Kubernetes requests and limits¶

Kubernetes requests and limits are the resource management mechanisms used to control the compute resources (CPU and memory) allocated to [[Pods|Pods]].^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]

These definitions are configured within the resources field of a container's specification.^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]

Requests¶

A request is the amount of resources guaranteed to a container.^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]

The kube-scheduler uses the request value to determine which Node has sufficient resources to place the Pod^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]. This practice follows the Borg philosophy, which assumes that jobs often use fewer resources than their maximum allowed capacity, allowing for higher cluster bin-packing density based on these lower request values^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Limits¶

A limit is the maximum amount of resources a container is allowed to use^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

The kubelet enforces this value by configuring the container's control groups (cgroups)^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Resource Types¶

Kubernetes categorizes resources based on how they handle resource exhaustion:

• CPU (Compressible Resources): If a container exceeds its CPU limit or if CPU resources are scarce, the Pod's performance degrades ("starvation"), but the process is not terminated^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].
• Memory (Incompressible Resources): If a container exceeds its memory limit or if the Node runs out of memory, the Pod may be terminated by the kernel via OOM (Out-Of-Memory) killing^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Quality of Service (QoS)¶

Based on the values set for requests and limits, Kubernetes assigns a QoS class to the Pod^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]. This classification determines the Pod's priority during [[Node]] memory pressure and Eviction processes^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Guaranteed¶

A Pod is classified as Guaranteed if every container in the Pod has both requests and limits set, and the values are identical for both CPU and memory^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

This is the highest priority tier. These Pods are only evicted if they exceed their limits or if the Node is under extreme Memory Pressure^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

CPU Isolation: When a Pod is Guaranteed and its CPU request equals an integer value, the kubelet may use cpuset to bind the Pod to exclusive CPU cores^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]. This eliminates CPU context switching overhead and can significantly improve application performance^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Burstable¶

A Pod is classified as Burstable if it does not meet the criteria for Guaranteed, but at least one container has set a request^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

BestEffort¶

A Pod is classified as BestEffort if no containers in the Pod have set any requests or limits^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

Eviction Priority¶

When the kubelet must reclaim resources, it targets Pods in the following order: BestEffort < Burstable < Guaranteed^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]

Configuration Example¶

Resources are specified per container^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md].

apiVersion: v1
kind: Pod
metadata:
  name: frontend
spec:
  containers:
  - name: db
    image: mysql
    resources:
      requests:
        memory: "64Mi"
        cpu: "250m"
      limits:
        memory: "128Mi"
        cpu: "500m"

Related Concepts¶

• [[Kubernetes Scheduling]]
• [[Eviction]]
• [[cgroups]]

Sources¶

^[400-devops__06-Kubernetes__k8s-paas__原理及源码解析__Kubernetes调度机制.md]