ComputeClass: Prioritization, Logic & Fallbacks

Traversal & Tie-Breaking

• Sequential: Tried top-to-bottom. Unobtainable shapes get a 5-minute cooldown. Max ~10 entries (prevents infinite loops).
• Tie-break (No Score): Top entry wins. If multiple shapes match one rule, lowest unit cost wins.
• Tie-break (priorityScore): Int 1–1000 (Higher = Preferred). If one rule has a score, all must. Max 3 rules per score. Tied rules evaluated together; lowest cost wins. (GKE 1.35.2+).
• Equal-Score Zonal Balancing (Round-Robin): Since GKE reservations are zonal, to achieve balanced scale-up across multiple zones (e.g., us-central1-a, b, and c), you can define separate priority rules for each zone and assign them the exact same priorityScore. GKE will evaluate these tied zonal rules together, performing a round-robin selection to achieve roughly equal zonal distribution of nodes. Note that this requires using specific reservation names per zone.

Fallback Patterns

Key Rules

• No repetition: Doesn't improve obtainability.
• Vary dimensions: Zone, Family, Capacity (Spot/OD), machine size (cores).
• Size obtainability (large shapes are scarce): Shapes >32 vCPU draw from thinner capacity pools and hit out.of.resources stockouts far more than ≤32-core shapes. A ComputeClass pinned to large machines only has no escape hatch → Pending. Add smaller-core fallback priorities if the workload allows it. Gate on Pod requests: node auto-creation sizes nodes to Pod requests, so a single pod requesting >32 vCPU cannot land on a smaller node — only horizontally-scalable workloads (many small pods that bin-pack) benefit. For a genuinely large single pod, vary zone/family instead, not cores.
• Always include a floor: End with high-availability OD (e.g., N4/E2) to prevent Pending.
• Stateful Gen Isolation: For PV workloads, do NOT mix hardware generations (e.g., all Gen 4 OR all Gen 2) in priorities[]. Mixing causes Hyperdisk vs PD attachment failures. Exception (GKE 1.35.3-gke.1290000+): with the built-in dynamic-rwo StorageClass (type: dynamic + use-allowed-disk-topology: "true") the autoscaler scales up only disk-compatible nodes, so it skips the incompatible-generation priority instead of attach-failing — mixing generations is then safe.
• Mixed Architectures: Mix ARM (n4a) and x86 (n4) in priorities[]. Autoscaler skips incompatible shapes based on Pod constraints. Must use multi-platform image builds.
• Spot Availability: For CPU, if OD is out, Spot usually is too. For Accelerators, Spot often has capacity when OD doesn't.