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Limit Ranges

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By default, containers run with unbounded compute resources on a Kubernetes cluster. With Resource quotas, cluster administrators can restrict the resource consumption and creation on a namespace basis. Within a namespace, a Pod or Container can consume as much CPU and memory as defined by the namespace's resource quota. There is a concern that one Pod or Container could monopolize all of the resources. Limit Range is a policy to constrain resource by Pod or Container in a namespace.

A limit range, defined by a LimitRange object, provides constraints that can:

Enforce minimum and maximum compute resources usage per Pod or Container in a namespace.
Enforce minimum and maximum storage request per PersistentVolumeClaim in a namespace.
Enforce a ratio between request and limit for a resource in a namespace.
Set default request/limit for compute resources in a namespace and automatically inject them to Containers at runtime.

Enabling Limit Range

Limit Range support is enabled by default for many Kubernetes distributions. It is enabled when the apiserver --enable-admission-plugins= flag has LimitRanger admission controller as one of its arguments.

A limit range is enforced in a particular namespace when there is a LimitRange object in that namespace.

Overview of Limit Range:

The administrator creates one LimitRange in one namespace.
Users create resources like Pods, Containers, and PersistentVolumeClaims in the namespace.
The LimitRanger admission controller enforces defaults limits for all Pods and Container that do not set compute resource requirements and tracks usage to ensure it does not exceed resource minimum , maximum and ratio defined in any LimitRange present in the namespace.
If creating or updating a resource (Pod, Container, PersistentVolumeClaim) violates a limit range constraint, the request to the API server will fail with HTTP status code 403 FORBIDDEN and a message explaining the constraint that would have been violated.
If limit range is activated in a namespace for compute resources like cpu and memory, users must specify requests or limits for those values; otherwise, the system may reject pod creation.
LimitRange validations occurs only at Pod Admission stage, not on Running pods.

Examples of policies that could be created using limit range are:

In a 2 node cluster with a capacity of 8 GiB RAM, and 16 cores, constrain Pods in a namespace to request 100m and not exceeds 500m for CPU , request 200Mi and not exceed 600Mi
Define default CPU limits and request to 150m and Memory default request to 300Mi for containers started with no cpu and memory requests in their spec.

In the case where the total limits of the namespace is less than the sum of the limits of the Pods/Containers, there may be contention for resources; The Containers or Pods will not be created.

Neither contention nor changes to limitrange will affect already created resources.

Limiting Container compute resources

The following section discusses the creation of a LimitRange acting at Container Level. A Pod with 04 containers is first created; each container within the Pod has a specific spec.resource configuration each container within the pod is handled differently by the LimitRanger admission controller.

Create a namespace limitrange-demo using the following kubectl command:

kubectl create namespace limitrange-demo

To avoid passing the target limitrange-demo in your kubectl commands, change your context with the following command:

kubectl config set-context --current --namespace=limitrange-demo

Here is the configuration file for a LimitRange object: {{< codenew file="admin/resource/limit-mem-cpu-container.yaml" >}}

This object defines minimum and maximum Memory/CPU limits, default cpu/Memory requests and default limits for CPU/Memory resources to be apply to containers.

Create the limit-mem-cpu-per-container LimitRange in the limitrange-demo namespace with the following kubectl command:

kubectl create -f https://k8s.io/examples/admin/resource/limit-mem-cpu-container.yaml -n limitrange-demo

kubectl describe limitrange/limit-mem-cpu-per-container -n limitrange-demo

Type        Resource  Min   Max   Default Request  Default Limit  Max Limit/Request Ratio
----        --------  ---   ---   ---------------  -------------  -----------------------
Container   cpu       100m  800m  110m             700m           -
Container   memory    99Mi  1Gi   111Mi            900Mi          -

Here is the configuration file for a Pod with 04 containers to demonstrate LimitRange features : {{< codenew file="admin/resource/limit-range-pod-1.yaml" >}}

Create the busybox1 Pod:

kubectl apply -f https://k8s.io/examples/admin/resource/limit-range-pod-1.yaml -n limitrange-demo

Container spec with valid CPU/Memory requests and limits

View the busybox-cnt01 resource configuration:

kubectl get po/busybox1 -n limitrange-demo -o json | jq ".spec.containers[0].resources"

{
  "limits": {
    "cpu": "500m",
    "memory": "200Mi"
  },
  "requests": {
    "cpu": "100m",
    "memory": "100Mi"
  }
}

The busybox-cnt01 Container inside busybox Pod defined requests.cpu=100m and requests.memory=100Mi.
100m <= 500m <= 800m , The container cpu limit (500m) falls inside the authorized CPU limit range.
99Mi <= 200Mi <= 1Gi , The container memory limit (200Mi) falls inside the authorized Memory limit range.
No request/limits ratio validation for CPU/Memory , thus the container is valid and created.

