Kubernetes 1.8抢占式调度Preemption源码分析
Kubernetes 1.8抢占式调度Preemption源码分析
WaltonWang 发表于2周前
Kubernetes 1.8抢占式调度Preemption源码分析
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摘要: 本博文是对Kubernetes 1.8中基于Pod优先级抢占式调度Preemption的源码分析,整个过程主要围绕这scheduler的相关源码进行分析,从源码角度解读抢占式调度的全过程,相信从中你会学到很多在官方文档中你看不到但又很重要的细节,比如抢占的最佳节点到底是怎么选出来的?竟然还跟scheduler extender相关?等等

Author: xidianwangtao@gmail.com

阅读本博文前,建议先阅读解析Kubernetes 1.8中的基于Pod优先级的抢占式调度

ScheduleAlgorithm的变化

在Kubernetes 1.8中,对ScheduleAlgorithm Interface的定义发生了改变,多了一个Preempt(...)。因此,我在博文Kubernetes Scheduler原理解析(当时是基于kubernetes 1.5)中对scheduler调度过程开的一句话概括“将PodSpec.NodeName为空的Pods逐个地,经过预选(Predicates)和优选(Priorities)两个步骤,挑选最合适的Node作为该Pod的Destination。”将不再准确了。

现在应该一句话这样描述才算准确了:“将PodSpec.NodeName为空的Pods逐个地,经过预选(Predicates)和优选(Priorities)两个步骤,挑选最合适的Node作为该Pod的Destination。如果经过预选和优选仍然没有找到合适的节点,并且启动了Pod Priority,那么该Pod将会进行Preempt抢占式调度找到最合适的节点及需要Evict的Pods。”

// ScheduleAlgorithm is an interface implemented by things that know how to schedule pods
// onto machines.
type ScheduleAlgorithm interface {
	Schedule(*v1.Pod, NodeLister) (selectedMachine string, err error)
	// Preempt receives scheduling errors for a pod and tries to create room for
	// the pod by preempting lower priority pods if possible.
	// It returns the node where preemption happened, a list of preempted pods, and error if any.
	Preempt(*v1.Pod, NodeLister, error) (selectedNode *v1.Node, preemptedPods []*v1.Pod, err error)
	// Predicates() returns a pointer to a map of predicate functions. This is
	// exposed for testing.
	Predicates() map[string]FitPredicate
	// Prioritizers returns a slice of priority config. This is exposed for
	// testing.
	Prioritizers() []PriorityConfig
}

我的博文Kubernetes Scheduler源码分析(当时是基于kubernetes 1.5)对schedule的全过程做过全面的代码解读,当时的描述是这样子的:Scheduler.scheduleOne开始真正的调度逻辑,每次负责一个Pod的调度,逻辑如下:

  • 从PodQueue中获取一个Pod。
  • 执行对应Algorithm的Schedule,进行预选和优选。
  • AssumePod
  • Bind Pod, 如果Bind Failed,ForgetPod。

在1.8中,但预选和优选调度完整没有找到合适node时(其实一定会是预选没有找到nodes,优选只是挑更好的),还会调用sched.preempt进行抢占式调度。

plugin/pkg/scheduler/scheduler.go:293

func (sched *Scheduler) scheduleOne() {
	pod := sched.config.NextPod()
	if pod.DeletionTimestamp != nil {
		sched.config.Recorder.Eventf(pod, v1.EventTypeWarning, "FailedScheduling", "skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
		glog.V(3).Infof("Skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
		return
	}

	glog.V(3).Infof("Attempting to schedule pod: %v/%v", pod.Namespace, pod.Name)

	// Synchronously attempt to find a fit for the pod.
	start := time.Now()
	suggestedHost, err := sched.schedule(pod)
	metrics.SchedulingAlgorithmLatency.Observe(metrics.SinceInMicroseconds(start))
	if err != nil {
		// schedule() may have failed because the pod would not fit on any host, so we try to
		// preempt, with the expectation that the next time the pod is tried for scheduling it
		// will fit due to the preemption. It is also possible that a different pod will schedule
		// into the resources that were preempted, but this is harmless.
		if fitError, ok := err.(*core.FitError); ok {
			sched.preempt(pod, fitError)
		}
		return
	}

