GKE ComputeClasses: Debugging

First Check: GKE Version

If fields are ignored or fail with "not supported," the control plane is likely too old.

• Verify CRD: kubectl describe crd computeclasses.cloud.google.com
• Check Versions: gcloud container clusters describe <CLUSTER> --format="value(currentMasterVersion,currentNodeVersion)"

Symptom 1: ComputeClass Config Error

Check status.conditions on the ComputeClass object via kubectl describe ComputeClass <NAME>.

• Common Error: location config with specific reservations enabled.
• Fix: Remove location.zones from the reservation priority — zones come from reservations.specific[].zones instead. Only location.zones collides; a policy-only location.locationPolicy (e.g. BALANCED) may remain.

Symptom 2: Scale-Up Failure (Pods Pending)

Check Autoscaler Visibility logs (docs).

• Log Filter: log_id("container.googleapis.com/cluster-autoscaler-visibility")
• Asset: assets/log-autoscaler-events.sh <cluster-name> (Live tail).

Symptom 3: Trapped in Pending (GPU Tolerations Missing)

• Symptom: Pod requesting a GPU ComputeClass is stuck in Pending with noScaleUp logs.

• Cause: GKE auto-taints GPU nodes (nvidia.com/gpu:NoSchedule). Scheduler refuses placement without a toleration.

• Fix: Add toleration to pod spec:

  tolerations:
  - key: "nvidia.com/gpu"
    operator: "Exists"
    effect: "NoSchedule"
  

Symptom 4: Wrong Nodes Provisioned (E2 Fallback Trap)

• Symptom: Requested specific nodes (e.g., C3 or N4), but GKE provisions default E2 nodes.
• Cause: whenUnsatisfiable: ScaleUpAnyway provisions generic E2 nodes to start the pod if preferred hardware fails.
• Fix: Set whenUnsatisfiable: DoNotScaleUp to strictly enforce hardware list.

Symptom 5: Active Migration Blocked

• Symptom: Spot capacity returned, but pods stuck on On-Demand nodes.
• Cause: Pod Disruption Budgets (PDBs) block eviction. Active migration strictly honors PDBs.
• Fix: Ensure PDBs allow at least 1 disruption. maxUnavailable: 0 blocks migration.
• Common GKE blocker — system-managed pods: Non-DaemonSet pods in system namespaces (kube-system, gke-managed-*, gmp-system) often carry tight PDBs + low replicas, so the source node cannot drain (also blocks ordinary scale-down). Check kubectl get pdb -A and the autoscaler noScaleDown reason; raise replicas to add PDB headroom or isolate them onto a separate ComputeClass.
• Note: PDBs / safe-to-evict only gate voluntary disruption; Spot preemption is involuntary and ignores both.

Symptom 6: ImageType Fragmentation Bug (Pre-1.33.5)

• Symptom: Autoscaler creates hundreds of tiny, fragmented node pools.
• Cause: Explicitly defining imageType: UBUNTU_CONTAINERD (or COS) on versions older than 1.33.5-gke.1862000 (and 1.34.1-gke.2541000).
• Fix: Upgrade cluster or temporarily remove imageType.

Symptom 7: Pods Ignoring ComputeClass

• Fixes: Ensure pod has nodeSelector: cloud.google.com/compute-class: <NAME>. Translate non-GKE node selectors — a generic/AWS-style machine-family: c4 won't match; use GKE-native cloud.google.com/machine-family: c4 (family) or node.kubernetes.io/instance-type (shape), or better, move the constraint into the ComputeClass priorities[]. Verify manual pools have correct label/taint. Check if Pod requests exceed priority bounds.

Symptom 8: "ANY" Reservation Bypasses Fallbacks

• Cause: reservations.affinity: AnyBestEffort falls back to On-Demand at GCE layer.
• Fix: Use affinity: Specific with named reservations.

Symptom 9: Disk/PV Attachment Fail

• Cause: Mixing Gen 4 VMs (Hyperdisk) and Gen 2 (PD) in the same priority list.
• Fix: Do not mix generations for workloads with attached PVs. Or (GKE 1.35.3-gke.1290000+): back the data PVs with the built-in dynamic-rwo StorageClass (type: dynamic + use-allowed-disk-topology: "true") — the autoscaler becomes disk-topology-aware and scales up only compatible nodes, so a mixed-generation priorities[] no longer attach-fails.

Symptom 10: Zonal PV Deadlock (Pending Pods)

• Symptom: StatefulSet pod is Pending because disk is in zone B but node is in zone A.
• Fix: Do not hardcode location in priorities. Use a StorageClass with volumeBindingMode: WaitForFirstConsumer so the disk provisions in the chosen node's zone — the built-in dynamic-rwo (GKE 1.35.3-gke.1290000+) already sets this plus use-allowed-disk-topology: "true".

Symptom 11: List Loops / Backoff

• Cause: >10 priorities. Unobtainable shapes enter a 5-minute cooldown. Long lists expire upper-tier cooldowns before reaching the bottom, causing an infinite loop.
• Fix: Trim list; remove redundant rules.

Symptom 12: Pods on Low-Priority Nodes

• Symptom: Pods land on existing low-priority nodes (e.g., On-Demand) instead of triggering scale-up for available high-priority nodes (e.g., Spot).
• Cause: ComputeClass controls node provisioning, not pod scheduling. K8s schedules pods on any existing node with capacity before scaling up.
• Fix:
  1. ActiveMigration: Set optimizeRulePriority: true to eventually move workloads to higher-priority nodes.
  2. PriorityClass: Use native K8s PriorityClass for pod-level preemption.
  3. Kueue: Use Kueue for complex batch/AI/ML fair-sharing and queueing.

Useful Commands

kubectl get nodes -L cloud.google.com/compute-class
kubectl get pods -A -o json | jq -r '.items[] | select(.spec.nodeSelector["cloud.google.com/compute-class"]=="<name>") | .metadata.name'
gcloud logging read 'log_id("container.googleapis.com/cluster-autoscaler-visibility")' --freshness=1h --limit=50