Assign CPU Resources to Containers and Pods

Specify a CPU request and a CPU limit

To specify a CPU request for a Container, include the resources:requests field in the Container resource manifest. To specify a CPU limit, include resources:limits.

In this exercise, you create a Pod that has one Container. The Container has a request of 0.5 CPU and a limit of 1 CPU. Here is the configuration file for the Pod:

apiVersion: v1
kind: Pod
metadata:
  name: syrus-cpu-demo
  namespace: syrusk8s
spec:
  containers:
  - name: syrus-cpu-demo-ctr
    image: vish/stress
    resources:
      limits:
        cpu: "1"
      requests:
        cpu: "0.5"
    args:
    - -cpus
    - "2"

The args section of the configuration file provides arguments for the Container when it starts. The -cpus "2" argument tells the Container to attempt to use 2 CPUs.

Create the Pod:

kubectl apply -f cpu-request-limit.yaml --namespace=syrusk8s

Verify that the Pod Container is running:

kubectl get pod syrus-cpu-demo --namespace=syrusk8s

View detailed information about the Pod:

kubectl get pod syrus-cpu-demo --output=yaml --namespace=syrusk8s

The output shows that the one Container in the Pod has a CPU request of 500 milliCPU and a CPU limit of 1 CPU.

resources:
  limits:
    cpu: "1"
  requests:
    cpu: 500m

Use kubectl top to fetch the metrics for the pod:

kubectl top pod syrus-cpu-demo --namespace=syrusk8s

The output shows that the Pod is using 974 milliCPU, which is just a bit less than the limit of 1 CPU specified in the Pod configuration file.

NAME                        CPU(cores)   MEMORY(bytes)
syrus-cpu-demo                    974m         <something>

Recall that by setting -cpu "2", you configured the Container to attempt to use 2 CPUs, but the Container is only being allowed to use about 1 CPU. The Container CPU use is being throttled, because the Container is attempting to use more CPU resources than its limit.

Note: Another possible explanation for the CPU throttling is that the Node might not have enough CPU resources available. Recall that the prerequisites for this exercise require each of your Nodes to have at least 1 CPU. If your Container runs on a Node that has only 1 CPU, the Container cannot use more than 1 CPU regardless of the CPU limit specified for the Container.

CPU units

The CPU resource is 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 values are allowed. A Container that requests 0.5 CPU is guaranteed half as much CPU as a Container that requests 1 CPU. You can use the suffix m to mean milli. For example 100m CPU, 100 milliCPU, and 0.1 CPU are all the same. Precision finer than 1m is not allowed.

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.

Delete your Pod:

kubectl delete pod syrus-cpu-demo --namespace=syrusk8s

Specify a CPU request that is too big for your Nodes

CPU requests and limits are associated with Containers, but it is useful to think of a Pod as having a CPU request and limit. The CPU request for a Pod is the sum of the CPU requests for all the Containers in the Pod. Likewise, the CPU limit for a Pod is the sum of the CPU limits for all the Containers in the Pod.

Pod scheduling is based on requests. A Pod is scheduled to run on a Node only if the Node has enough CPU resources available to satisfy the Pod CPU request.

In this exercise, you create a Pod that has a CPU request so big that it exceeds the capacity of any Node in your cluster. Here is the configuration file for a Pod that has one Container. The Container requests 100 CPU, which is likely to exceed the capacity of any Node in your cluster.

kind: Pod
metadata:
  name: syrus-cpu-demo-2
  namespace: syrusk8s
spec:
  containers:
  - name: syrus-cpu-demo-ctr-2
    image: vish/stress
    resources:
      limits:
        cpu: "100"
      requests:
        cpu: "100"
    args:
    - -cpus
    - "2"

Create the Pod:

kubectl apply -f cpu-request-limit-2.yaml --namespace=syrusk8s

View the Pod status:

kubectl get pod syrus-cpu-demo-2 --namespace=syrusk8s

The output shows that the Pod status is Pending. That is, the Pod has not been scheduled to run on any Node, and it will remain in the Pending state indefinitely:

kubectl get pod syrus-cpu-demo-2 --namespace=syrusk8s
NAME         READY     STATUS    RESTARTS   AGE
syrus-cpu-demo-2   0/1       Pending   0          7m

View detailed information about the Pod, including events:

kubectl describe pod syrus-cpu-demo-2 --namespace=syrusk8s

The output shows that the Container cannot be scheduled because of insufficient CPU resources on the Nodes:

Events:
  Reason			Message
  ------			-------
  FailedScheduling	No nodes are available that match all of the following predicates:: Insufficient cpu (3).

Delete your Pod:

kubectl delete pod syrus-cpu-demo-2 --namespace=syrusk8s

If you do not specify a CPU limit

If you do not specify a CPU limit for a Container, then one of these situations applies:

• The Container has no upper bound on the CPU resources it can use. The Container could use all of the CPU resources available on the Node where it is running.

• The Container is running in a namespace that has a default CPU limit, and the Container is automatically assigned the default limit. Cluster administrators can use a LimitRange to specify a default value for the CPU limit.

Motivation for CPU requests and limits

By configuring the CPU requests and limits of the Containers that run in your cluster, you can make efficient use of the CPU resources available on your cluster Nodes. By keeping a Pod CPU request low, you give the Pod a good chance of being scheduled. By having a CPU limit that is greater than the CPU request, you accomplish two things:

• The Pod can have bursts of activity where it makes use of CPU resources that happen to be available.

• The amount of CPU resources a Pod can use during a burst is limited to some reasonable amount.

Clean up

Delete your namespace:

kubectl delete namespace syrusk8s