HPA not triggering when Istio sidecar injected

Networking
1

We have Kubernetes HPA enabled for our pods as such

spec:
  maxReplicas: 4
  minReplicas: 1
  targetCPUUtilizationPercentage: 80

and pods resource requests configured as such:

app:
    resources:
      requests:
        cpu: 75m

Using this configuration we would expect the HPA to scale a new pod at 60m ( 80% of 75m). However, we found that with the Istio sidecar injected scaling does not happen when expected at 60m.
I can see in the pod both containers have resource requests as such:

app:
    resources:
      requests:
        cpu: 75m
istio-proxy
    resources:
      requests:
        istio-proxy: 100m

I understand that k8s calculates the HPA scale out differently when you have multiple containers in a pod.

At what point will HPA scale a new pod with Istio sidecar injected, and what should the targetCPUUtilizationPercentage be adjusted to to reach the same effect as prior to sidecar injection

2

You need to use custom metrics and probably want to scale on the CPU of both containers.

3

Why would you need custom metrics? You can still use the standard metrics cant you?

4

Looks like hpa is based on all containers: https://stackoverflow.com/questions/48172151/kubernetes-pod-cpu-usage-calculation-method-for-hpa. So you will need to take that into account when using hpa