5.9 : Pod Priority & Preemption

https://drive.google.com/file/d/1lnSO1RKmS6n6JtSRh9jsYmmg2EbxfWrH/view?usp=sharing

🔍 Part 1: PriorityClass Definition

# mission-critical-priority-class.yaml
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
meta
  name: mission-critical-apps
value: 1000000                 # ← Higher = more important
preemptionPolicy: Never        # ← Key setting!
globalDefault: false
description: "For mission-critical apps"

🔑 Key Fields:

value: Priority score (1 to 1,000,000,000). Higher = scheduled first.

preemptionPolicy:

PreemptLowerPriority (default): Evict lower-priority Pods to make room

Never: Don’t evict — just wait in queue (your example)

globalDefault: false: Don’t apply to Pods without priorityClassName

💡 Default Critical Classes (built into Kubernetes):

system-cluster-critical → value: 2000000000 (CoreDNS, CNI, Metrics Server)

system-node-critical → value: 2000001000 (etcd, kubelet, API server)

🔍 Part 2: Pod Using PriorityClass

# mission-critical-pod.yaml
apiVersion: v1
kind: Pod
meta
  name: topgun
  labels:
    app: topgun
    env: prod
spec:
  containers:
  - name: boxone
    image: nginx
    imagePullPolicy: IfNotPresent
  priorityClassName: mission-critical-apps  # ← Links to PriorityClass

✅ This Pod will:

Be placed ahead of lower-priority Pods in scheduling queue

NOT preempt others (because preemptionPolicy: Never)

Wait indefinitely if no resources are available

📌 How Priority & Preemption Work

Scenario	Behavior
Resources available	High-priority Pod schedules immediately
No resources, `preemptionPolicy: PreemptLowerPriority`	Scheduler evicts lowest-priority Pods to free space
No resources, `preemptionPolicy: Never`	Pod waits in queue — no eviction
All Pods have same priority	First-come, first-served

⚠️ Preemption is a last resort — scheduler tries to find space without eviction first.

🧪 k3s Lab: Priority & Preemption in Action

✅ Assumption: Your k3s cluster has limited CPU/memory (or we’ll simulate scarcity).

🔧 Step 1: Create PriorityClass

# Save as mission-critical-priority-class.yaml
cat <<EOF | kubectl apply -f -
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
meta
  name: mission-critical-apps
value: 1000000
preemptionPolicy: PreemptLowerPriority  # ← Changed to test preemption!
globalDefault: false
description: "For mission-critical apps"
EOF

💡 We set preemptionPolicy: PreemptLowerPriority to see eviction in action.

🔍 Part 1: PriorityClass Definition

🔍 Part 2: Pod Using PriorityClass

📌 How Priority & Preemption Work

🧪 k3s Lab: Priority & Preemption in Action

🔧 Step 1: Create PriorityClass

🔧 Step 2: Deploy Low-Priority "Filler" Pods