215 lines
9.0 KiB
Markdown
215 lines
9.0 KiB
Markdown
---
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assignees:
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- derekwaynecarr
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- janetkuo
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---
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By default, pods run with unbounded CPU and memory limits. This means that any pod in the
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system will be able to consume as much CPU and memory on the node that executes the pod.
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Users may want to impose restrictions on the amount of resources a single pod in the system may consume
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for a variety of reasons.
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For example:
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1. Each node in the cluster has 2GB of memory. The cluster operator does not want to accept pods
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that require more than 2GB of memory since no node in the cluster can support the requirement. To prevent a
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pod from being permanently unscheduled to a node, the operator instead chooses to reject pods that exceed 2GB
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of memory as part of admission control.
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2. A cluster is shared by two communities in an organization that runs production and development workloads
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respectively. Production workloads may consume up to 8GB of memory, but development workloads may consume up
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to 512MB of memory. The cluster operator creates a separate namespace for each workload, and applies limits to
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each namespace.
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3. Users may create a pod which consumes resources just below the capacity of a machine. The left over space
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may be too small to be useful, but big enough for the waste to be costly over the entire cluster. As a result,
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the cluster operator may want to set limits that a pod must consume at least 20% of the memory and cpu of their
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average node size in order to provide for more uniform scheduling and to limit waste.
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This example demonstrates how limits can be applied to a Kubernetes [namespace](/docs/admin/namespaces/walkthrough/) to control
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min/max resource limits per pod. In addition, this example demonstrates how you can
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apply default resource limits to pods in the absence of an end-user specified value.
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See [LimitRange design doc](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/admission_control_limit_range.md) for more information. For a detailed description of the Kubernetes resource model, see [Resources](/docs/user-guide/compute-resources/)
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## Step 0: Prerequisites
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This example requires a running Kubernetes cluster. See the [Getting Started guides](/docs/getting-started-guides/) for how to get started.
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Change to the `<kubernetes>` directory if you're not already there.
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## Step 1: Create a namespace
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This example will work in a custom namespace to demonstrate the concepts involved.
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Let's create a new namespace called limit-example:
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```shell
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$ kubectl create namespace limit-example
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namespace "limit-example" created
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```
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Note that `kubectl` commands will print the type and name of the resource created or mutated, which can then be used in subsequent commands:
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```shell
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$ kubectl get namespaces
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NAME STATUS AGE
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default Active 51s
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limit-example Active 45s
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```
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## Step 2: Apply a limit to the namespace
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Let's create a simple limit in our namespace.
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```shell
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$ kubectl create -f docs/admin/limitrange/limits.yaml --namespace=limit-example
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limitrange "mylimits" created
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```
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Let's describe the limits that we have imposed in our namespace.
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```shell
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$ kubectl describe limits mylimits --namespace=limit-example
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Name: mylimits
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Namespace: limit-example
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Type Resource Min Max Default Request Default Limit Max Limit/Request Ratio
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---- -------- --- --- --------------- ------------- -----------------------
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Pod cpu 200m 2 - - -
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Pod memory 6Mi 1Gi - - -
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Container cpu 100m 2 200m 300m -
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Container memory 3Mi 1Gi 100Mi 200Mi -
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```
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In this scenario, we have said the following:
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1. If a max constraint is specified for a resource (2 CPU and 1Gi memory in this case), then a limit
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must be specified for that resource across all containers. Failure to specify a limit will result in
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a validation error when attempting to create the pod. Note that a default value of limit is set by
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*default* in file `limits.yaml` (300m CPU and 200Mi memory).
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2. If a min constraint is specified for a resource (100m CPU and 3Mi memory in this case), then a
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request must be specified for that resource across all containers. Failure to specify a request will
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result in a validation error when attempting to create the pod. Note that a default value of request is
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set by *defaultRequest* in file `limits.yaml` (200m CPU and 100Mi memory).
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3. For any pod, the sum of all containers memory requests must be >= 6Mi and the sum of all containers
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memory limits must be <= 1Gi; the sum of all containers CPU requests must be >= 200m and the sum of all
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containers CPU limits must be <= 2.
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## Step 3: Enforcing limits at point of creation
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The limits enumerated in a namespace are only enforced when a pod is created or updated in
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the cluster. If you change the limits to a different value range, it does not affect pods that
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were previously created in a namespace.
