Kubernetes' Magic Numbers

Here is just a small auxiliary note. You may return to the main text.

Resource types

So, we know CPU and memory are each a resource type in Kubernetes. A resource type has a base unit. CPU is specified in units of cores, and memory is specified in units of bytes.

CPU and memory are collectively referred to as compute resources, or just resources. Compute resources are measurable quantities that can be requested, allocated, and consumed. They are distinct from API resources. API resources, such as Pods and Services, are objects that can be read and modified through the Kubernetes API server.

Resource requests and limits of Pod and Container

Each Container of a Pod can specify one or more of the following:

• spec.containers[].resources.limits.cpu
• spec.containers[].resources.limits.memory
• spec.containers[].resources.requests.cpu
• spec.containers[].resources.requests.memory

Although requests and limits can only be specified on individual Containers, it is convenient to talk about Pod resource requests and limits. A Pod resource request/limit for a particular resource type is the sum of the resource requests/limits of that type for each Container in the Pod.

Meaning of CPU

Limits and requests for CPU resources are measured in cpu units. One cpu, in Kubernetes, is equivalent to:

• 1 ☁️ AWS vCPU
• 1 ☁️ GCP Core
• 1 ☁️ Azure vCore
• 1 Hyperthread on a bare-metal Intel processor with Hyperthreading

Fractional requests are allowed. A Container with spec.containers[].resources.requests.cpu of 0.5 is guaranteed half as much CPU as one that asks for 1 CPU. The expression 0.1 is equivalent to the expression 100m, which can be read as "one hundred millicpu".

Some people say "one hundred millicores", and this is understood to mean the same thing. A request with a decimal point, like 0.1, is converted to 100m by the API, and precision finer than 1m is not allowed. For this reason, the form 100m might be preferred.

CPU is always requested as an absolute quantity, never as a relative quantity; 0.1 is the same amount of CPU on a single-core, dual-core, or 48-core machine.

If you are not already familiar with the concept of millicores, and how does the operating system kernel even enforce this measure, we at Otomato recommend you to read this article written by Will Hegedus.

Meaning of memory

Limits and requests for memory are measured in bytes. You can express memory as a plain integer or as a fixed-point integer using one of these SI suffixes: E, P, T, G, M, K. You can also use the power-of-two equivalents: Ei, Pi, Ti, Gi, Mi, Ki. For example, the following represent roughly the same value: 128974848, 129e6, 129M, 123Mi

Here's a YAML example. The following Pod has two Containers. Each Container has a request of 0.25 cpu and 64MiB (2^26 bytes) of memory Each Container has a limit of 0.5 cpu and 128MiB of memory. You can say the Pod has a request of 0.5 cpu and 128 MiB of memory, and a limit of 1 cpu and 256MiB of memory.

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

If you want to find out the actual resource limit/request, then you can see the value by describing the deployment or the pod, for example: kubectl describe deployment <name> or kubectl describe pod <name>.

You may return to the main text.