	// Tell the cache to assume that a pod now is running on a given node, even though it hasn't been bound yet.
	// This allows us to keep scheduling without waiting on binding to occur.
	assumedPod := *pod
	// assume modifies `assumedPod` by setting NodeName=suggestedHost
	err = sched.assume(&assumedPod, suggestedHost)
	if err != nil {
		return
	}

	// bind the pod to its host asynchronously (we can do this b/c of the assumption step above).
	go func() {
		err := sched.bind(&assumedPod, &v1.Binding{
			ObjectMeta: metav1.ObjectMeta{Namespace: assumedPod.Namespace, Name: assumedPod.Name, UID: assumedPod.UID},
			Target: v1.ObjectReference{
				Kind: "Node",
				Name: suggestedHost,
			},
		})
		metrics.E2eSchedulingLatency.Observe(metrics.SinceInMicroseconds(start))
		if err != nil {
			glog.Errorf("Internal error binding pod: (%v)", err)
		}
	}()
}

Scheduler.preemt

好的,关于预选和优选,我这里不做过多解读,因为整个源码逻辑和1.5是一样,不同的是1.8增加了更多的Predicate和Priority Policys及其实现。下面只看抢占式调度Preempt的代码。

plugin/pkg/scheduler/scheduler.go:191

func (sched *Scheduler) preempt(preemptor *v1.Pod, scheduleErr error) (string, error) {
	if !utilfeature.DefaultFeatureGate.Enabled(features.PodPriority) {
		glog.V(3).Infof("Pod priority feature is not enabled. No preemption is performed.")
		return "", nil
	}
	preemptor, err := sched.config.PodPreemptor.GetUpdatedPod(preemptor)
	if err != nil {
		glog.Errorf("Error getting the updated preemptor pod object: %v", err)
		return "", err
	}
	node, victims, err := sched.config.Algorithm.Preempt(preemptor, sched.config.NodeLister, scheduleErr)
	if err != nil {
		glog.Errorf("Error preempting victims to make room for %v/%v.", preemptor.Namespace, preemptor.Name)
		return "", err
	}
	if node == nil {
		return "", err
	}
	glog.Infof("Preempting %d pod(s) on node %v to make room for %v/%v.", len(victims), node.Name, preemptor.Namespace, preemptor.Name)
	annotations := map[string]string{core.NominatedNodeAnnotationKey: node.Name}
	err = sched.config.PodPreemptor.UpdatePodAnnotations(preemptor, annotations)
	if err != nil {
		glog.Errorf("Error in preemption process. Cannot update pod %v annotations: %v", preemptor.Name, err)
		return "", err
	}
	for _, victim := range victims {
		if err := sched.config.PodPreemptor.DeletePod(victim); err != nil {
			glog.Errorf("Error preempting pod %v/%v: %v", victim.Namespace, victim.Name, err)
			return "", err
		}
		sched.config.Recorder.Eventf(victim, v1.EventTypeNormal, "Preempted", "by %v/%v on node %v", preemptor.Namespace, preemptor.Name, node.Name)
	}
	return node.Name, err
}
  • 检查FeaturesGate中是否开启了PodPriority,如果没开启,则不会进行后续Preemption操作;
  • 由于该Pod在Predicate/Priortiy调度过程失败后,会更新PodCondition,记录调度失败状态及失败原因。因此需要从apiserver中获取PodCondition更新后的Pod Object;
  • 调用ScheduleAlgorithm.Preempt进行抢占式调度,选出最佳node和待preempt pods(称为victims);
  • 调用apiserver给该pod(称为Preemptor)打上Annotation:NominatedNodeName=nodeName;
  • 遍历victims,调用apiserver进行逐个删除这些pods;

注意:在scheduler调用shed.schedule(pod)进行预选和优选调度失败时,Pod Bind Node失败,该Pod会requeue unscheduled Cache podqueue中,如果在这个pod调度过程中又有新的pod加入到待调度队列,那么该pod requeue时它前面就有其他pod,下一次调度就是先调度在它前面的pod,而这些pod的调度有可能会调度到刚刚通过Preempt释放资源的Node上,导致把刚才Preemptor释放的resource消耗掉。当再次轮到上次的Preemptor调度时,可能又需要触发一次某个节点的Preempt。

genericScheduler.Preempt

ScheduleAlgorithm.Preempt是抢占式调度的关键实现,其对应的实现在genericScheduler中:

