How many cores do kubernetes pods use when it's CPU usage is limited by policy?

Question

Kubernetes allows to limit pod resource usage.

requests:
  cpu: 100m
  memory: 128Mi
limits:
  cpu: 200m   # which is 20% of 1 core
  memory: 256Mi

Let's say my kubernetes node has 2 core. And I run this pod with limit of CPU: 200m on this node. In this case, will my pod use it's underlying node's 1Core's 200m or 2Core's 100m+100m?

This calculation is needed for my gunicorn worker's number formula, or nginx worker's number etc.. In gunicorn documentation it says

Generally we recommend (2 x $num_cores) + 1 as the number of workers to start off with.

So should I use 5 workers? (my node has 2 cores). Or it doesn't even matter since my pod has only allocated 200m cpu and I should consider my pod has 1 core?

TLDR: How many cores do pods use when its cpu usage is limited by kubernetes? If I run top inside pod, I'm seeing 2 cores available. But I'm not sure my application is using this 2 core's 10%+10% or 1core's 20%..

Answer 1

It will limit to 20% of one core, i.e. 200m. Also, limit means a pod can touch a maximum of that much CPU and no more. So pod CPU utilization will not always touch the limit.

Total CPU limit of a cluster is the total amount of cores used by all nodes present in cluster.

If you have a 2 node cluster and the first node has 2 cores and second node has 1 core, K8s CPU capacity will be 3 cores (2 core + 1 core). If you have a pod which requests 1.5 cores, then it will not be scheduled to the second node, as that node has a capacity of only 1 core. It will instead be scheduled to first node, since it has 2 cores.

Answer 2

CPU is measured in units called millicores. Each node in the cluster introspects the operating system to determine the amount of CPU cores on the node and then multiples that value by 1000 to express its total capacity. For example, if a node has 2 cores, the node’s CPU capacity would be represented as 2000m. If you wanted to use a 1/10 of a single core, you would represent that as 100m.

So, if in your cluster you provided 200m milicores, then it will stick to one core and take up the 20 percent of that core. Now if you provided another pod with 1.5m, then only it will take up more than one core.