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Merge pull request #39118 from dshebib/qos_detail
Clarify QOS class effect
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‎content/en/docs/concepts/configuration/manage-resources-containers.md

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and its [resource requirements](/docs/reference/kubernetes-api/workload-resources/pod-v1/#resources)
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* Read about [project quotas](https://xfs.org/index.php/XFS_FAQ#Q:_Quota:_Do_quotas_work_on_XFS.3F) in XFS
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* Read more about the [kube-scheduler configuration reference (v1beta3)](/docs/reference/config-api/kube-scheduler-config.v1beta3/)
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* Read more about [Quality of Service classes for Pods](/docs/concepts/workloads/pods/pod-quality/)
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‎content/en/docs/concepts/workloads/pods/pod-qos.md

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---
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title: Pod Quality of Service Classes
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content_type: concept
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weight: 30
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---
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<!-- overview -->
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This page introduces _Quality of Service (QoS) classes_ in Kubernetes, and explains
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how Kubernetes assigns a QoS class to each Pods as a consequence of the resource
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constraints that you specify for the containers in that Pod. Kubernetes relies on this
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classification to make decisions about which Pods to evict when there are not enough
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available resources on a Node.
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<!-- body -->
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## Quality of Service classes
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Kubernetes classifies the Pods that you run and allocates each Pod into a specific
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_quality of service (QoS) class_. Kubernetes uses that classification to influence how different
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pods are handled. Kubernetes does this classification based on the
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[resource requests](/docs/concepts/configuration/manage-resources-containers/)
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of the {{< glossary_tooltip text="Containers" term_id="container" >}} in that Pod, along with
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how those requests relate to resource limits.
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This is known as {{< glossary_tooltip text="Quality of Service" term_id="qos-class" >}}
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(QoS) class. Kubernetes assigns every Pod a QoS class based on the resource requests
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and limits of its component Containers. QoS classes are used by Kubernetes to decide
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which Pods to evict from a Node experiencing
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[Node Pressure](/docs/concepts/scheduling-eviction/node-pressure-eviction/). The possible
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QoS classes are `Guaranteed`, `Burstable`, and `BestEffort`. When a Node runs out of resources,
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Kubernetes will first evict `BestEffort` Pods running on that Node, followed by `Burstable` and
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finally `Guaranteed` Pods. When this eviction is due to resource pressure, only Pods exceeding
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resource requests are candidates for eviction.
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### Guaranteed
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Pods that are `Guaranteed` have the strictest resource limits and are least likely
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to face eviction. They are guaranteed not to be killed until they exceed their limits
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or there are no lower-priority Pods that can be preempted from the Node. They may
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not acquire resources beyond their specified limits. These Pods can also make
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use of exclusive CPUs using the
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[`static`](/docs/tasks/administer-cluster/cpu-management-policies/#static-policy) CPU management policy.
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#### Criteria
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For a Pod to be given a QoS class of `Guaranteed`:
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* Every Container in the Pod must have a memory limit and a memory request.
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* For every Container in the Pod, the memory limit must equal the memory request.
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* Every Container in the Pod must have a CPU limit and a CPU request.
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* For every Container in the Pod, the CPU limit must equal the CPU request.
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### Burstable
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Pods that are `Burstable` have some lower-bound resource guarantees based on the request, but
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do not require a specific limit. If a limit is not specified, it defaults to a
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limit equivalent to the capacity of the Node, which allows the Pods to flexibly increase
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their resources if resources are available. In the event of Pod eviction due to Node
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resource pressure, these Pods are evicted only after all `BestEffort` Pods are evicted.
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Because a `Burstable` Pod can include a Container that has no resource limits or requests, a Pod
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that is `Burstable` can try to use any amount of node resources.
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#### Criteria
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A Pod is given a QoS class of `Burstable` if:
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* The Pod does not meet the criteria for QoS class `Guaranteed`.
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* At least one Container in the Pod has a memory or CPU request or limit.
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### BestEffort
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Pods in the `BestEffort` QoS class can use node resources that aren't specifically assigned
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to Pods in other QoS classes. For example, if you have a node with 16 CPU cores available to the
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kubelet, and you assign assign 4 CPU cores to a `Guaranteed` Pod, then a Pod in the `BestEffort`
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QoS class can try to use any amount of the remaining 12 CPU cores.
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The kubelet prefers to evict `BestEffort` Pods if the node comes under resource pressure.
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#### Criteria
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A Pod has a QoS class of `BestEffort` if it doesn't meet the criteria for either `Guaranteed`
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or `Burstable`. In other words, a Pod is `BestEffort` only if none of the Containers in the Pod have a
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memory limit or a memory request, and none of the Containers in the Pod have a
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CPU limit or a CPU request.
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Containers in a Pod can request other resources (not CPU or memory) and still be classified as
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`BestEffort`.
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## Some behavior is independent of QoS class {#class-independent-behavior}
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Certain behavior is independent of the QoS class assigned by Kubernetes. For example:
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* Any Container exceeding a resource limit will be killed and restarted by the kubelet without
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affecting other Containers in that Pod.
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* If a Container exceeds its resource request and the node it runs on faces
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resource pressure, the Pod it is in becomes a candidate for [eviction](/docs/concepts/scheduling-eviction/node-pressure-eviction/).
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If this occurs, all Containers in the Pod will be terminated. Kubernetes may create a
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replacement Pod, usually on a different node.
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* The resource request of a Pod is equal to the sum of the resource requests of
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its component Containers, and the resource limit of a Pod is equal to the sum of
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the resource limits of its component Containers.
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* The kube-scheduler does not consider QoS class when selecting which Pods to
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[preempt](/docs/concepts/scheduling-eviction/pod-priority-preemption/#preemption).
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Preemption can occur when a cluster does not have enough resources to run all the Pods
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you defined.
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## {{% heading "whatsnext" %}}
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* Learn about [resource management for Pods and Containers](/docs/concepts/configuration/manage-resources-containers/).
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* Learn about [Node-pressure eviction](/docs/concepts/scheduling-eviction/node-pressure-eviction/).
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* Learn about [Pod priority and preemption](/docs/concepts/scheduling-eviction/pod-priority-preemption/).
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* Learn about [Pod disruptions](/docs/concepts/workload/pods/disruptions/).
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* Learn how to [assign memory resources to containers and pods](/docs/tasks/configure-pod-container/assign-memory-resource/).
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* Learn how to [assign CPU resources to containers and pods](/docs/tasks/configure-pod-container/assign-cpu-resource/).
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* Learn how to [configure Quality of Service for Pods](/docs/tasks/configure-pod-container/quality-service-pod/).

‎content/en/docs/reference/glossary/qos-class.md

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title: QoS Class
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id: qos-class
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date: 2019-04-15
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full_link:
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full_link: /docs/concepts/workloads/pods/pod-quality/
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short_description: >
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QoS Class (Quality of Service Class) provides a way for Kubernetes to classify pods within the cluster into several classes and make decisions about scheduling and eviction.
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‎content/en/docs/tasks/configure-pod-container/quality-service-pod.md

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<!-- overview -->
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This page shows how to configure Pods so that they will be assigned particular
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Quality of Service (QoS) classes. Kubernetes uses QoS classes to make decisions about
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scheduling and evicting Pods.
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Quality of Service (QoS) classes. Kubernetes uses QoS classes to make decisions about evicting Pods when Node resources are exceeded.
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