plugin/pkg/scheduler/core/generic_scheduler.go:181

// preempt finds nodes with pods that can be preempted to make room for "pod" to
// schedule. It chooses one of the nodes and preempts the pods on the node and
// returns the node and the list of preempted pods if such a node is found.
// TODO(bsalamat): Add priority-based scheduling. More info: today one or more
// pending pods (different from the pod that triggered the preemption(s)) may
// schedule into some portion of the resources freed up by the preemption(s)
// before the pod that triggered the preemption(s) has a chance to schedule
// there, thereby preventing the pod that triggered the preemption(s) from
// scheduling. Solution is given at:
// https://github.com/kubernetes/community/blob/master/contributors/design-proposals/pod-preemption.md#preemption-mechanics
func (g *genericScheduler) Preempt(pod *v1.Pod, nodeLister algorithm.NodeLister, scheduleErr error) (*v1.Node, []*v1.Pod, error) {
	// Scheduler may return various types of errors. Consider preemption only if
	// the error is of type FitError.
	fitError, ok := scheduleErr.(*FitError)
	if !ok || fitError == nil {
		return nil, nil, nil
	}
	err := g.cache.UpdateNodeNameToInfoMap(g.cachedNodeInfoMap)
	if err != nil {
		return nil, nil, err
	}
	if !podEligibleToPreemptOthers(pod, g.cachedNodeInfoMap) {
		glog.V(5).Infof("Pod %v is not eligible for more preemption.", pod.Name)
		return nil, nil, nil
	}
	allNodes, err := nodeLister.List()
	if err != nil {
		return nil, nil, err
	}
	if len(allNodes) == 0 {
		return nil, nil, ErrNoNodesAvailable
	}
	potentialNodes := nodesWherePreemptionMightHelp(pod, allNodes, fitError.FailedPredicates)
	if len(potentialNodes) == 0 {
		glog.V(3).Infof("Preemption will not help schedule pod %v on any node.", pod.Name)
		return nil, nil, nil
	}
	nodeToPods, err := selectNodesForPreemption(pod, g.cachedNodeInfoMap, potentialNodes, g.predicates, g.predicateMetaProducer)
	if err != nil {
		return nil, nil, err
	}
	for len(nodeToPods) > 0 {
		node := pickOneNodeForPreemption(nodeToPods)
		if node == nil {
			return nil, nil, err
		}
		passes, pErr := nodePassesExtendersForPreemption(pod, node.Name, nodeToPods[node], g.cachedNodeInfoMap, g.extenders)
		if passes && pErr == nil {
			return node, nodeToPods[node], err
		}
		if pErr != nil {
			glog.Errorf("Error occurred while checking extenders for preemption on node %v: %v", node, pErr)
		}
		// Remove the node from the map and try to pick a different node.
		delete(nodeToPods, node)
	}
	return nil, nil, err
}

sched.schedule error检查

  • 只有前面sched.schedule()返回的error为FitError类型时,才会触发后续的Preemption。FitError就是表示pod在Predicate阶段进行某些PredicateFunc筛选时不通过。也就是说只有预选失败的Pod才会进行抢占式调度。

更新scheduler cache中的NodeInfo

  • 更新scheduler cache中NodeInfo,主要是更新Node上scheduled 和Assumed Pods,作为后续Preempt Pods时的考虑范围,确保Preemption是正确的。

podEligibleToPreemptOthers检查pod是否有资格进行抢占式调度

  • invoke podEligibleToPreemptOthers来判断该pod是否适合进行后续的Preemption,判断逻辑是:
    • 如果该Pod已经包含Annotation:NominatedNodeName=nodeName(说明该pod之前已经Preempted),并且Annotation中的这个Node有比该pod优先级更低的pod正在Terminating,则认为该pod不适合进行后续的Preemption,流程结束。

    • 除此之外,继续后续的流程。

    • 对应代码如下:

      plugin/pkg/scheduler/core/generic_scheduler.go:756
      
      func podEligibleToPreemptOthers(pod *v1.Pod, nodeNameToInfo map[string]*schedulercache.NodeInfo) bool {
      	if nodeName, found := pod.Annotations[NominatedNodeAnnotationKey]; found {
      		if nodeInfo, found := nodeNameToInfo[nodeName]; found {
      			for _, p := range nodeInfo.Pods() {
      				if p.DeletionTimestamp != nil && util.GetPodPriority(p) < util.GetPodPriority(pod) {
      					// There is a terminating pod on the nominated node.
      					return false
      				}
      			}
      		}
      	}
      	return true
      }
      

nodesWherePreemptionMightHelp筛选出Potential Nodes

  • invoke nodesWherePreemptionMightHelp来获取potential nodes。nodesWherePreemptionMightHelp的逻辑是:
    • 遍历所有的nodes,对每个nodes在sched.schedule()在预选阶段失败的Predicate策略(failedPredicates)进行扫描,如果failedPredicates包含以下Policy,则说明该node不适合作为Preempt的备选节点。