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If a resource (cpu or memory) is being restricted by a limit, the user will get an error at time
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of creation explaining why.
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Let's first spin up a [Deployment](/docs/user-guide/deployments) that creates a single container Pod to demonstrate
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how default values are applied to each pod.
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```shell
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$ kubectl run nginx --image=nginx --replicas=1 --namespace=limit-example
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deployment "nginx" created
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```
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Note that `kubectl run` creates a Deployment named "nginx" on Kubernetes cluster >= v1.2. If you are running older versions, it creates replication controllers instead.
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If you want to obtain the old behavior, use `--generator=run/v1` to create replication controllers. See [`kubectl run`](/docs/user-guide/kubectl/kubectl_run/) for more details.
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The Deployment manages 1 replica of single container Pod. Let's take a look at the Pod it manages. First, find the name of the Pod:
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```shell
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$ kubectl get pods --namespace=limit-example
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NAME READY STATUS RESTARTS AGE
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nginx-2040093540-s8vzu 1/1 Running 0 11s
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```
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Let's print this Pod with yaml output format (using `-o yaml` flag), and then `grep` the `resources` field. Note that your pod name will be different.
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```shell
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$ kubectl get pods nginx-2040093540-s8vzu --namespace=limit-example -o yaml | grep resources -C 8
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resourceVersion: "57"
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selfLink: /api/v1/namespaces/limit-example/pods/nginx-2040093540-ivimu
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uid: 67b20741-f53b-11e5-b066-64510658e388
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spec:
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containers:
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- image: nginx
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imagePullPolicy: Always
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name: nginx
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resources:
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limits:
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cpu: 300m
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memory: 200Mi
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requests:
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cpu: 200m
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memory: 100Mi
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terminationMessagePath: /dev/termination-log
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volumeMounts:
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```
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Note that our nginx container has picked up the namespace default cpu and memory resource *limits* and *requests*.
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Let's create a pod that exceeds our allowed limits by having it have a container that requests 3 cpu cores.
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```shell
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$ kubectl create -f docs/admin/limitrange/invalid-pod.yaml --namespace=limit-example
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Error from server: error when creating "docs/admin/limitrange/invalid-pod.yaml": Pod "invalid-pod" is forbidden: [Maximum cpu usage per Pod is 2, but limit is 3., Maximum cpu usage per Container is 2, but limit is 3.]
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```
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Let's create a pod that falls within the allowed limit boundaries.
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```shell
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$ kubectl create -f docs/admin/limitrange/valid-pod.yaml --namespace=limit-example
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pod "valid-pod" created
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```
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Now look at the Pod's resources field:
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```shell
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$ kubectl get pods valid-pod --namespace=limit-example -o yaml | grep -C 6 resources
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uid: 3b1bfd7a-f53c-11e5-b066-64510658e388
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spec:
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containers:
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- image: gcr.io/google_containers/serve_hostname
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imagePullPolicy: Always
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name: kubernetes-serve-hostname
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resources:
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limits:
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cpu: "1"
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memory: 512Mi
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requests:
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cpu: "1"
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memory: 512Mi
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```
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Note that this pod specifies explicit resource *limits* and *requests* so it did not pick up the namespace
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default values.
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Note: The *limits* for CPU resource are enforced in the default Kubernetes setup on the physical node
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that runs the container unless the administrator deploys the kubelet with the folllowing flag:
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```shell
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$ kubelet --help
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Usage of kubelet
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....
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--cpu-cfs-quota[=true]: Enable CPU CFS quota enforcement for containers that specify CPU limits
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$ kubelet --cpu-cfs-quota=false ...
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```
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## Step 4: Cleanup
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To remove the resources used by this example, you can just delete the limit-example namespace.
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```shell
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$ kubectl delete namespace limit-example
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namespace "limit-example" deleted
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$ kubectl get namespaces
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NAME STATUS AGE
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default Active 12m
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```
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## Summary
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Cluster operators that want to restrict the amount of resources a single container or pod may consume
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are able to define allowable ranges per Kubernetes namespace. In the absence of any explicit assignments,
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the Kubernetes system is able to apply default resource *limits* and *requests* if desired in order to
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constrain the amount of resource a pod consumes on a node.
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