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Reserve Compute Resources for System Daemons

Kubernetes nodes can be scheduled to Capacity. Pods can consume all the available capacity on a node by default. This is an issue because nodes typically run quite a few system daemons that power the OS and Kubernetes itself. Unless resources are set aside for these system daemons, pods and system daemons compete for resources and lead to resource starvation issues on the node.

The kubelet exposes a feature named Node Allocatable that helps to reserve compute resources for system daemons. Kubernetes recommends cluster administrators to configure Node Allocatable based on their workload density on each node.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

Your Kubernetes server must be at or later than version 1.8. To check the version, enter kubectl version.

Your Kubernetes server must be at or later than version 1.17 to use the kubelet command line option --reserved-cpus to set an explicitly reserved CPU list.

Node Allocatable

      Node Capacity
---------------------------
|     kube-reserved       |
|-------------------------|
|     system-reserved     |
|-------------------------|
|    eviction-threshold   |
|-------------------------|
|                         |
|      allocatable        |
|   (available for pods)  |
|                         |
|                         |
---------------------------

Allocatable on a Kubernetes node is defined as the amount of compute resources that are available for pods. The scheduler does not over-subscribe Allocatable. CPU, memory and ephemeral-storage are supported as of now.

Node Allocatable is exposed as part of v1.Node object in the API and as part of kubectl describe node in the CLI.

Resources can be reserved for two categories of system daemons in the kubelet.

Enabling QoS and Pod level cgroups

To properly enforce node allocatable constraints on the node, you must enable the new cgroup hierarchy via the --cgroups-per-qos flag. This flag is enabled by default. When enabled, the kubelet will parent all end-user pods under a cgroup hierarchy managed by the kubelet.

Configuring a cgroup driver

The kubelet supports manipulation of the cgroup hierarchy on the host using a cgroup driver. The driver is configured via the --cgroup-driver flag.

The supported values are the following:

  • cgroupfs is the default driver that performs direct manipulation of the cgroup filesystem on the host in order to manage cgroup sandboxes.
  • systemd is an alternative driver that manages cgroup sandboxes using transient slices for resources that are supported by that init system.

Depending on the configuration of the associated container runtime, operators may have to choose a particular cgroup driver to ensure proper system behavior. For example, if operators use the systemd cgroup driver provided by the docker runtime, the kubelet must be configured to use the systemd cgroup driver.

Kube Reserved

  • Kubelet Flag: --kube-reserved=[cpu=100m][,][memory=100Mi][,][ephemeral-storage=1Gi][,][pid=1000]
  • Kubelet Flag: --kube-reserved-cgroup=

kube-reserved is meant to capture resource reservation for kubernetes system daemons like the kubelet, container runtime, node problem detector, etc. It is not meant to reserve resources for system daemons that are run as pods. kube-reserved is typically a function of pod density on the nodes.

In addition to cpu, memory, and ephemeral-storage, pid may be specified to reserve the specified number of process IDs for kubernetes system daemons.

To optionally enforce kube-reserved on system daemons, specify the parent control group for kube daemons as the value for --kube-reserved-cgroup kubelet flag.

It is recommended that the kubernetes system daemons are placed under a top level control group (runtime.slice on systemd machines for example). Each system daemon should ideally run within its own child control group. Refer to this doc for more details on recommended control group hierarchy.

Note that Kubelet does not create --kube-reserved-cgroup if it doesn’t exist. Kubelet will fail if an invalid cgroup is specified.

System Reserved

  • Kubelet Flag: --system-reserved=[cpu=100m][,][memory=100Mi][,][ephemeral-storage=1Gi][,][pid=1000]
  • Kubelet Flag: --system-reserved-cgroup=

system-reserved is meant to capture resource reservation for OS system daemons like sshd, udev, etc. system-reserved should reserve memory for the kernel too since kernel memory is not accounted to pods in Kubernetes at this time. Reserving resources for user login sessions is also recommended (user.slice in systemd world).

In addition to cpu, memory, and ephemeral-storage, pid may be specified to reserve the specified number of process IDs for OS system daemons.

To optionally enforce system-reserved on system daemons, specify the parent control group for OS system daemons as the value for --system-reserved-cgroup kubelet flag.

It is recommended that the OS system daemons are placed under a top level control group (system.slice on systemd machines for example).

Note that Kubelet does not create --system-reserved-cgroup if it doesn’t exist. Kubelet will fail if an invalid cgroup is specified.

