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优先级抢占（Preemption scheduling）策略

Volcano 通过 Priority 插件实现了优先级抢占策略，即 Preemption scheduling 策略。在集群资源有限且多个 Job 等待调度时， 如果使用 Kubernetes 默认调度器，可能会导致具有更多 Pod 数量的 Job 分得更多资源。而 Volcano-scheduler 提供了算法，支持不同的 Job 以 fair-share 的形式共享集群资源。

Priority 插件允许用户自定义 Job 和 Task 的优先级，并根据需求在不同层次上定制调度策略。 例如，对于金融场景、物联网监控场景等需要较高实时性的应用，Priority 插件能够确保其优先得到调度。

使用方式

优先级的决定基于配置的 PriorityClass 中的 Value 值，值越大优先级越高。默认已启用，无需修改。可通过以下命令确认或修改。

kubectl -n volcano-system edit configmaps volcano-scheduler-configmap

使用案例

假设集群中存在两个空闲节点，并有三个优先级不同的工作负载：high-priority、med-priority 和 low-priority。 当 high-priority 工作负载运行并占满集群资源后，再提交 med-priority 和 low-priority 工作负载。 由于集群资源全部被更高优先级的工作负载占用，med-priority 和 low-priority 工作负载将处于 pending 状态。 当 high-priority 工作负载结束后，根据优先级调度原则，med-priority 工作负载将优先被调度。

  1. 通过 priority.yaml 创建 3 个优先级定义，分别为：high-priority，med-priority，low-priority。

    查看 priority.yaml
    cat <<EOF | kubectl apply -f - 
apiVersion: scheduling.k8s.io/v1 
kind: PriorityClass 
items: 
  - metadata: 
      name: high-priority 
    value: 100 
    globalDefault: false 
    description: "This priority class should be used for volcano job only." 
  - metadata: 
      name: med-priority 
    value: 50 
    globalDefault: false 
    description: "This priority class should be used for volcano job only." 
  - metadata: 
      name: low-priority 
    value: 10 
    globalDefault: false 
    description: "This priority class should be used for volcano job only." 
EOF
    2. 查看优先级定义信息。

    kubectl get PriorityClass

    NAME                      VALUE        GLOBAL-DEFAULT   AGE  
high-priority             100          false            97s  
low-priority              10           false            97s  
med-priority              50           false            97s  
system-cluster-critical   2000000000   false            6d6h  
system-node-critical      2000001000   false            6d6h

  2. 创建高优先级工作负载 high-priority-job，占用集群的全部资源。

    查看 high-priority-job
    cat <<EOF | kubectl apply -f -  
apiVersion: batch.volcano.sh/v1alpha1  
kind: Job  
metadata:  
  name: priority-high  
spec:  
  schedulerName: volcano  
  minAvailable: 4  
  priorityClassName: high-priority  
  tasks:  
    - replicas: 4  
      name: "test"  
      template:  
        spec:  
          containers:  
            - image: alpine  
              command: ["/bin/sh", "-c", "sleep 1000"]  
              imagePullPolicy: IfNotPresent  
              name: running  
              resources:  
                requests:  
                  cpu: "4"  
          restartPolicy: OnFailure  
EOF

    通过 kubectl get pod 查看 Pod运行 信息：

    kubectl get pods

    NAME                   READY   STATUS    RESTARTS   AGE  
priority-high-test-0   1/1     Running   0          3s  
priority-high-test-1   1/1     Running   0          3s  
priority-high-test-2   1/1     Running   0          3s  
priority-high-test-3   1/1     Running   0          3s

    此时，集群节点资源已全部被占用。

  3. 创建中优先级工作负载 med-priority-job 和低优先级工作负载 low-priority-job。

    med-priority-job
    cat <<EOF | kubectl apply -f -  
apiVersion: batch.volcano.sh/v1alpha1  
kind: Job  
metadata:  
  name: priority-medium  
spec:  
  schedulerName: volcano  
  minAvailable: 4  
  priorityClassName: med-priority  
  tasks:  
    - replicas: 4  
      name: "test"  
      template:  
        spec:  
          containers:  
            - image: alpine  
              command: ["/bin/sh", "-c", "sleep 1000"]  
              imagePullPolicy: IfNotPresent  
              name: running  
              resources:  
                requests:  
                  cpu: "4"  
          restartPolicy: OnFailure  
EOF
    low-priority-job
    cat <<EOF | kubectl apply -f -  
apiVersion: batch.volcano.sh/v1alpha1  
kind: Job  
metadata:  
  name: priority-low  
spec:  
  schedulerName: volcano  
  minAvailable: 4  
  priorityClassName: low-priority  
  tasks:  
    - replicas: 4  
      name: "test"  
      template:  
        spec:  
          containers:  
            - image: alpine  
              command: ["/bin/sh", "-c", "sleep 1000"]  
              imagePullPolicy: IfNotPresent  
              name: running  
              resources:  
                requests:  
                  cpu: "4"  
          restartPolicy: OnFailure  
EOF

    通过 kubectl get pod 查看 Pod 运行信息，集群资源不足，Pod 处于 Pending 状态：

    kubectl get pods

    NAME                     READY   STATUS    RESTARTS   AGE  
priority-high-test-0     1/1     Running   0          3m29s  
priority-high-test-1     1/1     Running   0          3m29s  
priority-high-test-2     1/1     Running   0          3m29s  
priority-high-test-3     1/1     Running   0          3m29s  
priority-low-test-0      0/1     Pending   0          2m26s  
priority-low-test-1      0/1     Pending   0          2m26s  
priority-low-test-2      0/1     Pending   0          2m26s  
priority-low-test-3      0/1     Pending   0          2m26s  
priority-medium-test-0   0/1     Pending   0          2m36s  
priority-medium-test-1   0/1     Pending   0          2m36s  
priority-medium-test-2   0/1     Pending   0          2m36s  
priority-medium-test-3   0/1     Pending   0          2m36s

  4. 删除 high_priority_job 工作负载，释放集群资源，med_priority_job 会被优先调度。 执行 kubectl delete -f high_priority_job.yaml 释放集群资源，查看 Pod 的调度信息：

    kubectl get pods

    NAME                     READY   STATUS    RESTARTS   AGE  
priority-low-test-0      0/1     Pending   0          5m18s  
priority-low-test-1      0/1     Pending   0          5m18s  
priority-low-test-2      0/1     Pending   0          5m18s  
priority-low-test-3      0/1     Pending   0          5m18s  
priority-medium-test-0   1/1     Running   0          5m28s  
priority-medium-test-1   1/1     Running   0          5m28s  
priority-medium-test-2   1/1     Running   0          5m28s  
priority-medium-test-3   1/1     Running   0          5m28s