      • NodeSelectorNotMatch,
      • PodNotMatchHostName,
      • TaintsTolerationsNotMatch,
      • NodeLabelPresenceViolated,
      • NodeNotReady,
      • NodeNetworkUnavailable,
      • NodeUnschedulable,
      • NodeUnknownCondition
    • 除此之外的Node均作为Potential Nodes。

    • 对应代码如下:

      func nodesWherePreemptionMightHelp(pod *v1.Pod, nodes []*v1.Node, failedPredicatesMap FailedPredicateMap) []*v1.Node {
      	potentialNodes := []*v1.Node{}
      	for _, node := range nodes {
      		unresolvableReasonExist := false
      		failedPredicates, found := failedPredicatesMap[node.Name]
      		// If we assume that scheduler looks at all nodes and populates the failedPredicateMap
      		// (which is the case today), the !found case should never happen, but we'd prefer
      		// to rely less on such assumptions in the code when checking does not impose
      		// significant overhead.
      		for _, failedPredicate := range failedPredicates {
      			switch failedPredicate {
      			case
      				predicates.ErrNodeSelectorNotMatch,
      				predicates.ErrPodNotMatchHostName,
      				predicates.ErrTaintsTolerationsNotMatch,
      				predicates.ErrNodeLabelPresenceViolated,
      				predicates.ErrNodeNotReady,
      				predicates.ErrNodeNetworkUnavailable,
      				predicates.ErrNodeUnschedulable,
      				predicates.ErrNodeUnknownCondition:
      				unresolvableReasonExist = true
      				break
      				// TODO(bsalamat): Please add affinity failure cases once we have specific affinity failure errors.
      			}
      		}
      		if !found || !unresolvableReasonExist {
      			glog.V(3).Infof("Node %v is a potential node for preemption.", node.Name)
      			potentialNodes = append(potentialNodes, node)
      		}
      	}
      	return potentialNodes
      }
      

selectNodesForPreemption和selectVictimsOnNode选出可行Nodes及其对应的victims

  • invoke selectNodesForPreemption从Potential Nodes中找出所有可行的Nodes及对应的victim Pods,其对应的逻辑如为:启动max(16, potentialNodesNum)个worker(对应goruntine)通过WaitGroups并发等待所有node的check完成:
    • 遍历该node上所有的scheduled pods(包括assumed pods),将优先级比Preemptor更低的Pods都加入到Potential victims List中,并且将这些victims从NodeInfoCopy中删除,下次进行Predicate时就意味着Node上有更多资源可用。

    • 对Potential victims中元素进行排序,排序规则是按照优先级从高到底排序的,index为0的对应的优先级最高。

    • 检查Preemptor是否能scheduler配置的所有Predicates Policy(基于前面将这些victims从NodeInfoCopy中删除,将所有更低优先级的pods资源全部释放了),如果不通过则返回,表示该node不合适。All Predicate通过后,继续下面流程。

    • 遍历所有的Potential victims list item(已经按照优先级从高到底排序),试着把Potential victims中第一个Pod(优先级最高)加回到NodeInfoCopy中,再检查Preemptor是否能scheduler配置的所有Predicates Policy,如果不满足就把该pod再从NodeInfoCopy中删除,并且正式加入到victims list中。接着对Potential victims中第2,3...个Pod进行同样处理。这样做,是为了保证尽量保留优先级更高的Pods,尽量删除更少的Pods。

