:260] successfully acquired lease kube-system/my-new-scheduler

Scheduler Priority and Plugins

We know that pods are sorted based on the priority defined on the pods, you can read more from this page kube-scheduler.

To set a priority for a pod, you need to create a priority class first and apply it to the pod. Reference

priority-class.yaml

apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: high-priority
value: 1000000 # the higher the value, the higher the priority
globalDefault: false
description: "This priority class should be used for XYZ service pods only."

sample-pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: sample-pod
spec:
  priorityClassName: high-priority
  containers:
    - name: sample
      image: ubuntu

priorityClassName - It determines the scheduling priority of the Pod. Pods with higher priority are scheduled before Pods with lower priority.

Steps of how a pod is scheduled:

The pods with higher priority are scheduled first by performing sorting based on values (beginning of the queue).
The pod will enter the filtering phase where the scheduler will check the nodes based on the node selector and affinity rules. Also, the scheduler will check the resources (CPU, memory) of the nodes to ensure that the pods can be scheduled on the nodes.
Then the node will enter the scoring phase where the scheduler will score the nodes based on the resources. The node with the highest score will be selected to schedule the pod.
Binding the pod to the node with the highest score.

Scheduler Plugins

Reference

Actually, every steps that I mentioned got its own plugins, for example

Scheduling Queue plugins
- PrioritySort - Sort the pods based on the priority of the pods.
Filtering
- NodeResourcesFit - Identify the nodes that have enough resources to run the pod.
- NodeName - Check the pod has a specific node name mentioned in the pod spec.
- NodeUnschedulable - Check the node is unschedulable or not. You can use this command to check the node unschedulable status kubectl describe node <node-name>
Scoring
- NodeResourcesFit - Score the nodes based on the resources. Remember a single plugin can be used in multiple phases.
- ImageLocality - Score the nodes based on the container image that the pod runs. Meaning, it will select the node that has the container image cached. What if there is no nodes available? It will still place the pod on a node that doesn't have the container image cached.
Binding
- DefaultBinder - Bind the pod to the node.

Actually, you can write your own plugins to extend the scheduler functionalities. We call it as extension points. For example, you can write a plugin to check the node health on the filtering phase. Reference

Scheduling Profiles

Reference

my-new-scheduler.yaml

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
profiles:
  - schedulerName: my-new-scheduler-1
  - schedulerName: my-new-scheduler-2
  - schedulerName: my-new-scheduler-3

test-scheduler.yaml

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
profiles:
  - schedulerName: test-scheduler

Let's said we deploy separate schedulers each with a separate scheduler binary and configuration** file**. It's a lot of work required to manage these separate processes and due to separate processes, the other scheduler may schedule a pod on a node without considering the other scheduler's decision (race condition).

So, we can use scheduling profiles to configure multiple schedulers in a single process.

scheduler-config.yaml

apiVersion: kubescheduler.config.k8s.io/v1
kind: KubeSchedulerConfiguration
profiles:
  - schedulerName: default-scheduler
  - schedulerName: my-new-scheduler-1
    plugins:
      score:
        disabled: 
          - name: TaintToleration
        enabled:
          - name: CustomPlugin1
          - name: CustomPlugin2
          - name: CustomPlugin3
  - schedulerName: no-scoring-scheduler
    plugins:
      preScore:
        disabled:
        - name: '*'
      score:
        disabled:
        - name: '*'

Multiple Schedulers

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