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Descheduler for Kubernetes

Scheduling in Kubernetes is the process of binding pending pods to nodes, and is performed by a component of Kubernetes called kube-scheduler. The scheduler's decisions, whether or where a pod can or can not be scheduled, are guided by its configurable policy which comprises of set of rules, called predicates and priorities. The scheduler's decisions are influenced by its view of a Kubernetes cluster at that point of time when a new pod appears for scheduling. As Kubernetes clusters are very dynamic and their state changes over time, there may be desire to move already running pods to some other nodes for various reasons:

• Some nodes are under or over utilized.
• The original scheduling decision does not hold true any more, as taints or labels are added to or removed from nodes, pod/node affinity requirements are not satisfied any more.
• Some nodes failed and their pods moved to other nodes.
• New nodes are added to clusters.

Consequently, there might be several pods scheduled on less desired nodes in a cluster. Descheduler, based on its policy, finds pods that can be moved and evicts them. Please note, in current implementation, descheduler does not schedule replacement of evicted pods but relies on the default scheduler for that.

Table of Contents

• Quick Start
  • Run As A Job
  • Run As A CronJob
  • Run As A Deployment
  • Install Using Helm
  • Install Using Kustomize
• User Guide
• Policy and Strategies
  • RemoveDuplicates
  • LowNodeUtilization
  • HighNodeUtilization
  • RemovePodsViolatingInterPodAntiAffinity
  • RemovePodsViolatingNodeAffinity
  • RemovePodsViolatingNodeTaints
  • RemovePodsViolatingTopologySpreadConstraint
  • RemovePodsHavingTooManyRestarts
  • PodLifeTime
  • RemoveFailedPods
• Filter Pods
  • Namespace filtering
  • Priority filtering
  • Label filtering
  • Node Fit filtering
• Pod Evictions
  • Pod Disruption Budget (PDB)
• High Availability
  • Configure HA Mode
• Metrics
• Compatibility Matrix
• Getting Involved and Contributing
  • Communicating With Contributors
• Roadmap
  • Code of conduct

Quick Start

The descheduler can be run as a Job, CronJob, or Deployment inside of a k8s cluster. It has the advantage of being able to be run multiple times without needing user intervention. The descheduler pod is run as a critical pod in the kube-system namespace to avoid being evicted by itself or by the kubelet.

Run As A Job

kubectl create -f kubernetes/base/rbac.yaml
kubectl create -f kubernetes/base/configmap.yaml
kubectl create -f kubernetes/job/job.yaml

Run As A CronJob

kubectl create -f kubernetes/base/rbac.yaml
kubectl create -f kubernetes/base/configmap.yaml
kubectl create -f kubernetes/cronjob/cronjob.yaml

Run As A Deployment

kubectl create -f kubernetes/base/rbac.yaml
kubectl create -f kubernetes/base/configmap.yaml
kubectl create -f kubernetes/deployment/deployment.yaml

Install Using Helm

Starting with release v0.18.0 there is an official helm chart that can be used to install the descheduler. See the helm chart README for detailed instructions.

The descheduler helm chart is also listed on the artifact hub.

Install Using Kustomize

You can use kustomize to install descheduler. See the resources | Kustomize for detailed instructions.

Run As A Job

kustomize build 'github.com/kubernetes-sigs/descheduler/kubernetes/job?ref=v0.24.1' | kubectl apply -f -

Run As A CronJob

kustomize build 'github.com/kubernetes-sigs/descheduler/kubernetes/cronjob?ref=v0.24.1' | kubectl apply -f -

Run As A Deployment

kustomize build 'github.com/kubernetes-sigs/descheduler/kubernetes/deployment?ref=v0.24.1' | kubectl apply -f -

User Guide

See the user guide in the /docs directory.

Policy and Strategies

Descheduler's policy is configurable and includes strategies that can be enabled or disabled. By default, all strategies are enabled.

The policy includes a common configuration that applies to all the strategies:

As part of the policy, the parameters associated with each strategy can be configured. See each strategy for details on available parameters.