    • 最终返回每个可行node及其对应victims list。

    • selectNodesForPreemption代码如下,其实核心代码在selectVictimsOnNode

      plugin/pkg/scheduler/core/generic_scheduler.go:583
      
      func selectNodesForPreemption(pod *v1.Pod,
      	nodeNameToInfo map[string]*schedulercache.NodeInfo,
      	potentialNodes []*v1.Node,
      	predicates map[string]algorithm.FitPredicate,
      	metadataProducer algorithm.PredicateMetadataProducer,
      ) (map[*v1.Node][]*v1.Pod, error) {
      
      	nodeNameToPods := map[*v1.Node][]*v1.Pod{}
      	var resultLock sync.Mutex
      
      	// We can use the same metadata producer for all nodes.
      	meta := metadataProducer(pod, nodeNameToInfo)
      	checkNode := func(i int) {
      		nodeName := potentialNodes[i].Name
      		var metaCopy algorithm.PredicateMetadata
      		if meta != nil {
      			metaCopy = meta.ShallowCopy()
      		}
      		pods, fits := selectVictimsOnNode(pod, metaCopy, nodeNameToInfo[nodeName], predicates)
      		if fits {
      			resultLock.Lock()
      			nodeNameToPods[potentialNodes[i]] = pods
      			resultLock.Unlock()
      		}
      	}
      	workqueue.Parallelize(16, len(potentialNodes), checkNode)
      	return nodeNameToPods, nil
      }
      
      plugin/pkg/scheduler/core/generic_scheduler.go:659
      
      func selectVictimsOnNode(
      	pod *v1.Pod,
      	meta algorithm.PredicateMetadata,
      	nodeInfo *schedulercache.NodeInfo,
      	fitPredicates map[string]algorithm.FitPredicate) ([]*v1.Pod, bool) {
      	potentialVictims := util.SortableList{CompFunc: util.HigherPriorityPod}
      	nodeInfoCopy := nodeInfo.Clone()
      
      	removePod := func(rp *v1.Pod) {
      		nodeInfoCopy.RemovePod(rp)
      		if meta != nil {
      			meta.RemovePod(rp)
      		}
      	}
      	addPod := func(ap *v1.Pod) {
      		nodeInfoCopy.AddPod(ap)
      		if meta != nil {
      			meta.AddPod(ap, nodeInfoCopy)
      		}
      	}
      	// As the first step, remove all the lower priority pods from the node and
      	// check if the given pod can be scheduled.
      	podPriority := util.GetPodPriority(pod)
      	for _, p := range nodeInfoCopy.Pods() {
      		if util.GetPodPriority(p) < podPriority {
      			potentialVictims.Items = append(potentialVictims.Items, p)
      			removePod(p)
      		}
      	}
      	potentialVictims.Sort()
      	// If the new pod does not fit after removing all the lower priority pods,
      	// we are almost done and this node is not suitable for preemption. The only condition
      	// that we should check is if the "pod" is failing to schedule due to pod affinity
      	// failure.
      	// TODO(bsalamat): Consider checking affinity to lower priority pods if feasible with reasonable performance.
      	if fits, _, err := podFitsOnNode(pod, meta, nodeInfoCopy, fitPredicates, nil); !fits {
      		if err != nil {
      			glog.Warningf("Encountered error while selecting victims on node %v: %v", nodeInfo.Node().Name, err)
      		}
      		return nil, false
      	}
      	victims := []*v1.Pod{}
      	// Try to reprieve as many pods as possible starting from the highest priority one.
      	for _, p := range potentialVictims.Items {
      		lpp := p.(*v1.Pod)
      		addPod(lpp)
      		if fits, _, _ := podFitsOnNode(pod, meta, nodeInfoCopy, fitPredicates, nil); !fits {
      			removePod(lpp)
      			victims = append(victims, lpp)
      			glog.V(5).Infof("Pod %v is a potential preemption victim on node %v.", lpp.Name, nodeInfo.Node().Name)
      		}
      	}
      	return victims, true
      }    
      

pickOneNodeForPreemption从可行Nodes中找出最合适的一个Node

  • 如果上一步至少找到一个可行node,则调用pickOneNodeForPreemption按照以下逻辑选择一个最合适的node:
    • 选择victims中最高pod优先级最低的那个Node。

