mirrors/kubernetes-hands-on
mirrors
/
kubernetes-hands-on
mirror of https://github.com/algolia/kubernetes-hands-on.git
1
0
Fork 0
Code Issues Projects Releases Wiki Activity
kubernetes-hands-on/12-resources/README.md

3.7 KiB
Raw Blame History

Resources, and how it impacts the scheduling

Introduction

When you declare a pod you can declare what resources it will use. Those resources are how much CPU and RAM your pods will use.

Specifying those will help k8s to schedule more efficently your pods on the available nodes.

For each resource you can define the limits and the requests.

Resources definition

The CPU resource is measured in a number of CPU the pod will use for a given amount of time. It can be inferior to 0. Specifying 0.5 (or 500m, which means 500 millicpu), will give half of a CPU to your pod.

The RAM resource is measured in the number of bytes of RAM the pod will use. Specifying 123Mi (or 128974848, which means 128974848 bytes), will give that many RAM to your pod.

requests vs limits

k8s let's you configure the requests and the limits for each resource.

requests

The requests is the number that will help k8s schedule your pod on a node were the resources are available.

Let's take an example. You have 3 nodes:

  • node 1 has 100m CPU available, and 1Gi RAM available
  • node 2 has 1000m CPU available, and 100Mi RAM available
  1. You start a pod with only a CPU request of 10m. It can be scheduled on any node, so k8s will take either of them.
  2. You start a pod with only a CPU request of 500m. It can be scheduled only on node 2. Node 1 has only 100m CPU available.
  3. You start a pod with a CPU request of 500m and a RAM request of 500Mi. k8s will not be able to schedule your pod as no node has both requests at the same time. The pod will be in the state Unschedulable.

limits

The limits is the maximum utilization of a resource k8s will allow for a given pod. If your pod goes above this limit it will be restarted.

Note on resources

On any computer, you can overuse CPU, but you cannot overuse RAM.

If running all your programs require more processing power than you have available your computer will just be slow.

On the other hand, if running all your program require more RAM than you have available, your computer will kill randomly processes that ask for too much RAM (at least on linux).

In kubernetes it's a bit the same. You can ask for limits that are higher than the node CPU, but not the RAM.

Furthermore, you can ask for less than a CPU. But what does that mean? When you ask for 100 millicpu, kubernetes will give you 100 milliseconds of CPU during a second of time. If your container wants to use more it'll be throttled during the remaining 900 milliseconds of the second.

Good practices

Only the requests is taken into account into the scheduling. So it's possible for a k8s node to go on "overcommit", to have the sum of resources used above the physical resource limitation of a node. In this case k8s will look for pods to terminate. Its algorithm is to look at pods that are above what they requested and terminates them. If a pod has no requests, so they are using more than what they requested, they will be the first to be terminated. Other candidates are the pods that have gone over their request but are still under their limit.

Unless your applications are designed to use multiple cores, it is usually a best practice to keep the CPU request at "1" or below.

Most of the time you could define request == limit. But careful as your pod will be terminated if it goes above the limit.

Exercices

Nothing to see here.

Clean up

kubectl delete service,deployment,pod --all

Links