Convert site to Hugo (#8316)
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Bjørn Erik Pedersen
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---
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title: Control CPU Management Policies on the Node
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reviewers:
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- sjenning
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- ConnorDoyle
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- balajismaniam
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---
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{{< feature-state state="beta" >}}
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{{< toc >}}
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Kubernetes keeps many aspects of how pods execute on nodes abstracted
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from the user. This is by design. However, some workloads require
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stronger guarantees in terms of latency and/or performance in order to operate
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acceptably. The kubelet provides methods to enable more complex workload
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placement policies while keeping the abstraction free from explicit placement
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directives.
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## CPU Management Policies
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By default, the kubelet uses [CFS quota](https://en.wikipedia.org/wiki/Completely_Fair_Scheduler)
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to enforce pod CPU limits. When the node runs many CPU-bound pods,
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the workload can move to different CPU cores depending on
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whether the pod is throttled and which CPU cores are available at
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scheduling time. Many workloads are not sensitive to this migration and thus
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work fine without any intervention.
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However, in workloads where CPU cache affinity and scheduling latency
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significantly affect workload performance, the kubelet allows alternative CPU
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management policies to determine some placement preferences on the node.
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### Configuration
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The CPU Manager is an alpha feature in Kubernetes v1.8. It was enabled by
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default as a beta feature since v1.10.
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The CPU Manager policy is set with the `--cpu-manager-policy` kubelet
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option. There are two supported policies:
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* `none`: the default, which represents the existing scheduling behavior.
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* `static`: allows pods with certain resource characteristics to be
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granted increased CPU affinity and exclusivity on the node.
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The CPU manager periodically writes resource updates through the CRI in
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order to reconcile in-memory CPU assignments with cgroupfs. The reconcile
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frequency is set through a new Kubelet configuration value
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`--cpu-manager-reconcile-period`. If not specified, it defaults to the same
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duration as `--node-status-update-frequency`.
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### None policy
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The `none` policy explicitly enables the existing default CPU
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affinity scheme, providing no affinity beyond what the OS scheduler does
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automatically. Limits on CPU usage for
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[Guaranteed pods](/docs/tasks/configure-pod-container/quality-service-pod/)
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are enforced using CFS quota.
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### Static policy
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The `static` policy allows containers in `Guaranteed` pods with integer CPU
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`requests` access to exclusive CPUs on the node. This exclusivity is enforced
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using the [cpuset cgroup controller](https://www.kernel.org/doc/Documentation/cgroup-v1/cpusets.txt).
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{{< note >}}
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**Note:** System services such as the container runtime and the kubelet itself can continue to run on these exclusive CPUs. The exclusivity only extends to other pods.
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{{< /note >}}
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{{< note >}}
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**Note:** The alpha version of this policy does not guarantee static
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exclusive allocations across Kubelet restarts.
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{{< /note >}}
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This policy manages a shared pool of CPUs that initially contains all CPUs in the
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node. The amount of exclusively allocatable CPUs is equal to the total
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number of CPUs in the node minus any CPU reservations by the kubelet `--kube-reserved` or
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`--system-reserved` options. CPUs reserved by these options are taken, in
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integer quantity, from the initial shared pool in ascending order by physical
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core ID. This shared pool is the set of CPUs on which any containers in
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`BestEffort` and `Burstable` pods run. Containers in `Guaranteed` pods with fractional
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CPU `requests` also run on CPUs in the shared pool. Only containers that are
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both part of a `Guaranteed` pod and have integer CPU `requests` are assigned
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exclusive CPUs.
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{{< note >}}
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**Note:** The kubelet requires a CPU reservation greater than zero be made
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using either `--kube-reserved` and/or `--system-reserved` when the static
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policy is enabled. This is because zero CPU reservation would allow the shared
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pool to become empty.
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{{< /note >}}
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As `Guaranteed` pods whose containers fit the requirements for being statically
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assigned are scheduled to the node, CPUs are removed from the shared pool and
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placed in the cpuset for the container. CFS quota is not used to bound
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the CPU usage of these containers as their usage is bound by the scheduling domain
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itself. In others words, the number of CPUs in the container cpuset is equal to the integer
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CPU `limit` specified in the pod spec. This static assignment increases CPU
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affinity and decreases context switches due to throttling for the CPU-bound
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workload.
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Consider the containers in the following pod specs:
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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```
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This pod runs in the `BestEffort` QoS class because no resource `requests` or
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`limits` are specified. It runs in the shared pool.
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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resources:
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limits:
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memory: "200Mi"
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requests:
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memory: "100Mi"
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```
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This pod runs in the `Burstable` QoS class because resource `requests` do not
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equal `limits` and the `cpu` quantity is not specified. It runs in the shared
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pool.
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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resources:
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limits:
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memory: "200Mi"
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cpu: "2"
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requests:
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memory: "100Mi"
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cpu: "1"
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```
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This pod runs in the `Burstable` QoS class because resource `requests` do not
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equal `limits`. It runs in the shared pool.
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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resources:
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limits:
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memory: "200Mi"
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cpu: "2"
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requests:
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memory: "200Mi"
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cpu: "2"
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```
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This pod runs in the `Guaranteed` QoS class because `requests` are equal to `limits`.
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And the container's resource limit for the CPU resource is an integer greater than
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or equal to one. The `nginx` container is granted 2 exclusive CPUs.
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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resources:
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limits:
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memory: "200Mi"
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cpu: "1.5"
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requests:
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memory: "200Mi"
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cpu: "1.5"
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```
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This pod runs in the `Guaranteed` QoS class because `requests` are equal to `limits`.
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But the container's resource limit for the CPU resource is a fraction. It runs in
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the shared pool.
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```yaml
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spec:
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containers:
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- name: nginx
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image: nginx
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resources:
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limits:
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memory: "200Mi"
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cpu: "2"
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```
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This pod runs in the `Guaranteed` QoS class because only `limits` are specified
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and `requests` are set equal to `limits` when not explicitly specified. And the
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container's resource limit for the CPU resource is an integer greater than or
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equal to one. The `nginx` container is granted 2 exclusive CPUs.
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