Update pod priority and preemption user docs (#9172)
* Update pod priority and preemption user docs * Copyedit
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Bobby (Babak) Salamat
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Misty Linville
Misty Linville
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@@ -9,61 +9,77 @@ weight: 70
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{{% capture overview %}}
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{{< feature-state state="alpha" >}}
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{{< feature-state for_k8s_version="1.8" state="alpha" >}}
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{{< feature-state for_k8s_version="1.11" state="beta" >}}
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[Pods](/docs/user-guide/pods) in Kubernetes 1.8 and later can have priority. Priority
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indicates the importance of a Pod relative to other Pods. When a Pod cannot be scheduled,
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[Pods](/docs/user-guide/pods) can have _priority_. Priority
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indicates the importance of a Pod relative to other Pods. If a Pod cannot be scheduled,
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the scheduler tries to preempt (evict) lower priority Pods to make scheduling of the
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pending Pod possible. In Kubernetes 1.9 and later, Priority also affects scheduling
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pending Pod possible.
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In Kubernetes 1.9 and later, Priority also affects scheduling
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order of Pods and out-of-resource eviction ordering on the Node.
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Pod priority and preemption are moved to beta since Kubernetes 1.11 and are enabled by default in
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this release and later.
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In Kubernetes versions where Pod priority and preemption is still an alpha-level
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feature, you need to explicitly enable it. To use these features in the older versions of
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Kubernetes, follow the instructions in the documentation for your Kubernetes version, by
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going to the documentation archive version for your Kubernetes version.
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| Kubernetes Version | Priority and Preemption State | Enabled by default |
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| -------- |:-----:|:----:|
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| 1.8 | alpha | no |
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| 1.9 | alpha | no |
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| 1.10 | alpha | no |
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| 1.11 | beta | yes |
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{{< warning >}}
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**Warning**: In a cluster where not all users are trusted, a malicious
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user could create pods at the highest possible priorities, causing
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other pods to be evicted/not get scheduled. To resolve this issue,
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[ResourceQuota](https://kubernetes.io/docs/concepts/policy/resource-quotas/) is augmented to support
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Pod priority. An admin can create ResourceQuota for users at specific priority levels, preventing
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them from creating pods at high priorities. However, this feature is in alpha as of Kubernetes 1.11.
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{{< /warning >}}
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{{% /capture %}}
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{{% capture body %}}
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## How to use priority and preemption
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To use priority and preemption in Kubernetes 1.8 and later, follow these steps:
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To use priority and preemption in Kubernetes 1.11 and later, follow these steps:
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1. Enable the feature.
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1. Add one or more [PriorityClasses](#priorityclass).
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1. Add one or more PriorityClasses.
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1. Create Pods with `priorityClassName` set to one of the added PriorityClasses.
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1. Create Pods with[`priorityClassName`](#pod-priority) set to one of the added PriorityClasses.
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Of course you do not need to create the Pods directly; normally you would add
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`priorityClassName` to the Pod template of a collection object like a Deployment.
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The following sections provide more information about these steps.
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## Enabling priority and preemption
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Pod priority and preemption is disabled by default in Kubernetes 1.8.
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To enable the feature, set this command-line flag for the API server, scheduler and kubelet:
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```
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--feature-gates=PodPriority=true
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```
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Also enable scheduling.k8s.io/v1alpha1 API and Priority [admission controller](/docs/admin/admission-controllers/) in API server:
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```
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--runtime-config=scheduling.k8s.io/v1alpha1=true --enable-admission-plugins=Controller-Foo,Controller-Bar,...,Priority
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```
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After the feature is enabled, you can create [PriorityClasses](#priorityclass)
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and create Pods with [`priorityClassName`](#pod-priority) set.
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Keep reading for more information about these steps.
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If you try the feature and then decide to disable it, you must remove the PodPriority
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command-line flag or set it to false, and then restart the API server and
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command-line flag or set it to `false`, and then restart the API server and
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scheduler. After the feature is disabled, the existing Pods keep their priority
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fields, but preemption is disabled, and priority fields are ignored, and you
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cannot set `priorityClassName` in new Pods.
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fields, but preemption is disabled, and priority fields are ignored. If the feature
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is disabled, you cannot set `priorityClassName` in new Pods.
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## How to disable preemption
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In Kubernetes 1.11 and later, preemption is controlled by a kube-scheduler flag `disablePreemption`, which is set to `false` by default.
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{{< note >}}
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**Note**: In Kubernetes 1.11, critical pods (except DaemonSet pods, which are
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still scheduled by the DaemonSet controller) rely on scheduler preemption to be
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scheduled when a cluster is under resource pressure. For this reason, we do not
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recommend disabling this feature. If you still have to disable this feature,
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follow the instructions below.
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{{< /note >}}
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If you want to disable preemption, just set `disablePreemption` to true. This will keep pod priority enabled while preemption is disabled. Here is a sample configuration:
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In Kubernetes 1.11 and later, preemption is controlled by a kube-scheduler flag
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`disablePreemption`, which is set to `false` by default.
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To disable preemption, set `disablePreemption` to true. This keeps pod priority
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enabled but disables preemption. Here is a sample configuration:
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```yaml
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apiVersion: componentconfig/v1alpha1
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@@ -77,7 +93,8 @@ disablePreemption: true
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```
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Please note: although preemption of scheduler is enabled by default, preemption will not happen if `PodPriority` feature is not available.
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Although preemption of the scheduler is enabled by default, it is disabled if `PodPriority`
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feature is disabled.
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## PriorityClass
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@@ -100,22 +117,17 @@ with `globalDefault` set, the priority of Pods with no `priorityClassName` is ze
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The `description` field is an arbitrary string. It is meant to tell users of
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the cluster when they should use this PriorityClass.
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{{< note >}}
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**Note 1**: If you upgrade your existing cluster and enable this feature, the priority
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of your existing Pods will be considered to be zero.
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{{< /note >}}
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### Notes about PodPriority and existing clusters
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- If you upgrade your existing cluster and enable this feature, the priority
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of your existing Pods is effectively zero.
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{{< note >}}
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**Note 2**: Addition of a PriorityClass with `globalDefault` set to true does not
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- Addition of a PriorityClass with `globalDefault` set to `true` does not
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change the priorities of existing Pods. The value of such a PriorityClass is used only
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for Pods created after the PriorityClass is added.
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{{< /note >}}
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{{< note >}}
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**Note 3**: If you delete a PriorityClass, existing Pods that use the name of the
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deleted priority class remain unchanged, but you are not able to create more Pods
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- If you delete a PriorityClass, existing Pods that use the name of the
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deleted PriorityClass remain unchanged, but you cannot create more Pods
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that use the name of the deleted PriorityClass.
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{{< /note >}}
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### Example PriorityClass
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