A Brief Analysis on the Implementation of the Kubernetes Scheduler

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By Xiao Yuan

Kubernetes Scheduler

Kubernetes is a container-based distributed scheduler that implements its own scheduling module.

In a Kubernetes cluster, the scheduler runs as an independent module through the Pod. This article describes the Kubernetes scheduler from several perspectives.

How the Scheduler Works

The scheduler in Kubernetes runs as an independent component (usually in a master) and the number of schedulers remains consistent with that of masters. An instance is selected to work as a Leader through the Raft protocol and the other instances are backup instances. When the master fails, the Raft protocol selects a new master from the other instances.

The scheduler basically works like this:

  • The scheduler maintains a scheduled podQueue and listens to the APIServer.
  • When we create a Pod, we first write Pod metadata to etcd through the APIServer.
  • The scheduler listens to the Pod status through Informer. When a new Pod is added, the Pod is added to the podQueue.
  • The main process continuously extracts Pods from the podQueue and assigns nodes to Pods.
  • The scheduling process consists of two steps: Filter matching nodes and prioritize these nodes based on Pod configuration (for example, by metrics like resource usage and affinity) to score nodes and select the node with the highest score.
  • After a node is assigned successfully, invoke the binding pod interface of the apiServer and set pod.Spec.NodeName to the assigned pod.
  • The kubelet on the node also listens to the ApiServer. If it finds that a new Pod is scheduled to that node, the local dockerDaemon is invoked to run the container.
  • If the scheduler fails to schedule a Pod, if priority and preemption is enabled, first a preemption attempt is made, Pods with low priority on the node are deleted and Pods to be scheduled will be scheduled to the node. If the preemption is not enabled or the preemption attempt fails, related information will be recorded in logs and Pods will be added to the end of the podQueue.

Implementation Details

kube-scheduling is a component that runs independently and is mainly in charge of running functions.

This involves several things:

  • Initialize a Scheduler instance sched, pass various Informers and set up a listener for a target resource and register the handler, such as maintaining podQuene
  • Register the “events” component and set logs.
  • Register the HTTP/HTTPS listeners to provide health checks and metric requests.
  • Run sched.run(), the main entry of the scheduled content. If --leader-elect is set to true, multiple instances are started and a Leader will be selected through the Raft protocol. An instance will run sched.run only when it is selected as the master.
The core content of a scheduling task lies in the `sched.run()` function, which will start a "go routine" to continuously run `sched.scheduleOne`, each run representing a scheduling recurrence.func (sched *Scheduler) Run() {
if ! sched.config.WaitForCacheSync() {
go wait.Until(sched.scheduleOne, 0, sched.config.StopEverything)

Let’s see what sched.scheduleOne mainly does.

func (sched *Scheduler) scheduleOne() {
pod := sched.config.NextPod()
.... // do some pre check
scheduleResult, err := sched.schedule(pod)
if err ! = nil {
if fitError, ok := err.(*core.FitError); ok {
if ! util.PodPriorityEnabled() || sched.config.DisablePreemption {
..... // do some log
} else {
sched.preempt(pod, fitError)
// Assume volumes first before assuming the pod.
allBound, err := sched.assumeVolumes(assumedPod, scheduleResult.SuggestedHost)
fo func() {
// Bind volumes first before Pod
if ! allBound {
err := sched.bindVolumes(assumedPod)
if err ! = nil {
klog.Errorf("error binding volumes: %v", err)
err := sched.bind(assumedPod, &v1. Binding{
ObjectMeta: metav1. ObjectMeta{Namespace: assumedPod.Namespace, Name: assumedPod.Name, UID: assumedPod.UID},
Target: v1. ObjectReference{
Kind: "Node",
Name: scheduleResult.SuggestedHost,

Several things are done in sched.scheduleOne.

  • Pods are extracted from podQuene through sched.config.NextPod().
  • sched.schedule is run to make a scheduling attempt.
  • If the scheduling fails and the preemption feature is enabled, sched.preempt starts a preemption attempt and evicts some Pods to reserve space for the scheduled pods. The preemption will take effect in the next scheduling instance.
  • If the scheduling is successful, interface binding will start. Before the interface binding, the PVC declared in the pod volume will be provisioned.

sched.schedule is the main Pod scheduling logic.

func (g *genericScheduler) Schedule(pod *v1. Pod, nodeLister algorithm.NodeLister) (result ScheduleResult, err error) {
// Get node list
nodes, err := nodeLister.List()
// Filter
filteredNodes, failedPredicateMap, err := g.findNodesThatFit(pod, nodes)
if err ! = nil {
return result, err
// Priority
priorityList, err := PrioritizeNodes(pod, g.cachedNodeInfoMap, metaPrioritiesInterface, g.prioritizers, filteredNodes, g.extenders)
if err ! = nil {
return result, err

// SelectHost
host, err := g.selectHost(priorityList)
return ScheduleResult{
SuggestedHost: host,
EvaluatedNodes: len(filteredNodes) + len(failedPredicateMap),
FeasibleNodes: len(filteredNodes),
}, err

A scheduling task is completed in three steps:

  • Filters: Filter nodes that do not meet the specified conditions.
  • PrioritizeNodes: Score matching nodes and obtain a final score list called priorityList.
  • selectHost: Select a group of nodes with the highest score from the priorityList and then select the most appropriate node by using the round-robin algorithm.
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Next, let’s take a closer look at the three steps.


