In IBM PowerVM, uncapped partitions improve utilization and flexibility—but they also make CPU fairness conditional rather than absolute.

🚀 Core Idea

• Capped LPAR → fixed CPU limit (strict fairness)
• Uncapped LPAR → can borrow unused CPU from the shared pool

👉 Fairness shifts from “equal limits” → “guaranteed minimum + weighted sharing of excess”

⚙️ How Fairness Works with Uncapped Partitions

1. Entitlement-Based Fairness (Baseline)

• Every LPAR gets its entitled capacity first

👉 This ensures:

• No LPAR is starved
• Minimum fairness is always preserved

2. Weight-Based Fairness (Beyond Entitlement)

• Extra CPU is distributed using uncapped weights

👉 Example:

• LPAR A (weight 200)
• LPAR B (weight 100)

➡️ A gets ~2× more of the spare CPU than B

3. Opportunistic Behavior

• Uncapped LPARs can temporarily consume:
  • Idle CPU from others

👉 Result:

• High efficiency
• But fairness becomes dynamic, not fixed

📊 Impact on Fairness

✅ Positive Effects

Better Resource Utilization

• Idle CPU doesn’t go to waste
• Active workloads scale up dynamically

Proportional Fairness

• CPU distributed according to:
  • Entitlement
  • Weight

⚠️ Negative Effects

Relative Fairness (Not Absolute)

• LPARs may not get equal CPU even if similar
• Depends on:
  • Weights
  • Activity of other LPARs

Potential Dominance by High-Weight LPARs

• High-weight partitions can consume most spare CPU

👉 Lower-weight LPARs:

• May see reduced performance during contention

Performance Variability (Jitter)

• CPU availability fluctuates based on:
  • System load
  • Other LPAR activity

👉 Impacts:

• Latency-sensitive workloads

🔄 Behavior Under Different Conditions

⚡ Example Scenario

👉 If C is idle:

• A gets the largest share of extra CPU
• B gets some
• Fairness = proportional, not equal

🔗 Interaction with I/O

• If high-weight LPAR dominates CPU:
  • Others may experience:
    • Increased I/O latency
• Especially important for:
  • Virtual I/O Server (VIOS)

👉 VIOS must have:

• Sufficient entitlement to maintain fairness system-wide

🧠 Key Insight

Uncapped partitions redefine fairness as:

“Guaranteed minimum for all + proportional sharing of what remains”

—not equal distribution.

🎯 When to Use Uncapped Mode

Best for:

• Mixed workloads
• Cloud / consolidation environments
• Applications that can scale with CPU availability

Use Caution for:

• Real-time systems
• Strict SLA workloads
• Latency-sensitive databases

🔑 Final Takeaway

• Uncapped partitions improve efficiency but make fairness dynamic
• True fairness depends on:
  • Entitlements (guarantee)
  • Weights (priority)
  • System load (contention level)