Explicitly Reserved CPU List

FEATURE STATE: Kubernetes v1.17 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.
  • Kubelet Flag: --reserved-cpus=0-3

reserved-cpus is meant to define an explicit CPU set for OS system daemons and kubernetes system daemons. reserved-cpus is for systems that do not intend to define separate top level cgroups for OS system daemons and kubernetes system daemons with regard to cpuset resource. If the Kubelet does not have --system-reserved-cgroup and --kube-reserved-cgroup, the explicit cpuset provided by reserved-cpus will take precedence over the CPUs defined by --kube-reserved and --system-reserved options.

This option is specifically designed for Telco/NFV use cases where uncontrolled interrupts/timers may impact the workload performance. you can use this option to define the explicit cpuset for the system/kubernetes daemons as well as the interrupts/timers, so the rest CPUs on the system can be used exclusively for workloads, with less impact from uncontrolled interrupts/timers. To move the system daemon, kubernetes daemons and interrupts/timers to the explicit cpuset defined by this option, other mechanism outside Kubernetes should be used. For example: in Centos, you can do this using the tuned toolset.

Eviction Thresholds

  • Kubelet Flag: --eviction-hard=[memory.available<500Mi]

Memory pressure at the node level leads to System OOMs which affects the entire node and all pods running on it. Nodes can go offline temporarily until memory has been reclaimed. To avoid (or reduce the probability of) system OOMs kubelet provides Out of Resource management. Evictions are supported for memory and ephemeral-storage only. By reserving some memory via --eviction-hard flag, the kubelet attempts to evict pods whenever memory availability on the node drops below the reserved value. Hypothetically, if system daemons did not exist on a node, pods cannot use more than capacity - eviction-hard. For this reason, resources reserved for evictions are not available for pods.

Enforcing Node Allocatable

  • Kubelet Flag: --enforce-node-allocatable=pods[,][system-reserved][,][kube-reserved]

The scheduler treats Allocatable as the available capacity for pods.

kubelet enforce Allocatable across pods by default. Enforcement is performed by evicting pods whenever the overall usage across all pods exceeds Allocatable. More details on eviction policy can be found here. This enforcement is controlled by specifying pods value to the kubelet flag --enforce-node-allocatable.

Optionally, kubelet can be made to enforce kube-reserved and system-reserved by specifying kube-reserved & system-reserved values in the same flag. Note that to enforce kube-reserved or system-reserved, --kube-reserved-cgroup or --system-reserved-cgroup needs to be specified respectively.

General Guidelines

System daemons are expected to be treated similar to Guaranteed pods. System daemons can burst within their bounding control groups and this behavior needs to be managed as part of kubernetes deployments. For example, kubelet should have its own control group and share Kube-reserved resources with the container runtime. However, Kubelet cannot burst and use up all available Node resources if kube-reserved is enforced.

Be extra careful while enforcing system-reserved reservation since it can lead to critical system services being CPU starved, OOM killed, or unable to fork on the node. The recommendation is to enforce system-reserved only if a user has profiled their nodes exhaustively to come up with precise estimates and is confident in their ability to recover if any process in that group is oom_killed.

  • To begin with enforce Allocatable on pods.
  • Once adequate monitoring and alerting is in place to track kube system daemons, attempt to enforce kube-reserved based on usage heuristics.
  • If absolutely necessary, enforce system-reserved over time.

The resource requirements of kube system daemons may grow over time as more and more features are added. Over time, kubernetes project will attempt to bring down utilization of node system daemons, but that is not a priority as of now. So expect a drop in Allocatable capacity in future releases.

Example Scenario

Here is an example to illustrate Node Allocatable computation:

  • Node has 32Gi of memory, 16 CPUs and 100Gi of Storage
  • --kube-reserved is set to cpu=1,memory=2Gi,ephemeral-storage=1Gi
  • --system-reserved is set to cpu=500m,memory=1Gi,ephemeral-storage=1Gi
  • --eviction-hard is set to memory.available<500Mi,nodefs.available<10%

Under this scenario, Allocatable will be 14.5 CPUs, 28.5Gi of memory and 88Gi of local storage. Scheduler ensures that the total memory requests across all pods on this node does not exceed 28.5Gi and storage doesn’t exceed 88Gi. Kubelet evicts pods whenever the overall memory usage across pods exceeds 28.5Gi, or if overall disk usage exceeds 88Gi If all processes on the node consume as much CPU as they can, pods together cannot consume more than 14.5 CPUs.

If kube-reserved and/or system-reserved is not enforced and system daemons exceed their reservation, kubelet evicts pods whenever the overall node memory usage is higher than 31.5Gi or storage is greater than 90Gi

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