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Graduate Pod Priority and Preemption to GA #12428

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Feb 20, 2019
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Graduate Pod Priority and Preemption to GA #12428

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41 changes: 14 additions & 27 deletions content/en/docs/concepts/configuration/pod-priority-preemption.md
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Expand Up @@ -9,7 +9,7 @@ weight: 70

{{% capture overview %}}

{{< feature-state for_k8s_version="1.11" state="beta" >}}
{{< feature-state for_k8s_version="1.14" state="stable" >}}

[Pods](/docs/user-guide/pods) can have _priority_. Priority indicates the
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Hello @bsalamat, would you update the state attribute to one of:
"alpha, beta, deprecated, stable"? Hopefully, this will fix the build.

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done! Thanks.

importance of a Pod relative to other Pods. If a Pod cannot be scheduled, the
Expand All @@ -19,8 +19,8 @@ pending Pod possible.
In Kubernetes 1.9 and later, Priority also affects scheduling order of Pods and
out-of-resource eviction ordering on the Node.

Pod priority and preemption are moved to beta since Kubernetes 1.11 and are
enabled by default in this release and later.
Pod priority and preemption graduated to beta in Kubernetes 1.11 and to GA in
Kubernetes 1.14. They have been enabled by default since 1.11.

In Kubernetes versions where Pod priority and preemption is still an alpha-level
feature, you need to explicitly enable it. To use these features in the older
Expand All @@ -34,6 +34,7 @@ Kubernetes Version | Priority and Preemption State | Enabled by default
1.9 | alpha | no
1.10 | alpha | no
1.11 | beta | yes
1.14 | GA | yes

{{< warning >}}In a cluster where not all users are trusted, a
malicious user could create pods at the highest possible priorities, causing
Expand Down Expand Up @@ -71,15 +72,15 @@ Pods.
## How to disable preemption

{{< note >}}
In Kubernetes 1.11, critical pods (except DaemonSet pods, which are
still scheduled by the DaemonSet controller) rely on scheduler preemption to be
scheduled when a cluster is under resource pressure. For this reason, you will
need to run an older version of Rescheduler if you decide to disable preemption.
More on this is provided below.
In Kubernetes 1.12+, critical pods rely on scheduler preemption to be scheduled
when a cluster is under resource pressure. For this reason, it is not
recommended to disable preemption.
{{< /note >}}

In Kubernetes 1.11 and later, preemption is controlled by a kube-scheduler flag
`disablePreemption`, which is set to `false` by default.
If you want to disable preemption despite the above note, you can set
`disablePreemption` to `true`.

This option is available in component configs only and is not available in
old-style command line options. Below is a sample component config to disable
Expand All @@ -96,20 +97,6 @@ algorithmSource:
disablePreemption: true
```

### Start an older version of Rescheduler in the cluster

When priority or preemption is disabled, we must run Rescheduler v0.3.1 (instead
of v0.4.0) to ensure that critical Pods are scheduled when nodes or cluster are
under resource pressure. Since critical Pod annotation is still supported in
this release, running Rescheduler should be enough and no other changes to the
configuration of Pods should be needed.

Rescheduler images can be found at:
[gcr.io/k8s-image-staging/rescheduler](http://gcr.io/k8s-image-staging/rescheduler).

In the code, changing the Rescheduler version back to v.0.3.1 is the reverse of
[this PR](https://github.com/kubernetes/kubernetes/pull/65454).

## PriorityClass

A PriorityClass is a non-namespaced object that defines a mapping from a
Expand Down Expand Up @@ -364,11 +351,11 @@ Pod priority and
[QoS](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/resource-qos.md)
are two orthogonal features with few interactions and no default restrictions on
setting the priority of a Pod based on its QoS classes. The scheduler's
preemption logic does consider QoS when choosing preemption targets. Preemption
considers Pod priority and attempts to choose a set of targets with the lowest
priority. Higher-priority Pods are considered for preemption only if the removal
of the lowest priority Pods is not sufficient to allow the scheduler to schedule
the preemptor Pod, or if the lowest priority Pods are protected by
preemption logic does not consider QoS when choosing preemption targets.
Preemption considers Pod priority and attempts to choose a set of targets with
the lowest priority. Higher-priority Pods are considered for preemption only if
the removal of the lowest priority Pods is not sufficient to allow the scheduler
to schedule the preemptor Pod, or if the lowest priority Pods are protected by
`PodDisruptionBudget`.

The only component that considers both QoS and Pod priority is
Expand Down