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+---
+reviewers:
+- davidopp
+- filipg
+- piosz
+title: Guaranteed Scheduling For Critical Add-On Pods
+---
+
+{{< toc >}}
+
+## Overview
+
+In addition to Kubernetes core components like api-server, scheduler, controller-manager running on a master machine
+there are a number of add-ons which, for various reasons, must run on a regular cluster node (rather than the Kubernetes master).
+Some of these add-ons are critical to a fully functional cluster, such as Heapster, DNS, and UI.
+A cluster may stop working properly if a critical add-on is evicted (either manually or as a side effect of another operation like upgrade)
+and becomes pending (for example when the cluster is highly utilized and either there are other pending pods that schedule into the space
+vacated by the evicted critical add-on pod or the amount of resources available on the node changed for some other reason).
+
+## Rescheduler: guaranteed scheduling of critical add-ons
+**Rescheduler is deprecated as of Kubernetes 1.10 and will be removed in version 1.11 in
+accordance with the [deprecation policy](/docs/reference/deprecation-policy) for beta features.**
+
+**To avoid eviction of critical pods, you must
+[enable priorities in scheduler](/docs/concepts/configuration/pod-priority-preemption/)
+before upgrading to Kubernetes 1.10 or higher.**
+
+Rescheduler ensures that critical add-ons are always scheduled
+(assuming the cluster has enough resources to run the critical add-on pods in the absence of regular pods).
+If the scheduler determines that no node has enough free resources to run the critical add-on pod
+given the pods that are already running in the cluster
+(indicated by critical add-on pod's pod condition PodScheduled set to false, the reason set to Unschedulable)
+the rescheduler tries to free up space for the add-on by evicting some pods; then the scheduler will schedule the add-on pod.
+
+To avoid situation when another pod is scheduled into the space prepared for the critical add-on,
+the chosen node gets a temporary taint "CriticalAddonsOnly" before the eviction(s)
+(see [more details](https://git.k8s.io/community/contributors/design-proposals/scheduling/taint-toleration-dedicated.md)).
+Each critical add-on has to tolerate it,
+while the other pods shouldn't tolerate the taint. The taint is removed once the add-on is successfully scheduled.
+
+*Warning:* currently there is no guarantee which node is chosen and which pods are being killed
+in order to schedule critical pods, so if rescheduler is enabled your pods might be occasionally
+killed for this purpose. Please ensure that rescheduler is not enabled along with priorities & preemptions in default-scheduler as rescheduler is oblivious to priorities and it may evict high priority pods, instead of low priority ones.
+
+## Config
+
+Rescheduler doesn't have any user facing configuration (component config) or API.
+
+### Marking pod as critical when using Rescheduler. 
+
+To be considered critical, the pod has to run in the `kube-system` namespace (configurable via flag) and
+
+* have the `scheduler.alpha.kubernetes.io/critical-pod` annotation set to empty string, and
+* have the PodSpec's `tolerations` field set to `[{"key":"CriticalAddonsOnly", "operator":"Exists"}]`.
+
+The first one marks a pod a critical. The second one is required by Rescheduler algorithm.
+
+### Marking pod as critical when priorites are enabled.
+
+To be considered critical, the pod has to run in the `kube-system` namespace (configurable via flag) and
+
+* Have the priorityClass set as "system-cluster-critical" or "system-node-critical", the latter being the highest for entire cluster and `scheduler.alpha.kubernetes.io/critical-pod` annotation set to empty string(This will be deprecated too).