Policy:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
nodeSelector: prod=dev
evictFailedBarePods: false
evictLocalStoragePods: true
evictSystemCriticalPods: true
maxNoOfPodsToEvictPerNode: 40
ignorePvcPods: false
strategies:
  ...

The following diagram provides a visualization of most of the strategies to help categorize how strategies fit together.

RemoveDuplicates

This strategy makes sure that there is only one pod associated with a ReplicaSet (RS), ReplicationController (RC), StatefulSet, or Job running on the same node. If there are more, those duplicate pods are evicted for better spreading of pods in a cluster. This issue could happen if some nodes went down due to whatever reasons, and pods on them were moved to other nodes leading to more than one pod associated with a RS or RC, for example, running on the same node. Once the failed nodes are ready again, this strategy could be enabled to evict those duplicate pods.

It provides one optional parameter, excludeOwnerKinds, which is a list of OwnerRef Kinds. If a pod has any of these Kinds listed as an OwnerRef, that pod will not be considered for eviction. Note that pods created by Deployments are considered for eviction by this strategy. The excludeOwnerKinds parameter should include ReplicaSet to have pods created by Deployments excluded.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemoveDuplicates":
     enabled: true
     params:
       removeDuplicates:
         excludeOwnerKinds:
         - "ReplicaSet"

LowNodeUtilization

This strategy finds nodes that are under utilized and evicts pods, if possible, from other nodes in the hope that recreation of evicted pods will be scheduled on these underutilized nodes. The parameters of this strategy are configured under nodeResourceUtilizationThresholds.

The under utilization of nodes is determined by a configurable threshold thresholds. The threshold thresholds can be configured for cpu, memory, number of pods, and extended resources in terms of percentage (the percentage is calculated as the current resources requested on the node vs total allocatable. For pods, this means the number of pods on the node as a fraction of the pod capacity set for that node).

If a node's usage is below threshold for all (cpu, memory, number of pods and extended resources), the node is considered underutilized. Currently, pods request resource requirements are considered for computing node resource utilization.

There is another configurable threshold, targetThresholds, that is used to compute those potential nodes from where pods could be evicted. If a node's usage is above targetThreshold for any (cpu, memory, number of pods, or extended resources), the node is considered over utilized. Any node between the thresholds, thresholds and targetThresholds is considered appropriately utilized and is not considered for eviction. The threshold, targetThresholds, can be configured for cpu, memory, and number of pods too in terms of percentage.

These thresholds, thresholds and targetThresholds, could be tuned as per your cluster requirements. Note that this strategy evicts pods from overutilized nodes (those with usage above targetThresholds) to underutilized nodes (those with usage below thresholds), it will abort if any number of underutilized nodes or overutilized nodes is zero.

Additionally, the strategy accepts a useDeviationThresholds parameter. If that parameter is set to true, the thresholds are considered as percentage deviations from mean resource usage. thresholds will be deducted from the mean among all nodes and targetThresholds will be added to the mean. A resource consumption above (resp. below) this window is considered as overutilization (resp. underutilization).

NOTE: Node resource consumption is determined by the requests and limits of pods, not actual usage. This approach is chosen in order to maintain consistency with the kube-scheduler, which follows the same design for scheduling pods onto nodes. This means that resource usage as reported by Kubelet (or commands like kubectl top) may differ from the calculated consumption, due to these components reporting actual usage metrics. Implementing metrics-based descheduling is currently TODO for the project.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "LowNodeUtilization":
     enabled: true
     params:
       nodeResourceUtilizationThresholds:
         thresholds:
           "cpu" : 20
           "memory": 20
           "pods": 20
         targetThresholds:
           "cpu" : 50
           "memory": 50
           "pods": 50

Policy should pass the following validation checks:

• Three basic native types of resources are supported: cpu, memory and pods. If any of these resource types is not specified, all its thresholds default to 100% to avoid nodes going from underutilized to overutilized.
• Extended resources are supported. For example, resource type nvidia.com/gpu is specified for GPU node utilization. Extended resources are optional, and will not be used to compute node's usage if it's not specified in thresholds and targetThresholds explicitly.
• thresholds or targetThresholds can not be nil and they must configure exactly the same types of resources.
• The valid range of the resource's percentage value is [0, 100]
• Percentage value of thresholds can not be greater than targetThresholds for the same resource.