Filters are relatively easy. By default, the scheduler registers a series of predicates. In the scheduling process, the predicate of each node is invoked in parallel. Then, a node list is obtained, containing nodes that meet the specified conditions.

func (g *genericScheduler) findNodesThatFit(pod *v1. Pod, nodes []*v1. Node) ([]*v1. Node, FailedPredicateMap, error) {
if len(g.predicates) == 0 {
filtered = nodes
} else {
allNodes := int32(g.cache.NodeTree(). NumNodes())
numNodesToFind := g.numFeasibleNodesToFind(allNodes)
checkNode := func(i int) {
nodeName := g.cache.NodeTree(). Next()
// All the predicates of this node are invoked at this point.
fits, failedPredicates, err := podFitsOnNode(
if fits {
length := atomic.AddInt32(&filteredLen, 1)
if length > numNodesToFind {
// If enough current nodes meet the conditions, the calculation will stop.
atomic.AddInt32(&filteredLen, -1)
} else {
filtered[length-1] = g.cachedNodeInfoMap[nodeName]. Node()
// Invoke the checkNode method in parallel.
workqueue.ParallelizeUntil(ctx, 16, int(allNodes), checkNode)
filtered = filtered[:filteredLen]
return filtered, failedPredicateMap, nil

Note that the FeasibleNodes mechanism was introduced in Kubernetes 1.13 to improving the performance for scheduling large-scale clusters. With this feature, we can set the node scoring percentage in the filter process (50% by default) by using the bad-percentage-of-nodes-to-score parameter. When the number of nodes exceeds 100, filtering will stop once the matching nodes exceed this percentage, and the calculation will not be performed on all the nodes.

For example, if the total number of nodes is 1,000 and the percentage is 30%, the scheduler only needs to find 300 matching nodes in the filtering process. When 300 matching nodes are found, the filtering process will stop. This makes it unnecessary to filter all the nodes and reduces the nodes to be prioritized. However, this also has a shortcoming: The Pod may be not scheduled to the most appropriate node.


Pod priority allows scoring nodes that meet the conditions and helps find the most appropriate node for a Pod. The scheduler also registers a series of Priority methods. This is the data structure of the Priority object:

// PriorityConfig is a config used for a priority function.
type PriorityConfig struct {
Name string
Map PriorityMapFunction
Reduce PriorityReduceFunction
// TODO: Remove it after migrating all functions to
// Map-Reduce pattern.
Function PriorityFunction
Weight int

Each PriorityConfig represents a scoring metric and takes account of factors like service balance and resource assignment for nodes. The main scoring process for a PriorityConfig includes Map and Reduce.

  • Map calculates the scores of individual nodes.
  • Reduce processes scores of all nodes in the current PriorityConfig again.

After all PriorityConfigs are calculated, the result for each PriorityConfig is multiplied by the corresponding weight to perform aggregation again by nodes.

workqueue.ParallelizeUntil(context.TODO(), 16, len(nodes), func(index int) {
nodeInfo := nodeNameToInfo[nodes[index]. Name]
for i := range priorityConfigs {
var err error
results[i][index], err = priorityConfigs[i]. Map(pod, meta, nodeInfo)
for i := range priorityConfigs {
go func(index int) {
defer wg.Done()
if err := priorityConfigs[index]. Reduce(pod, meta, nodeNameToInfo, results[index]);
// Summarize all scores.
result := make(schedulerapi.HostPriorityList, 0, len(nodes))
for i := range nodes {
result = append(result, schedulerapi.HostPriority{Host: nodes[i]. Name, Score: 0})
for j := range priorityConfigs {
result[i]. Score += results[j][i]. Score * priorityConfigs[j]. Weight

In addition, both the Filter and Prioritize support the invocation to the extener scheduler, which is not further described in this article.

Status Quo

Currently the main scheduling method of the kubernetes scheduler is Pod-by-Pod. This is also one of the current shortcomings of the scheduler. The main performance bottlenecks are as follows:

  • The Kubernetes scheduler currently evaluates each Pod for all nodes. Pod scheduling is extremely slow if a cluster is large and contains many nodes. This is the problem that percentage-of-nodes-to-score tries to solve.
  • The pod-by-pod method is not suitable for some machine learning scenarios. Kubernetes was initially designed to support online tasks. In some offline task scenarios, for example, distributed machine learning, we need a new algorithm called gang scheduling. Pods may not require too much scheduling immediacy. However, after tasks are submitted, computing will start only after all the workers in the batch of computing tasks run. In these scenario, the pod-by-pod scheduling method will easily lead to resource deadlock in the case of insufficient resources.
  • Currently the scheduler does not show excellent scalability. Scheduling in some specific scenarios has to be implemented in the main process through hard-coding. For example, the bindVolume causes the gang scheduler to be unable to be implemented in the current scheduling framework in a native manner.

Development of the Kubernetes Scheduler

Many schedulers are developed to solve these problems.

  • The scheduler V2 framework improves the scalability and opens the door for implementing gang scheduling in native schedulers.
  • Kube-batch: an implementation of gang scheduling. For more information, visit https://github.com/kubernetes-sigs/kube-batch
  • Poseidon: Firmament is a scheduler based on the network graph scheduling algorithm. Poseidon is to bring integration of Firmament Scheduler in Kubernetes. For more information, visit https://github.com/kubernetes-sigs/poseidon

Next, we will analyze the implementation of a specific scheduler to help you understand how the scheduler works. We will also pay attention to the trends in the scheduler community.



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