289 lines
12 KiB
Markdown
289 lines
12 KiB
Markdown
---
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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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content_type: task
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---
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<!-- overview -->
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{{< feature-state for_k8s_version="v1.12" state="beta" >}}
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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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## {{% heading "prerequisites" %}}
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{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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<!-- steps -->
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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 policy is set with the `--cpu-manager-policy` kubelet
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flag or the `cpuManagerPolicy` field in [KubeletConfiguration](/docs/reference/config-api/kubelet-config.v1beta1/).
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There are two supported policies:
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* [`none`](#none-policy): the default policy.
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* [`static`](#static-policy): 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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The behavior of the static policy can be fine-tuned using the `--cpu-manager-policy-options` flag.
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The flag takes a comma-separated list of `key=value` policy options.
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This feature can be disabled completely using the `CPUManagerPolicyOptions` feature gate.
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The policy options are split into two groups: alpha quality (hidden by default) and beta quality
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(visible by default). The groups are guarded respectively by the `CPUManagerPolicyAlphaOptions`
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and `CPUManagerPolicyBetaOptions` feature gates. Diverging from the Kubernetes standard, these
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feature gates guard groups of options, because it would have been too cumbersome to add a feature
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gate for each individual option.
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### Changing the CPU Manager Policy
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Since the CPU manger policy can only be applied when kubelet spawns new pods, simply changing from
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"none" to "static" won't apply to existing pods. So in order to properly change the CPU manager
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policy on a node, perform the following steps:
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1. [Drain](/docs/tasks/administer-cluster/safely-drain-node) the node.
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2. Stop kubelet.
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3. Remove the old CPU manager state file. The path to this file is
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`/var/lib/kubelet/cpu_manager_state` by default. This clears the state maintained by the
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CPUManager so that the cpu-sets set up by the new policy won’t conflict with it.
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4. Edit the kubelet configuration to change the CPU manager policy to the desired value.
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5. Start kubelet.
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Repeat this process for every node that needs its CPU manager policy changed. Skipping this
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process will result in kubelet crashlooping with the following error:
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```
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could not restore state from checkpoint: configured policy "static" differs from state checkpoint policy "none", please drain this node and delete the CPU manager checkpoint file "/var/lib/kubelet/cpu_manager_state" before restarting Kubelet
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```
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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/) and
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[Burstable 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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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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CPU Manager doesn't support offlining and onlining of
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CPUs at runtime. Also, if the set of online CPUs changes on the node,
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the node must be drained and CPU manager manually reset by deleting the
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state file `cpu_manager_state` in the kubelet root directory.
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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. From 1.17, the CPU reservation list can be specified
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explicitly by kubelet `--reserved-cpus` option. The explicit CPU list specified by
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`--reserved-cpus` takes precedence over the CPU reservation specified by
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`--kube-reserved` and `--system-reserved`. 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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The kubelet requires a CPU reservation greater than zero be made
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using either `--kube-reserved` and/or `--system-reserved` or `--reserved-cpus` when
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the static 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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#### Static policy options
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You can toggle groups of options on and off based upon their maturity level
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using the following feature gates:
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* `CPUManagerPolicyBetaOptions` default enabled. Disable to hide beta-level options.
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* `CPUManagerPolicyAlphaOptions` default disabled. Enable to show alpha-level options.
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You will still have to enable each option using the `CPUManagerPolicyOptions` kubelet option.
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The following policy options exist for the static `CPUManager` policy:
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* `full-pcpus-only` (beta, visible by default)
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* `distribute-cpus-across-numa` (alpha, hidden by default)
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If the `full-pcpus-only` policy option is specified, the static policy will always allocate full physical cores.
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By default, without this option, the static policy allocates CPUs using a topology-aware best-fit allocation.
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On SMT enabled systems, the policy can allocate individual virtual cores, which correspond to hardware threads.
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This can lead to different containers sharing the same physical cores; this behaviour in turn contributes
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to the [noisy neighbours problem](https://en.wikipedia.org/wiki/Cloud_computing_issues#Performance_interference_and_noisy_neighbors).
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With the option enabled, the pod will be admitted by the kubelet only if the CPU request of all its containers
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can be fulfilled by allocating full physical cores.
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If the pod does not pass the admission, it will be put in Failed state with the message `SMTAlignmentError`.
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If the `distribute-cpus-across-numa`policy option is specified, the static
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policy will evenly distribute CPUs across NUMA nodes in cases where more than
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one NUMA node is required to satisfy the allocation.
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By default, the `CPUManager` will pack CPUs onto one NUMA node until it is
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filled, with any remaining CPUs simply spilling over to the next NUMA node.
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This can cause undesired bottlenecks in parallel code relying on barriers (and
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similar synchronization primitives), as this type of code tends to run only as
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fast as its slowest worker (which is slowed down by the fact that fewer CPUs
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are available on at least one NUMA node).
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By distributing CPUs evenly across NUMA nodes, application developers can more
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easily ensure that no single worker suffers from NUMA effects more than any
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other, improving the overall performance of these types of applications.
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The `full-pcpus-only` option can be enabled by adding `full-pcups-only=true` to
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the CPUManager policy options.
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Likewise, the `distribute-cpus-across-numa` option can be enabled by adding
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`distribute-cpus-across-numa=true` to the CPUManager policy options.
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When both are set, they are "additive" in the sense that CPUs will be
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distributed across NUMA nodes in chunks of full-pcpus rather than individual
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cores.
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