QoS Classes

QoS Classes

The Three Quality of Service Classes

From requests and limits, Kubernetes 5,150 files every Pod into a QoS class, recorded in status.qosClass:

Quality of Service classes and eviction order
Class	Rule	Under memory pressure
Guaranteed	Every container's CPU and memory limits equal its requests	Evicted last
Burstable	Some request or limit, but not Guaranteed	By usage above request
BestEffort	No requests or limits at all	Evicted first

If the kernel's OOM killer strikes before the kubelet evicts, the class still steers it, through oom_score_adj:

QoS classes and OOM scores of three PodsShell

kubectl apply -f - <<'EOF' >/dev/null
apiVersion: v1
kind: Pod
metadata: { name: guaranteed }
spec:
  containers:
  - { name: c, image: localhost:33500/booknest-api:1.4, command: [sleep, infinity],
      resources: { limits: { cpu: 250m, memory: 64Mi } } }
EOF
kubectl wait --for=condition=Ready pod/guaranteed >/dev/null
A=$(kubectl get pod -l tier=api -o jsonpath='{.items[0].metadata.name}')
for p in guaranteed $A postgres-0; do
  echo "$p $(kubectl get pod $p -o jsonpath='{.status.qosClass}')" \
    "$(kubectl exec $p -- cat /proc/1/oom_score_adj)"
done

Output

guaranteed Guaranteed -997
api-776c666c56-28p22 Burstable 999
postgres-0 BestEffort 1000

The test Pod's limits became its requests too. PostgreSQL 1,289 , with no resources, is first to go; Tuning Probes fixes that.

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