Merge pull request #26993 from damemi/dev-1.21-random-downscale

Add description of replica controller scaledown sort logic

content/en/docs/concepts/workloads/controllers/replicaset.md

@@ -310,6 +310,17 @@ assuming that the number of replicas is not also changed).
 A ReplicaSet can be easily scaled up or down by simply updating the `.spec.replicas` field. The ReplicaSet controller
 ensures that a desired number of Pods with a matching label selector are available and operational.
 
+When scaling down, the ReplicaSet controller chooses which pods to delete by sorting the available pods to
+prioritize scaling down pods based on the following general algorithm:
+ 1. Pending (and unschedulable) pods are scaled down first
+ 2. If controller.kubernetes.io/pod-deletion-cost annotation is set, then
+    the pod with the lower value will come first.
+ 3. Pods on nodes with more replicas come before pods on nodes with fewer replicas.
+ 4. If the pods' creation times differ, the pod that was created more recently
+    comes before the older pod (the creation times are bucketed on an integer log scale)
+
+If all of the above match, then selection is random.
+
 ### ReplicaSet as a Horizontal Pod Autoscaler Target
 
 A ReplicaSet can also be a target for