Additional autoscaling configuration for Knative Pod Autoscaler

The following settings are specific to the Knative Pod Autoscaler (KPA).

Modes

The KPA acts on metrics (concurrency or rps) aggregated over time-based windows.

These windows define the amount of historical data that the Autoscaler takes into account, and are used to smooth the data over the specified amount of time. The shorter these windows are, the more quickly the Autoscaler will react.

The KPA's implementation has two modes: stable and panic.

Stable mode is used for general operation, while panic mode by default has a much shorter window, and will be used to quickly scale a revision up if a burst of traffic arrives.

NOTE: When using panic mode, the revision will not scale down to avoid churn. The Autoscaler will leave panic mode if there has been no reason to react quickly during the stable window's timeframe.

Stable window

  • Global key: stable-window
  • Per-revision annotation key: autoscaling.knative.dev/window
  • Possible values: Duration, 6s <= value <= 1h
  • Default: 60s

NOTE: During scale down, the last replica will only be removed after there has not been any traffic to the revision for the entire duration of the stable window.

Example: 

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-go
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/window: "40s"
    spec:
      containers:
        - image: gcr.io/knative-samples/helloworld-go

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 stable-window: "40s"

apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      stable-window: "40s"

Panic window

The panic window is defined as a percentage of the stable window to assure that both are relative to each other in a working way.

This value indicates how the window over which historical data is evaluated will shrink upon entering panic mode. For example, a value of 10.0 means that in panic mode the window will be 10% of the stable window size.

  • Global key: panic-window-percentage
  • Per-revision annotation key: autoscaling.knative.dev/panicWindowPercentage
  • Possible values: float, 1.0 <= value <= 100.0
  • Default: 10.0

Example: 

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-go
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/panicWindowPercentage: "20.0"
    spec:
      containers:
        - image: gcr.io/knative-samples/helloworld-go

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 panic-window-percentage: "20.0"

apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      panic-window-percentage: "20.0"

Panic mode threshold

This threshold defines when the Autoscaler will move from stable mode into panic mode.

This value is a percentage of the traffic that the current amount of replicas can handle.

NOTE: A value of 100.0 (100 percent) means that the Autoscaler is always in panic mode, therefore the minimum value should be higher than 100.0.

The default setting of 200.0 means that panic mode will be start if traffic is twice as high as the current replica population can handle.

  • Global key: panic-threshold-percentage
  • Per-revision annotation key: autoscaling.knative.dev/panicThresholdPercentage
  • Possible values: float, 110.0 <= value <= 1000.0
  • Default: 200.0

Example: 

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-go
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/panicThresholdPercentage: "150.0"
    spec:
      containers:
        - image: gcr.io/knative-samples/helloworld-go

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 panic-threshold-percentage: "150.0"

apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      panic-threshold-percentage: "150.0"

Scale rates

These settings control by how much the replica population can scale up or down in a single evaluation cycle.

A minimal change of one replica in each direction is always permitted, so the Autoscaler can scale to +/- 1 replica at any time, regardless of the scale rates set.

Scale up rate

This setting determines the maximum ratio of desired to existing pods. For example, with a value of 2.0, the revision can only scale from N to 2*N pods in one evaluation cycle.

  • Global key: max-scale-up-rate
  • Per-revision annotation key: n/a
  • Possible values: float
  • Default: 1000.0

Example: 

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 max-scale-up-rate: "500.0"

apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      max-scale-up-rate: "500.0"

Scale down rate

This setting determines the maximum ratio of existing to desired pods. For example, with a value of 2.0, the revision can only scale from N to N/2 pods in one evaluation cycle.

  • Global key: max-scale-down-rate
  • Per-revision annotation key: n/a
  • Possible values: float
  • Default: 2.0

Example: 

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 max-scale-down-rate: "4.0"

apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      max-scale-down-rate: "4.0"