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Scheduler Performance Tuning

FEATURE STATE: Kubernetes v1.14 [beta]

kube-scheduler is the Kubernetes default scheduler. It is responsible for placement of Pods on Nodes in a cluster.

Nodes in a cluster that meet the scheduling requirements of a Pod are called feasible Nodes for the Pod. The scheduler finds feasible Nodes for a Pod and then runs a set of functions to score the feasible Nodes, picking a Node with the highest score among the feasible ones to run the Pod. The scheduler then notifies the API server about this decision in a process called Binding.

This page explains performance tuning optimizations that are relevant for large Kubernetes clusters.

In large clusters, you can tune the scheduler’s behaviour balancing scheduling outcomes between latency (new Pods are placed quickly) and accuracy (the scheduler rarely makes poor placement decisions).

You configure this tuning setting via kube-scheduler setting percentageOfNodesToScore. This KubeSchedulerConfiguration setting determines a threshold for scheduling nodes in your cluster.

Setting the threshold

The percentageOfNodesToScore option accepts whole numeric values between 0 and 100. The value 0 is a special number which indicates that the kube-scheduler should use its compiled-in default. If you set percentageOfNodesToScore above 100, kube-scheduler acts as if you had set a value of 100.

To change the value, edit the kube-scheduler configuration file (this is likely to be /etc/kubernetes/config/kube-scheduler.yaml), then restart the scheduler.

After you have made this change, you can run

kubectl get componentstatuses

to verify that the kube-scheduler component is healthy. The output is similar to:

NAME                 STATUS    MESSAGE             ERROR
controller-manager   Healthy   ok
scheduler            Healthy   ok
...

Node scoring threshold

To improve scheduling performance, the kube-scheduler can stop looking for feasible nodes once it has found enough of them. In large clusters, this saves time compared to a naive approach that would consider every node.

You specify a threshold for how many nodes are enough, as a whole number percentage of all the nodes in your cluster. The kube-scheduler converts this into an integer number of nodes. During scheduling, if the kube-scheduler has identified enough feasible nodes to exceed the configured percentage, the kube-scheduler stops searching for more feasible nodes and moves on to the scoring phase.

How the scheduler iterates over Nodes describes the process in detail.

Default threshold

If you don’t specify a threshold, Kubernetes calculates a figure using a linear formula that yields 50% for a 100-node cluster and yields 10% for a 5000-node cluster. The lower bound for the automatic value is 5%.

This means that, the kube-scheduler always scores at least 5% of your cluster no matter how large the cluster is, unless you have explicitly set percentageOfNodesToScore to be smaller than 5.

If you want the scheduler to score all nodes in your cluster, set percentageOfNodesToScore to 100.

Example

Below is an example configuration that sets percentageOfNodesToScore to 50%.

apiVersion: kubescheduler.config.k8s.io/v1alpha1
kind: KubeSchedulerConfiguration
algorithmSource:
  provider: DefaultProvider

...

percentageOfNodesToScore: 50

Tuning percentageOfNodesToScore

percentageOfNodesToScore must be a value between 1 and 100 with the default value being calculated based on the cluster size. There is also a hardcoded minimum value of 50 nodes.

Note: In clusters with less than 50 feasible nodes, the scheduler still checks all the nodes, simply because there are not enough feasible nodes to stop the scheduler’s search early. In a small cluster, if you set a low value for percentageOfNodesToScore, your change will have no or little effect, for a similar reason. If your cluster has several hundred Nodes or fewer, leave this configuration option at its default value. Making changes is unlikely to improve the scheduler’s performance significantly.

An important detail to consider when setting this value is that when a smaller number of nodes in a cluster are checked for feasibility, some nodes are not sent to be scored for a given Pod. As a result, a Node which could possibly score a higher value for running the given Pod might not even be passed to the scoring phase. This would result in a less than ideal placement of the Pod.

You should avoid setting percentageOfNodesToScore very low so that kube-scheduler does not make frequent, poor Pod placement decisions. Avoid setting the percentage to anything below 10%, unless the scheduler’s throughput is critical for your application and the score of nodes is not important. In other words, you prefer to run the Pod on any Node as long as it is feasible.

How the scheduler iterates over Nodes

This section is intended for those who want to understand the internal details of this feature.

In order to give all the Nodes in a cluster a fair chance of being considered for running Pods, the scheduler iterates over the nodes in a round robin fashion. You can imagine that Nodes are in an array. The scheduler starts from the start of the array and checks feasibility of the nodes until it finds enough Nodes as specified by percentageOfNodesToScore. For the next Pod, the scheduler continues from the point in the Node array that it stopped at when checking feasibility of Nodes for the previous Pod.

If Nodes are in multiple zones, the scheduler iterates over Nodes in various zones to ensure that Nodes from different zones are considered in the feasibility checks. As an example, consider six nodes in two zones:

Zone 1: Node 1, Node 2, Node 3, Node 4
Zone 2: Node 5, Node 6

The Scheduler evaluates feasibility of the nodes in this order:

Node 1, Node 5, Node 2, Node 6, Node 3, Node 4

After going over all the Nodes, it goes back to Node 1.

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