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Resource Management

Kubernetes allows users to specify two different resource metrics. Resource requests specify the minimum amount of a resource required to run the application. Resource limits specify the maximum amount of a resource that an application can consume. 

Requests are used when scheduling Pods to nodes. The Kubernetes scheduler will ensure that the sum of all requests of all Pods on a node does not exceed the capacity of the node. Therefore, a Pod is guaranteed to have at least the requested resources when running on the node.

Imagine a container whose code attempts to use all available CPU cores. Suppose that we create a Pod with this container that requests 0.5 CPU. Kubernetes schedules this Pod onto a machine with a total of 2 CPU cores. As long as it is the only Pod on the machine, it will consume all 2.0 of the available cores, despite only requesting 0.5 CPU.

CPU requests are implemented using the cpu-shares functionality in the Linux kernel.

Memory requests are handled similarly to CPU, but there is an important difference. If a container is over its memory request, the OS can’t just remove memory from the process, because it’s been allocated. Consequently, when the system runs out of memory, the kubelet terminates containers whose memory usage is greater than their requested memory. 

requested a minimum of 0.5 of a core and 128 MB of memory and a limit of 1.0 CPU and 256 MB of memory.

apiVersion: v1
kind: Pod
metadata:
  name: kuard
spec:
  containers:
    - image: gcr.io/kuar-demo/kuard-amd64:blue
      name: kuard
      resources:
        requests:
          cpu: "500m"
          memory: "128Mi"
        limits:
          cpu: "1000m"
          memory: "256Mi"
      ports:
        - containerPort: 8080
          name: http
          protocol: TCP

When you establish limits on a container, the kernel is configured to ensure that consumption cannot exceed these limits. A container with a CPU limit of 0.5 cores will only ever get 0.5 cores, even if the CPU is otherwise idle. A container with a memory limit of 256 MB will not be allowed additional memory; for example, malloc will fail if its memory usage exceeds 256 MB.

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