There is another parameter associated with the LowNodeUtilization strategy, called numberOfNodes. This parameter can be configured to activate the strategy only when the number of under utilized nodes are above the configured value. This could be helpful in large clusters where a few nodes could go under utilized frequently or for a short period of time. By default, numberOfNodes is set to zero.

HighNodeUtilization

This strategy finds nodes that are under utilized and evicts pods from the nodes in the hope that these pods will be scheduled compactly into fewer nodes. Used in conjunction with node auto-scaling, this strategy is intended to help trigger down scaling of under utilized nodes. This strategy must be used with the scheduler scoring strategy MostAllocated. The parameters of this strategy are configured under nodeResourceUtilizationThresholds.

The under utilization of nodes is determined by a configurable threshold thresholds. The threshold thresholds can be configured for cpu, memory, number of pods, and extended resources in terms of percentage. The percentage is calculated as the current resources requested on the node vs total allocatable. For pods, this means the number of pods on the node as a fraction of the pod capacity set for that node.

If a node's usage is below threshold for all (cpu, memory, number of pods and extended resources), the node is considered underutilized. Currently, pods request resource requirements are considered for computing node resource utilization. Any node above thresholds is considered appropriately utilized and is not considered for eviction.

The thresholds param could be tuned as per your cluster requirements. Note that this strategy evicts pods from underutilized nodes (those with usage below thresholds) so that they can be recreated in appropriately utilized nodes. The strategy will abort if any number of underutilized nodes or appropriately utilized nodes is zero.

NOTE: Node resource consumption is determined by the requests and limits of pods, not actual usage. This approach is chosen in order to maintain consistency with the kube-scheduler, which follows the same design for scheduling pods onto nodes. This means that resource usage as reported by Kubelet (or commands like kubectl top) may differ from the calculated consumption, due to these components reporting actual usage metrics. Implementing metrics-based descheduling is currently TODO for the project.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "HighNodeUtilization":
     enabled: true
     params:
       nodeResourceUtilizationThresholds:
         thresholds:
           "cpu" : 20
           "memory": 20
           "pods": 20

Policy should pass the following validation checks:

• Three basic native types of resources are supported: cpu, memory and pods. If any of these resource types is not specified, all its thresholds default to 100%.
• Extended resources are supported. For example, resource type nvidia.com/gpu is specified for GPU node utilization. Extended resources are optional, and will not be used to compute node's usage if it's not specified in thresholds explicitly.
• thresholds can not be nil.
• The valid range of the resource's percentage value is [0, 100]

There is another parameter associated with the HighNodeUtilization strategy, called numberOfNodes. This parameter can be configured to activate the strategy only when the number of under utilized nodes is above the configured value. This could be helpful in large clusters where a few nodes could go under utilized frequently or for a short period of time. By default, numberOfNodes is set to zero.

RemovePodsViolatingInterPodAntiAffinity

This strategy makes sure that pods violating interpod anti-affinity are removed from nodes. For example, if there is podA on a node and podB and podC (running on the same node) have anti-affinity rules which prohibit them to run on the same node, then podA will be evicted from the node so that podB and podC could run. This issue could happen, when the anti-affinity rules for podB and podC are created when they are already running on node.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsViolatingInterPodAntiAffinity":
     enabled: true

RemovePodsViolatingNodeAffinity

This strategy makes sure all pods violating node affinity are eventually removed from nodes. Node affinity rules allow a pod to specify requiredDuringSchedulingIgnoredDuringExecution type, which tells the scheduler to respect node affinity when scheduling the pod but kubelet to ignore in case node changes over time and no longer respects the affinity. When enabled, the strategy serves as a temporary implementation of requiredDuringSchedulingRequiredDuringExecution and evicts pod for kubelet that no longer respects node affinity.

