Seperate priority and preemption

Author: resouer

docs/concepts/configuration/pod-priority-preemption.md

@@ -19,8 +19,8 @@ order of Pods and out-of-resource eviction ordering on the Node.
 
 {% capture body %}
 
-## How to use priority and preemption
-To use priority and preemption in Kubernetes 1.8 and later, follow these steps:
+## How to use priority
+To use priority in Kubernetes 1.8 and later, follow these steps:
 
 1. Enable the feature.
 
@@ -32,9 +32,9 @@ Of course you do not need to create the Pods directly; normally you would add
 
 The following sections provide more information about these steps.
 
-## Enabling priority and preemption
+## Enabling priority
 
-Pod priority and preemption is disabled by default in Kubernetes 1.8.
+Pod priority is disabled by default in Kubernetes 1.8.
 To enable the feature, set this command-line flag for the API server, scheduler and kubelet:
 
 ```
@@ -51,6 +51,30 @@ Also enable scheduling.k8s.io/v1alpha1 API and Priority [admission controller](/
 After the feature is enabled, you can create [PriorityClasses](#priorityclass)
 and create Pods with [`priorityClassName`](#pod-priority) set.
 
+
+## How to use preemption
+
+Before Kubernetes 1.11, preemption is bound with `PodPriority` feature gate and will be automatically enabled with priority. 
+
+But in Kubernetes 1.11 and later, preemption is controlled by a kube-scheduler flag `disablePreemption` which is `false` by default.
+Please note, although preemption is enabled by default, it will be ignored if priority feature is disabled.
+
+If you want to use priority without enabling preemption, you can set `disablePreemption` to `true` and enable `PodPriority` feature
+at the same time, here's a sample configuration:
+
+```yaml
+apiVersion: componentconfig/v1alpha1
+kind: KubeSchedulerConfiguration
+algorithmSource:
+  provider: DefaultProvider
+
+...
+
+disablePreemption: true
+
+```
+
+
 If you try the feature and then decide to disable it, you must remove the PodPriority
 command-line flag or set it to false, and then restart the API server and
 scheduler. After the feature is disabled, the existing Pods keep their priority