hvpa-controller

Weight based scaling

In this mode, deployment's scaling is divided into stages depending on number of replicas. A user can provide an array of intervals x1 to x2 and corresponding vpaWeights between and including 0 and 1. x1 and x2 are the number of replicas where that stage begins and ends respectively. In a particular stage, HVPA controller will consider VPA's recommendations according to the weights provided, and scale the deployment accordingly. The weights given to the HPA's recommendation will be 1 - vpaWeight.

According to VPA, new requests will be currentRequest + (targetRequest - currentRequest) * vpaWeight

According to HPA, new number of replicas will be:

• ceil of currentReplicas + (desiredReplicas - currentReplicas) * (1 - vpaWeight) if desired replicas is more than current replicas
• floor of currentReplicas + (desiredReplicas - currentReplicas) * (1 - vpaWeight) if desired replicas is less than current replicas

vpaWeight = 1 will result in pure vertical scaling vpaWeight = 0 will result in pure horizontal scaling

Resource request vs number of replicas curve could typically look like:
Here First stage Vpa weight = 0, second stage Vpa weight is a fraction, third stage Vpa weight = 0

resource
request
^
|
|                  ____________
|                 /|
|                /
|               /  |
|              /
|  ___________/    |
|
|-------------|----|-----------|----->
   min        x1    x2        max     #Replicas

Example of vpaWeight vs number of replicas.
Here first stage Vpa weight = 0, second stage Vpa weight = 0.4, third stage Vpa weight = 0

vpaWeight
   ^
   |
0.4|           |-------|
   |           |       |
  0|   ________|       |____________
   |------------------------------------->
               x1     x2               #Replicas

Pros

• Works even if HPA and VPA act on different metrices
• Gives better control to user on scaling of apps

Cons

• Need to define behavior when:
  • current number of replicas is x2, the deployment has not yet scaled up to maxAllowed, and user has chosen to scale only horizontally after x2. In this case, the deployment cannot scale up vertically anymore if the load increases.
  • current number of replicas is x1, the deployment has not yet scaled down to minAllowed, and user has chosen to scale only horizontally for lower values than x1. In this case, the deployment cannot scale down vertically anymore if the load decreases.
• Will need both VPA and HPA to be deployed in a recommendation-only mode.

Mitigation

• x1 and x2 will be optional, If either or both of them are not provided, vertical scaling will be done until minAllowed and/or maxAllowed correspondingly.
• Once replica count reaches maxReplicas, and VPA still recommends higher resource requirements, then vertical scaling will be done unconditionally

Spec

Here is the spec for HVPA controller

Example 1:

apiVersion: autoscaling.k8s.io/v1alpha1
kind: Hvpa
metadata:
  name: hvpa-sample
spec:
  scaleUpStabilizationWindow: "2m"
  scaleDownStabilizationWindow : "3m"
  minCpuChange:
    value: "200m"
    percentage: 70
  minMemChange:
    value: "200M"
    percentage: 80
  hpaTemplate:
    maxReplicas: 10
    minReplicas: 1
    metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 50
    - type: Resource
      resource:
        name: memory
        target:
          type: Utilization
          averageUtilization: 50
  vpaTemplate:
    resourcePolicy:
      containerPolicies:
      - MinAllowed:
          memory: 400Mi
        containerName: resource-consumer
        maxAllowed:
          memory: 3000Mi
  weightBasedScalingIntervals:
    - vpaWeight: 0
      startReplicaCount: 1
      lastReplicaCount: 2
    - vpaWeight: 0.6
      startReplicaCount: 3
      lastReplicaCount: 7
    - vpaWeight: 0
      startReplicaCount: 7
      lastReplicaCount: 10
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: resource-consumer

resource
request
^
|
|                  _________
|                 /|
|                /
|               /  |
|              /
|  ___________/    |
|
|-------------|----|--------|----->
   1          3    7        10     #Replicas

Example 2:

apiVersion: autoscaling.k8s.io/v1alpha1
kind: Hvpa
metadata:
  name: hvpa-sample
spec:
  weightBasedScalingIntervals:
    - vpaWeight: 1
      startReplicaCount: 1
      lastReplicaCount: 1
    - vpaWeight: 0.6
      startReplicaCount: 2
      lastReplicaCount: 4
    - vpaWeight: 0
      startReplicaCount: 5
      lastReplicaCount: 10
  hpaTemplate:
    .
    .
    .

resource
request
^
|       ______
|      /
|     /  
|    /
|   /  
|  |
|  |  
|  |
|
|--|----|-----|------->
   1   4       10     #Replicas

Example 3: After maxReplicas, even if weight is not mentioned, there is only vertical scaling because HPA is limited by maxReplicas

apiVersion: autoscaling.k8s.io/v1alpha1
kind: Hvpa
metadata:
  name: hvpa-sample
spec:
  weightBasedScalingIntervals:
    - vpaWeight: 0
      startReplicaCount: 1
      lastReplicaCount: 3
    - vpaWeight: 0.6
      startReplicaCount: 4
      lastReplicaCount: 10
  hpaTemplate:
    minReplicas: 1
    maxReplicas: 10
    .
    .
    .

resource
request
^
|
|           |
|           |
|           .     
|          /
|         /
|        / 
|       /
|  ____/ 
|
|--|---|----|------>
   1   3    10     #Replicas