    • 如果上一步有不止一个Nodes满足条件,则再对选择所有victims优先级之和最小的那个Node。

    • 如果上一步有不止一个Nodes满足条件,则再选择victims pod数最少的Node。

    • 如果上一步有不止一个Nodes满足条件,则再随机选择一个Node。

    • 以上每一步的Nodes列表,都是基于上一步筛选后的Nodes。

      plugin/pkg/scheduler/core/generic_scheduler.go:501
      
      func pickOneNodeForPreemption(nodesToPods map[*v1.Node][]*v1.Pod) *v1.Node {
      	type nodeScore struct {
      		node            *v1.Node
      		highestPriority int32
      		sumPriorities   int64
      		numPods         int
      	}
      	if len(nodesToPods) == 0 {
      		return nil
      	}
      	minHighestPriority := int32(math.MaxInt32)
      	minPriorityScores := []*nodeScore{}
      	for node, pods := range nodesToPods {
      		if len(pods) == 0 {
      			// We found a node that doesn't need any preemption. Return it!
      			// This should happen rarely when one or more pods are terminated between
      			// the time that scheduler tries to schedule the pod and the time that
      			// preemption logic tries to find nodes for preemption.
      			return node
      		}
      		// highestPodPriority is the highest priority among the victims on this node.
      		highestPodPriority := util.GetPodPriority(pods[0])
      		if highestPodPriority < minHighestPriority {
      			minHighestPriority = highestPodPriority
      			minPriorityScores = nil
      		}
      		if highestPodPriority == minHighestPriority {
      			minPriorityScores = append(minPriorityScores, &nodeScore{node: node, highestPriority: highestPodPriority, numPods: len(pods)})
      		}
      	}
      	if len(minPriorityScores) == 1 {
      		return minPriorityScores[0].node
      	}
      	// There are a few nodes with minimum highest priority victim. Find the
      	// smallest sum of priorities.
      	minSumPriorities := int64(math.MaxInt64)
      	minSumPriorityScores := []*nodeScore{}
      	for _, nodeScore := range minPriorityScores {
      		var sumPriorities int64
      		for _, pod := range nodesToPods[nodeScore.node] {
      			// We add MaxInt32+1 to all priorities to make all of them >= 0. This is
      			// needed so that a node with a few pods with negative priority is not
      			// picked over a node with a smaller number of pods with the same negative
      			// priority (and similar scenarios).
      			sumPriorities += int64(util.GetPodPriority(pod)) + int64(math.MaxInt32+1)
      		}
      		if sumPriorities < minSumPriorities {
      			minSumPriorities = sumPriorities
      			minSumPriorityScores = nil
      		}
      		nodeScore.sumPriorities = sumPriorities
      		if sumPriorities == minSumPriorities {
      			minSumPriorityScores = append(minSumPriorityScores, nodeScore)
      		}
      	}
      	if len(minSumPriorityScores) == 1 {
      		return minSumPriorityScores[0].node
      	}
      	// There are a few nodes with minimum highest priority victim and sum of priorities.
      	// Find one with the minimum number of pods.
      	minNumPods := math.MaxInt32
      	minNumPodScores := []*nodeScore{}
      	for _, nodeScore := range minSumPriorityScores {
      		if nodeScore.numPods < minNumPods {
      			minNumPods = nodeScore.numPods
      			minNumPodScores = nil
      		}
      		if nodeScore.numPods == minNumPods {
      			minNumPodScores = append(minNumPodScores, nodeScore)
      		}
      	}
      	// At this point, even if there are more than one node with the same score,
      	// return the first one.
      	if len(minNumPodScores) > 0 {
      		return minNumPodScores[0].node
      	}
      	glog.Errorf("Error in logic of node scoring for preemption. We should never reach here!")
      	return nil
      }
      

最合适的Node仍然要交给extender(if configed)检查

  • 如果scheduler配置extender scheduler,则还需要通过invoke nodePassesExtendersForPreemption再次将该pod和(假设)剔除victims的该node交给extender.Filter进行一下检查,只有检查通过了才返回该node作为最终选择的Preempt node。
  • 关于extender的理解,请参考如何对kubernetes scheduler进行二次开发Kubernetes Scheduler源码分析。其实用的场景不多,现在支持自定义调度器了,就更少需要使用scheduler extender了。

总结

整个抢占式调度的逻辑归纳为:

  • 检查FeaturesGate中是否开启了PodPriority;
  • 调用ScheduleAlgorithm.Preempt进行抢占式调度,选出最佳node和待preempt pods(称为victims);
    • podEligibleToPreemptOthers检查pod是否有资格进行抢占式调度;
    • nodesWherePreemptionMightHelp筛选出Potential Nodes;
    • selectNodesForPreemption和selectVictimsOnNode选出可行Nodes及其对应的victims;
    • pickOneNodeForPreemption从可行Nodes中找出最合适的一个Node;
    • 最合适的Node仍然要交给extender(if configed)检查;
  • 调用apiserver给该pod(称为Preemptor)打上Annotation:NominatedNodeName=nodeName;
  • 遍历victims,调用apiserver进行逐个删除这些pods;
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