For example, there is podA scheduled on nodeA which satisfies the node affinity rule requiredDuringSchedulingIgnoredDuringExecution at the time of scheduling. Over time nodeA stops to satisfy the rule. When the strategy gets executed and there is another node available that satisfies the node affinity rule, podA gets evicted from nodeA.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsViolatingNodeAffinity":
    enabled: true
    params:
      nodeAffinityType:
      - "requiredDuringSchedulingIgnoredDuringExecution"

RemovePodsViolatingNodeTaints

This strategy makes sure that pods violating NoSchedule taints on nodes are removed. For example there is a pod "podA" with a toleration to tolerate a taint key=value:NoSchedule scheduled and running on the tainted node. If the node's taint is subsequently updated/removed, taint is no longer satisfied by its pods' tolerations and will be evicted.

Node taints can be excluded from consideration by specifying a list of excludedTaints. If a node taint key or key=value matches an excludedTaints entry, the taint will be ignored.

For example, excludedTaints entry "dedicated" would match all taints with key "dedicated", regardless of value. excludedTaints entry "dedicated=special-user" would match taints with key "dedicated" and value "special-user".

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsViolatingNodeTaints":
    enabled: true
    params:
      excludedTaints:
      - dedicated=special-user # exclude taints with key "dedicated" and value "special-user"
      - reserved # exclude all taints with key "reserved"

RemovePodsViolatingTopologySpreadConstraint

This strategy makes sure that pods violating topology spread constraints are evicted from nodes. Specifically, it tries to evict the minimum number of pods required to balance topology domains to within each constraint's maxSkew. This strategy requires k8s version 1.18 at a minimum.

By default, this strategy only deals with hard constraints, setting parameter includeSoftConstraints to true will include soft constraints.

Strategy parameter labelSelector is not utilized when balancing topology domains and is only applied during eviction to determine if the pod can be evicted.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsViolatingTopologySpreadConstraint":
     enabled: true
     params:
       includeSoftConstraints: false

RemovePodsHavingTooManyRestarts

This strategy makes sure that pods having too many restarts are removed from nodes. For example a pod with EBS/PD that can't get the volume/disk attached to the instance, then the pod should be re-scheduled to other nodes. Its parameters include podRestartThreshold, which is the number of restarts (summed over all eligible containers) at which a pod should be evicted, and includingInitContainers, which determines whether init container restarts should be factored into that calculation.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsHavingTooManyRestarts":
     enabled: true
     params:
       podsHavingTooManyRestarts:
         podRestartThreshold: 100
         includingInitContainers: true

PodLifeTime

This strategy evicts pods that are older than maxPodLifeTimeSeconds.

You can also specify states parameter to only evict pods matching the following conditions:

• Pod Phase status of: Running, Pending
• Container State Waiting condition of: PodInitializing, ContainerCreating

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "PodLifeTime":
     enabled: true
     params:
       podLifeTime:
         maxPodLifeTimeSeconds: 86400
         states:
         - "Pending"
         - "PodInitializing"

RemoveFailedPods

This strategy evicts pods that are in failed status phase. You can provide an optional parameter to filter by failed reasons. reasons can be expanded to include reasons of InitContainers as well by setting the optional parameter includingInitContainers to true. You can specify an optional parameter minPodLifetimeSeconds to evict pods that are older than specified seconds. Lastly, you can specify the optional parameter excludeOwnerKinds and if a pod has any of these Kinds listed as an OwnerRef, that pod will not be considered for eviction.

Parameters:

Example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemoveFailedPods":
     enabled: true
     params:
       failedPods:
         reasons:
         - "NodeAffinity"
         includingInitContainers: true
         excludeOwnerKinds:
         - "Job"
         minPodLifetimeSeconds: 3600

Filter Pods

Namespace filtering

The following strategies accept a namespaces parameter which allows to specify a list of including, resp. excluding namespaces:

• PodLifeTime
• RemovePodsHavingTooManyRestarts
• RemovePodsViolatingNodeTaints
• RemovePodsViolatingNodeAffinity
• RemovePodsViolatingInterPodAntiAffinity
• RemoveDuplicates
• RemovePodsViolatingTopologySpreadConstraint
• RemoveFailedPods

For example:

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "PodLifeTime":
     enabled: true
     params:
        podLifeTime:
          maxPodLifeTimeSeconds: 86400
        namespaces:
          include:
          - "namespace1"
          -