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    Container Service for Kubernetes:Enable preemption

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    Container Service for Kubernetes:Enable preemption

    Last Updated:Mar 17, 2026

    When cluster resources become insufficient, high-priority tasks may fail to run. After you enable preemption, ACK Scheduler determines and evicts low-priority pods through resource simulation, releasing computing resources to launch high-priority pods.

    Before you begin

    Before enabling preemption, read Pod priority and preemption to understand the related concepts, including PriorityClass, priorityClassName, and how pod priority affects scheduling and eviction order.

    How preemption works

    When preemption is enabled, ACK Scheduler follows this procedure:

    1. Specify task priority: Add the priorityClassName parameter to pod configurations to declare the priority of each pod.

      System components (such as kube-proxy and etcd) are typically associated with the system-node-critical PriorityClass, which assigns a high priority. Most system components are deployed as DaemonSets and are not evicted by default.

    2. Simulate preemption: When ACK Scheduler detects a pending pod that cannot be scheduled due to insufficient resources, it simulates whether evicting certain low-priority pods would release enough resources for the high-priority pod.

    3. Securely evict low-priority pods: After ACK Scheduler triggers the preemption logic, kubelet evicts the identified low-priority pods from the node in a secure manner, releasing resources to complete the scheduling of the high-priority pod.

    4. Reschedule evicted pods: Evicted low-priority pods are recreated by their controllers (such as Deployments) and scheduled to other nodes. If cluster resources remain insufficient, the pods stay in a pending state.

    Preemption policies

    ACK Scheduler supports the following preemption policies by default. You can manage and configure them by using the PreemptionAlgorithm parameter.

    Policy

    Description

    Default

    The default Kubernetes preemption policy. ACK Scheduler first attempts direct scheduling. When resources are insufficient, ACK Scheduler simulates resource preemption to identify the low-priority pods to evict in order to free up resources for the high-priority pod. For more information, see Pod priority and preemption.

    ElasticQuota

    Performs preemption based on the ElasticQuotaTree. See ElasticQuota policy details below.

    Auto

    Adjusts the preemption policy based on cluster configuration. ACK Scheduler prioritizes the ElasticQuota policy and automatically falls back to the Default policy when the ElasticQuota policy does not take effect.

    None

    Disables preemption. No preemption behavior occurs in the cluster.

    When preemption is enabled, the policy takes effect according to the logic shown in the following figure.

    image

    ElasticQuota policy details

    When the ElasticQuota policy is active, ACK Scheduler performs preemption in three steps:

    1. Identify the preemption list: Unlike regular tasks, ACK Scheduler combines all pending pods of an entire gang into a single preemption list. The entire gang is processed only when all pods in the list can be scheduled through preemption. If any single pod cannot be scheduled, the gang is not processed. If any pod in the gang has a high priority, the entire gang is treated as high-priority for preemption purposes.

    2. Generate a list of feasible nodes: ACK Scheduler determines candidate nodes using the following rules:

      • If multiple pods in the gang require preemption, any node that matches one pod is added to the candidate pool. ACK Scheduler then attempts to free up resources for the remaining pods.

      • If only one pod requires preemption, ACK Scheduler filters out nodes marked as Unschedulable to accurately identify nodes with insufficient or mismatched resources.

    3. Simulate preemption: ACK Scheduler filters pods eligible for preemption from the feasible node list. A pod is eligible if it meets both of the following conditions:

      ACK Scheduler then applies the ElasticQuotaTree rules configured in the cluster:

      • If the preempting pod belongs to a Quota:

        • If current Quota usage is at or below its Min guarantee: The Quota has not yet consumed its minimum guaranteed resources. The pod is allowed to preempt pods from other Quotas that are currently using resources above their Min guarantee.

        • If current Quota usage exceeds its Min guarantee: The pod is only allowed to preempt lower-priority pods within its own Quota (intra-quota preemption).

      • If the preempting pod does not belong to any Quota: Preemption is not allowed.

      After identifying the pods to preempt, ACK Scheduler simulates removing those pods and rescheduling the preempting pod. If the simulation indicates successful scheduling, the identified pods are securely evicted and rescheduled to other nodes. If the simulation indicates failed scheduling, preemption is terminated.

    Enable preemption

    To enable preemption, customize the scheduler parameters by setting the preemptionAlgorithm parameter to the desired policy value (Default, ElasticQuota, Auto, or None). The default preemption policy is Auto.

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