Kubernetes

Interactions between Pod priority and quality of service(Qos)

[TOC]

Pod 优先级和服务质量之间的相互作用

集群中为了保障核心服务正常运行，有时候会舍弃掉不那么核心的服务。在通过Kubernetes实现的时候发现有两个手段：

  • 设置pod的服务质量。Guaranteed, Burstable, BestEffort
  • 设置pod的优先级

可是当这两个发生冲突的时候，优先考虑谁呢？比如现在这种情况：

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在资源不够用的情况下，一个高优先级，但是Qos属于Burstable或BestEffort的pod会抢占，低优先级但Qos属于Guaranteed的pod吗？

挠头不？

进过查找资料，发现官方给出的答案是这样的：

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02

其实这里是要分情况来看，简单来说是两个环节：pod调度环节 和 节点压力驱逐环节

pod调度环节

这里有最核心的一句话。

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The scheduler's preemption logic does not consider QoS when choosing preemption targets.

调度器的抢占逻辑在选择抢占目标时不考虑 QoS。只看pod的优先级，所以无论Guaranteed, Burstable, BestEffort，只要你的优先级最低，并且能腾出空间给新的pod运行，那很可能就是你了。

节点压力驱逐环节

而对于节点压力驱逐环节可能会繁琐一点

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As a result, kubelet ranks and evicts pods in the following order:
BestEffort or Burstable pods where the usage exceeds requests. These pods are evicted based on their Priority and then by how much their usage level exceeds the request.
Guaranteed pods and Burstable pods where the usage is less than requests are evicted last, based on their Priority.

第一条：usage exceeds requests

第一条，第一点Qos

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BestEffort or Burstable pods where the usage exceeds requests.

第一条，第二点Priority

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These pods are evicted based on their Priority

第一条，第三点usage level

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and then by how much their usage level exceeds the request.

第二条：usage is less than requests

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Guaranteed pods and Burstable pods where the usage is less than requests are evicted last, based on their Priority.

第二条，第一点Qos

第二条，第二点Priority

顺序 条件 举例
1 Qos: BestEffort
Priority: 0
内存 request: 0
内存usage：>0
2 Qos: BestEffort
Priority: >0
内存 request: 0
内存usage：>0		 Priority低的先驱逐，priority相同的，usage越高的先驱逐
3 Qos: Burstable
Priority: 0
内存usage：>request
4 Qos: Burstable
Priority: >0
内存usage：>request		 Priority低的先驱逐，priority相同的，usage越高的先驱逐
5 Qos: Burstable
Priority: 0
内存usage：<request
6 Qos: Burstable
Priority: >0
内存usage：<request		 Priority低的先驱逐，priority相同的，usage越高的先驱逐
7 Qos: Guaranteed
Priority: 0
8 Qos: Guaranteed
Priority: 0
 Priority低的先驱逐，priority相同的，request越高的先驱逐

所以，你学废了吗？

参考：

https://kubernetes.io/docs/concepts/scheduling-eviction/node-pressure-eviction/#pod-selection-for-kubelet-eviction

https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/#interactions-of-pod-priority-and-qos