Container spec with a valid CPU/Memory requests but no limits

View the busybox-cnt02 resource configuration

kubectl get po/busybox1 -n limitrange-demo -o json | jq ".spec.containers[1].resources"

{
  "limits": {
    "cpu": "700m",
    "memory": "900Mi"
  },
  "requests": {
    "cpu": "100m",
    "memory": "100Mi"
  }
}

The busybox-cnt02 Container inside busybox1 Pod defined requests.cpu=100m and requests.memory=100Mi but not limits for cpu and memory.
The container do not have a limits section, the default limits defined in the limit-mem-cpu-per-container LimitRange object are injected to this container limits.cpu=700mi and limits.memory=900Mi.
100m <= 700m <= 800m , The container cpu limit (700m) falls inside the authorized CPU limit range.
99Mi <= 900Mi <= 1Gi , The container memory limit (900Mi) falls inside the authorized Memory limit range.
No request/limits ratio set , thus the container is valid and created.

Container spec with a valid CPU/Memory limits but no requests

View the busybox-cnt03 resource configuration

kubectl get po/busybox1 -n limitrange-demo -o json | jq ".spec.containers[2].resources"

{
  "limits": {
    "cpu": "500m",
    "memory": "200Mi"
  },
  "requests": {
    "cpu": "500m",
    "memory": "200Mi"
  }
}

The busybox-cnt03 Container inside busybox1 Pod defined limits.cpu=500m and limits.memory=200Mi but no requests for cpu and memory.
The container do not define a request section, the defaultRequest defined in the limit-mem-cpu-per-container LimitRange is not used to fill its limits section but the limits defined by the container are set as requests limits.cpu=500m and limits.memory=200Mi.
100m <= 500m <= 800m , The container cpu limit (500m) falls inside the authorized CPU limit range.
99Mi <= 200Mi <= 1Gi , The container memory limit (200Mi) falls inside the authorized Memory limit range.
No request/limits ratio set , thus the container is valid and created.

Container spec with no CPU/Memory requests/limits

View the busybox-cnt04 resource configuration:

kubectl get po/busybox1 -n limitrange-demo -o json | jq ".spec.containers[3].resources"

{
  "limits": {
    "cpu": "700m",
    "memory": "900Mi"
  },
  "requests": {
    "cpu": "110m",
    "memory": "111Mi"
  }
}

The busybox-cnt04 Container inside busybox1 define neither limits nor requests.
The container do not define a limit section, the default limit defined in the limit-mem-cpu-per-container LimitRange is used to fill its request limits.cpu=700m and limits.memory=900Mi .
The container do not define a request section, the defaultRequest defined in the limit-mem-cpu-per-container LimitRange is used to fill its request section requests.cpu=110m and requests.memory=111Mi
100m <= 700m <= 800m , The container cpu limit (700m) falls inside the authorized CPU limit range.
99Mi <= 900Mi <= 1Gi , The container memory limit (900Mi) falls inside the authorized Memory limitrange .
No request/limits ratio set , thus the container is valid and created.

All containers defined in the busybox Pod passed LimitRange validations, this the Pod is valid and create in the namespace.

Limiting Pod compute resources

The following section discusses how to constrain resources at Pod level.

Without having to delete busybox1 Pod, create the limit-mem-cpu-pod LimitRange in the limitrange-demo namespace:

kubectl apply -f https://k8s.io/examples/admin/resource/limit-mem-cpu-pod.yaml -n limitrange-demo

The limitrange is created and limits CPU to 2 Core and Memory to 2Gi per Pod:

limitrange/limit-mem-cpu-per-pod created

Describe the limit-mem-cpu-per-pod limit object using the following kubectl command:

kubectl describe limitrange/limit-mem-cpu-per-pod

Name:       limit-mem-cpu-per-pod
Namespace:  limitrange-demo
Type        Resource  Min  Max  Default Request  Default Limit  Max Limit/Request Ratio
----        --------  ---  ---  ---------------  -------------  -----------------------
Pod         cpu       -    2    -                -              -
Pod         memory    -    2Gi  -                -              -

