Merge 35b9ab3f72 into a9292351c3
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Jérémy S
GitHub
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# AEP-8459: MemoryPerCPU
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<!-- toc -->
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- [Summary](#summary)
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- [Motivation](#motivation)
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- [Goals](#goals)
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- [Non-Goals](#non-goals)
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- [Proposal](#proposal)
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- [Design Details](#design-details)
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- [API Changes](#api-changes)
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- [Behavior](#behavior)
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- [Feature Enablement and Rollback](#feature-enablement-and-rollback)
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- [How can this feature be enabled / disabled in a live cluster?](#how-can-this-feature-be-enabled--disabled-in-a-live-cluster)
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- [Kubernetes Version Compatibility](#kubernetes-version-compatibility)
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- [Validation](#validation)
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- [Test Plan](#test-plan)
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- [Implementation History](#implementation-history)
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- [Future Work](#future-work)
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- [Alternatives](#alternatives)
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<!-- /toc -->
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## Summary
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This AEP proposes a new feature to allow enforcing a fixed memory-per-CPU ratio (`memoryPerCPU`) in Vertical Pod Autoscaler (VPA) recommendations.
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The feature is controlled by a new alpha feature gate `MemoryPerCPURatio` (default off).
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## Motivation
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Many workloads scale their memory requirements proportionally to CPU, but today VPA generates independent CPU and memory recommendations. This can lead to skewed configurations — for example, too much memory for a small CPU allocation, or too little memory for a high CPU allocation.
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The `memoryPerCPU` field addresses this by enforcing a predictable CPU-to-memory ratio in recommendations. This reduces the risk of misconfiguration, ensures consistency, and simplifies tuning for workloads where CPU and memory usage are tightly coupled.
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This feature is particularly useful in environments where services are billed primarily on CPU with a fixed CPU-to-memory ratio. In such cases, it allows VPA to be used for automatic vertical scaling while preserving the existing billing model and guarantees to customers.
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### Goals
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* Allow users to specify a `memoryPerCPU` ratio in `VerticalPodAutoscaler` objects.
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* Ensure VPA recommendations respect the ratio across Target, LowerBound, UpperBound, and UncappedTarget.
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* Provide a feature gate to enable/disable the feature cluster-wide.
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### Non-Goals
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* Redesign of the VPA recommender algorithm beyond enforcing the ratio.
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* Supporting multiple ratio policies per container (only one `memoryPerCPU` is supported).
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* Retroactive migration of existing VPAs without explicit user opt-in.
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## Proposal
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Extend `ContainerResourcePolicy` with a new optional field:
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```yaml
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apiVersion: autoscaling.k8s.io/v1
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kind: VerticalPodAutoscaler
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metadata:
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name: my-app
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spec:
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resourcePolicy:
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containerPolicies:
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- containerName: app
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minAllowed:
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cpu: 1
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memory: 4Gi
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maxAllowed:
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cpu: 4
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memory: 16Gi
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controlledResources: ["cpu", "memory"]
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controlledValues: RequestsAndLimits
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memoryPerCPU: "4Gi"
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```
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When enabled, VPA will adjust CPU or memory recommendations to maintain:
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```
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memory_bytes = cpu_cores * memoryPerCPU
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```
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## Design Details
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### API Changes
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* New field `memoryPerCPU` (`resource.Quantity`) in `ContainerResourcePolicy`.
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* Feature gate: `MemoryPerCPURatio` (alpha, default off).
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### Behavior
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* If both CPU and memory are controlled, VPA enforces the ratio.
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* Applies to Target, LowerBound, UpperBound, and UncappedTarget.
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* Ratio enforcement is strict:
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* If the memory recommendation would exceed `cpu * memoryPerCPU`, then **CPU is increased** to satisfy the ratio.
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* If the CPU recommendation would exceed `memory / memoryPerCPU`, then **memory is increased** to satisfy the ratio.
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* If ratio cannot be applied (e.g., missing CPU), fallback to standard recommendations.
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* With the `MemoryPerCPURatio` feature gate disabled, the `memoryPerCPU` field is ignored and recommendations fall back to standard VPA behavior.
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> [!IMPORTANT]
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> The enforced ratio values will be capped by
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> [`--container-recommendation-max-allowed-cpu`](https://github.com/kubernetes/autoscaler/blob/4d294562e505431d518a81e8833accc0ec99c9b8/vertical-pod-autoscaler/pkg/recommender/main.go#L122)
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> and
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> [`--container-recommendation-max-allowed-memory`](https://github.com/kubernetes/autoscaler/blob/4d294562e505431d518a81e8833accc0ec99c9b8/vertical-pod-autoscaler/pkg/recommender/main.go#L123)
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> flag values, if set.
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#### Examples
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* Example 1: `memoryPerCPU = 4Gi`
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* Baseline recommendation: 1 CPU, 8Gi memory
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* UncappedTarget (ratio enforced): 2 CPUs, 8Gi
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* Target (after policy/caps): 2 CPUs, 8Gi
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* Example 2: `memoryPerCPU = 4Gi`
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* Baseline recommendation: 2 CPUs, 4Gi memory
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* UncappedTarget (ratio enforced): 2 CPUs, 8Gi
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* Target (after policy/caps): 2 CPUs, 8Gi
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* Example 3: `memoryPerCPU = 4Gi`, with `--container-recommendation-max-allowed-memory=7Gi` or with `maxAllowed.memory=6Gi` set in VPA object
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* Baseline recommendation: 2 CPUs, 4Gi memory
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* UncappedTarget (ratio enforced): 2 CPUs, 8Gi
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* Target (capped): 2 CPUs, 7Gi ← memory capped by max-allowed-memory; ratio not fully satisfied
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### Feature Enablement and Rollback
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#### How can this feature be enabled / disabled in a live cluster?
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* Feature gate name: `MemoryPerCPURatio`
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* Default: Off (Alpha)
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* Components depending on the feature gate:
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* admission-controller
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* recommender
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**When enabled**:
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* VPA honors `memoryPerCPU` in recommendations.
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**When disabled**:
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* `memoryPerCPU` is ignored.
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* Recommendations behave as before.
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### Kubernetes Version Compatibility
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The `memoryPerCPU` feature requires VPA version 1.5.0 or higher. The feature is being introduced as alpha and will follow the standard Kubernetes feature gate graduation process:
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- Alpha: v1.5.0 (default off)
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- Beta: TBD (default on)
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- GA: TBD (default on)
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### Validation
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* `memoryPerCPU` must be > 0.
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* Value must be a valid `resource.Quantity` (e.g., `512Mi`, `4Gi`).
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### Test Plan
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* Unit tests covering:
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- ensuring ratio enforcement logic,
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- ensuring that when the feature gate is on or off the values and validation are applied accordingly.
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* E2E tests comparing behavior with different configurations.
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## Implementation History
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* 2025-08-19: Initial proposal
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## Future Work
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## Alternatives
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