Configuring the Autoscaler

Since Knative v0.2, per revision autoscalers have been replaced by a single shared autoscaler. This is, by default, the Knative Pod Autoscaler (KPA), which provides fast, request-based autoscaling capabilities out of the box.

Configuring Knative Pod Autoscaler

To modify the Knative Pod Autoscaler (KPA) configuration, you must modify a Kubernetes ConfigMap called config-autoscaler in the knative-serving namespace.

You can view the default contents of this ConfigMap using the following command.

kubectl -n knative-serving get cm config-autoscaler

Example of default ConfigMap

apiVersion: v1
kind: ConfigMap
metadata:
 name: config-autoscaler
 namespace: knative-serving
data:
 container-concurrency-target-default: 100
 container-concurrency-target-percentage: 1.0
 enable-scale-to-zero: true
 enable-vertical-pod-autoscaling: false
 max-scale-up-rate: 10
 panic-window: 6s
 scale-to-zero-grace-period: 30s
 stable-window: 60s
 tick-interval: 2s

Configuring scale to zero for KPA

To correctly configure autoscaling to zero for revisions, you must modify the following parameters in the ConfigMap.

scale-to-zero-grace-period

scale-to-zero-grace-period specifies the time an inactive revision is left running before it is scaled to zero (min: 30s).

scale-to-zero-grace-period: 30s

stable-window

When operating in a stable mode, the autoscaler operates on the average concurrency over the stable window.

stable-window: 60s

stable-window can also be configured in the Revision template as an annotation.

autoscaling.knative.dev/window: 60s

enable-scale-to-zero

Ensure that enable-scale-to-zero is set to true.

Termination period

The termination period is the time that the pod takes to shut down after the last request is finished. The termination period of the pod is equal to the sum of the values of the stable-window and scale-to-zero-grace-period parameters. In the case of this example, the termination period would be 90s.

Configuring concurrency

Concurrency for autoscaling can be configured using the following methods.

Configuring concurrent request limits

target

target defines how many concurrent requests are wanted at a given time (soft limit) and is the recommended configuration for autoscaling in Knative.

The default value for concurrency target is specified in the ConfigMap as 100.

`container-concurrency-target-default: 100`

This value can be configured by adding or modifying the autoscaling.knative.dev/target annotation value in the revision template.

autoscaling.knative.dev/target: 50

containerConcurrency

NOTE: containerConcurrency should only be used if there is a clear need to limit how many requests reach the app at a given time. Using containerConcurrency is only advised if the application needs to have an enforced constraint of concurrency.

containerConcurrency limits the amount of concurrent requests are allowed into the application at a given time (hard limit), and is configured in the revision template.

containerConcurrency: 0 | 1 | 2-N
  • A containerConcurrency value of 1 will guarantee that only one request is handled at a time by a given instance of the revision container.
  • A value of 2 or more will limit request concurrency to that value.
  • A value of 0 means the system should decide.

If there is no /target annotation, the autoscaler is configured as if /target == containerConcurrency.

Configuring scale bounds (minScale and maxScale)

The minScale and maxScale annotations can be used to configure the minimum and maximum number of pods that can serve applications. These annotations can be used to prevent cold starts or to help control computing costs.

minScale and maxScale can be configured as follows in the revision template;

spec:
  template:
    metadata:
      autoscaling.knative.dev/minScale: "2"
      autoscaling.knative.dev/maxScale: "10"

Using these annotations in the revision template will propagate this to PodAutoscaler objects. PodAutoscaler objects are mutable and can be further modified later without modifying anything else in the Knative Serving system.

edit podautoscaler <revision-name>

NOTE: These annotations apply for the full lifetime of a revision. Even when a revision is not referenced by any route, the minimal pod count specified by minScale will still be provided. Keep in mind that non-routeable revisions may be garbage collected, which enables Knative to reclaim the resources.

Default behavior

If the minScale annotation is not set, pods will scale to zero (or to 1 if enable-scale-to-zero is false per the ConfigMap mentioned above).

If the maxScale annotation is not set, there will be no upper limit for the number of pods created.

Configuring CPU-based autoscaling

NOTE: You can configure Knative autoscaling to work with either the default KPA or a CPU based metric, i.e. Horizontal Pod Autoscaler (HPA).

You can configure Knative to use CPU based autoscaling instead of the default request based metric by adding or modifying the autoscaling.knative.dev/class and autoscaling.knative.dev/metric values as annotations in the revision template.

spec:
  template:
    metadata:
      autoscaling.knative.dev/metric: concurrency
      autoscaling.knative.dev/class: hpa.autoscaling.knative.dev

Additional resources