Now create the busybox2 Pod:

kubectl apply -f https://k8s.io/examples/admin/resource/limit-range-pod-2.yaml -n limitrange-demo

The busybox2 Pod definition is identical to busybox1 but an error is reported since Pod's resources are now limited:

Error from server (Forbidden): error when creating "limit-range-pod-2.yaml": pods "busybox2" is forbidden: [maximum cpu usage per Pod is 2, but limit is 2400m., maximum memory usage per Pod is 2Gi, but limit is 2306867200.]

kubectl get po/busybox1 -n limitrange-demo -o json | jq ".spec.containers[].resources.limits.memory"
"200Mi"
"900Mi"
"200Mi"
"900Mi"

busybox2 Pod will not be admitted on the cluster since the total memory limit of its container is greater than the limit defined in the LimitRange. busybox1 will not be evicted since it was created and admitted on the cluster before the LimitRange creation.

Limiting Storage resources

You can enforce minimum and maximum size of storage resources that can be requested by each PersistentVolumeClaim in a namespace using a LimitRange:

Apply the YAML using kubectl create:

kubectl create -f https://k8s.io/examples/admin/resource/storagelimits.yaml -n limitrange-demo

limitrange/storagelimits created

Describe the created object:

kubectl describe limits/storagelimits

The output should look like:

Name:                  storagelimits
Namespace:             limitrange-demo
Type                   Resource  Min  Max  Default Request  Default Limit  Max Limit/Request Ratio
----                   --------  ---  ---  ---------------  -------------  -----------------------
PersistentVolumeClaim  storage   1Gi  2Gi  -                -              -

kubectl create -f https://k8s.io/examples/admin/resource//pvc-limit-lower.yaml -n limitrange-demo

While creating a PVC with requests.storage lower than the Min value in the LimitRange, an Error thrown by the server:

Error from server (Forbidden): error when creating "pvc-limit-lower.yaml": persistentvolumeclaims "pvc-limit-lower" is forbidden: minimum storage usage per PersistentVolumeClaim is 1Gi, but request is 500Mi.

Same behaviour is noted if the requests.storage is greater than the Max value in the LimitRange:

kubectl create -f https://k8s.io/examples/admin/resource/pvc-limit-greater.yaml -n limitrange-demo

Error from server (Forbidden): error when creating "pvc-limit-greater.yaml": persistentvolumeclaims "pvc-limit-greater" is forbidden: maximum storage usage per PersistentVolumeClaim is 2Gi, but request is 5Gi.

Limits/Requests Ratio

If LimitRangeItem.maxLimitRequestRatio is specified in the LimitRangeSpec, the named resource must have a request and limit that are both non-zero where limit divided by request is less than or equal to the enumerated value

The following LimitRange enforces memory limit to be at most twice the amount of the memory request for any pod in the namespace.

{{< codenew file="admin/resource/limit-memory-ratio-pod.yaml" >}}

kubectl apply -f https://k8s.io/examples/admin/resource/limit-memory-ratio-pod.yaml

Describe the LimitRange with the following kubectl command:

$ kubectl describe limitrange/limit-memory-ratio-pod

Name:       limit-memory-ratio-pod
Namespace:  limitrange-demo
Type        Resource  Min  Max  Default Request  Default Limit  Max Limit/Request Ratio
----        --------  ---  ---  ---------------  -------------  -----------------------
Pod         memory    -    -    -                -              2

Let's create a pod with requests.memory=100Mi and limits.memory=300Mi:

kubectl apply -f https://k8s.io/examples/admin/resource/limit-range-pod-3.yaml

The pod creation failed as the ratio here (3) is greater than the enforced limit (2) in limit-memory-ratio-pod LimitRange

Error from server (Forbidden): error when creating "limit-range-pod-3.yaml": pods "busybox3" is forbidden: memory max limit to request ratio per Pod is 2, but provided ratio is 3.000000.

Clean up

Delete the limitrange-demo namespace to free all resources:

kubectl delete ns limitrange-demo

Examples

See a tutorial on how to limit compute resources per namespace .
Check how to limit storage consumption.
See a detailed example on quota per namespace.

See LimitRanger design doc for more information.

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