Automatic merge from submit-queue
HPA: Consider unready pods separately
**Release note**:
``` release-note
The Horizontal Pod Autoscaler now takes the readiness of pods into account when calculating desired replicas.
```
Currently, the HPA considers unready pods the same as ready pods when
looking at their CPU and custom metric usage. However, pods frequently
use extra CPU during initialization, so we want to consider them
separately.
This commit causes the HPA to consider unready pods as having 0 CPU
usage when scaling up, and ignores them when scaling down. If, when
scaling up, factoring the unready pods as having 0 CPU would cause a
downscale instead, we simply choose not to scale. Otherwise, we simply
scale up at the reduced amount calculated by factoring the pods in at
zero CPU usage.
Similarly, if we are missing metrics for any pods, those pods will be
considered as having 0% CPU when scaling up, and 100% CPU when
scaling down. As with the unready pods calculation, this cannot change
the direction of the scale.
The effect is that unready pods cause the autoscaler to be a bit more
conservative -- large increases in CPU usage can still cause scales,
even with unready pods in the mix, but will not cause the scale factors
to be as large, in anticipation of the new pods later becoming ready and
handling load.
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