Merge pull request #49826 from jm-franc/kep-4951-autoscaling-configurable-tolerance

Add documentation for configurable autoscaling tolerance.

Author: k8s-ci-robot

Diff for: content/en/docs/reference/command-line-tools-reference/feature-gates/HPAConfigurableTolerance.md

@@ -0,0 +1,14 @@
+---
+title: HPAConfigurableTolerance
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: alpha
+    defaultValue: false
+    fromVersion: "1.33"
+---
+Enables setting a [tolerance threshold](/docs/tasks/run-application/horizontal-pod-autoscale.md#tolerance)
+for HorizontalPodAutoscaler metrics.

Diff for: content/en/docs/tasks/run-application/horizontal-pod-autoscale.md

@@ -143,8 +143,8 @@ is `100m`, the number of replicas will be doubled, since
 \\( { 200.0 \div 100.0 } = 2.0 \\).  
 If the current value is instead `50m`, you'll halve the number of
 replicas, since \\( { 50.0 \div 100.0 } = 0.5 \\). The control plane skips any scaling
-action if the ratio is sufficiently close to 1.0 (within a globally-configurable
-tolerance, 0.1 by default).
+action if the ratio is sufficiently close to 1.0 (within a
+[configurable tolerance](#tolerance), 0.1 by default).
 
 When a `targetAverageValue` or `targetAverageUtilization` is specified,
 the `currentMetricValue` is computed by taking the average of the given
@@ -388,9 +388,10 @@ to configure separate scale-up and scale-down behaviors.
 You specify these behaviours by setting `scaleUp` and / or `scaleDown`
 under the `behavior` field.
 
-You can specify a _stabilization window_ that prevents [flapping](#flapping)
-the replica count for a scaling target. Scaling policies also let you control the
-rate of change of replicas while scaling.
+Scaling policies let you control the rate of change of replicas while scaling.
+Also two settings can be used to prevent [flapping](#flapping): you can specify a
+_stabilization window_ for smoothing replica counts, and a tolerance to ignore
+minor metric fluctuations below a specified threshold.
 
 ### Scaling policies
 
@@ -452,6 +453,32 @@ interval. In the above example, all desired states from the past 5 minutes will
 This approximates a rolling maximum, and avoids having the scaling algorithm frequently
 remove Pods only to trigger recreating an equivalent Pod just moments later.
 
+### Tolerance {#tolerance}
+
+{{< feature-state feature_gate_name="HPAConfigurableTolerance" >}}
+
+The `tolerance` field configures a threshold for metric variations, preventing the
+autoscaler from scaling for changes below that value.
+
+This tolerance is defined as the amount of variation around the desired metric value under
+which no scaling will occur. For example, consider a HorizontalPodAutoscaler configured
+with a target memory consumption of 100MiB and a scale-up tolerance of 5%:
+
+```yaml
+behavior:
+  scaleUp:
+    tolerance: 0.05 # 5% tolerance for scale up
+```
+
+With this configuration, the HPA algorithm will only consider scaling up if the memory
+consumption is higher than 105MiB (that is: 5% above the target).
+
+If you don't set this field, the HPA applies the default cluster-wide tolerance of 10%. This
+default can be updated for both scale-up and scale-down using the
+[kube-controller-manager](/docs/reference/command-line-tools-reference/kube-controller-manager/)
+`--horizontal-pod-autoscaler-tolerance` command line argument. (You can't use the Kubernetes API
+to configure this default value.)
+
 ### Default Behavior
 
 To use the custom scaling not all fields have to be specified. Only values which need to be