前往小程序,Get更优阅读体验!
立即前往
首页
学习
活动
专区
工具
TVP
发布
社区首页 >专栏 > Kubernetes 1.8抢占式调

Kubernetes 1.8抢占式调

作者头像
Walton
发布2018-04-16 10:57:37
1.3K0
发布2018-04-16 10:57:37
举报
文章被收录于专栏:Kubernetes

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。”

代码语言:javascript
复制
// 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。

代码语言:javascript
复制
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。

代码语言:javascript
复制
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:

代码语言:javascript
复制
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。
代码语言:javascript
复制
 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。
代码语言:javascript
复制
 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。
代码语言:javascript
复制
 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
}

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。
代码语言:javascript
复制
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进行二次开发和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;
本文参与 腾讯云自媒体同步曝光计划,分享自作者个人站点/博客。
如有侵权请联系 cloudcommunity@tencent.com 删除

本文分享自 作者个人站点/博客 前往查看

如有侵权,请联系 cloudcommunity@tencent.com 删除。

本文参与 腾讯云自媒体同步曝光计划  ,欢迎热爱写作的你一起参与!

评论
登录后参与评论
0 条评论
热度
最新
推荐阅读
目录
  • ScheduleAlgorithm的变化
  • Scheduler.preemt
  • genericScheduler.Preempt
    • sched.schedule error检查
      • 更新scheduler cache中的NodeInfo
        • 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
                • 总结
                领券
                问题归档专栏文章快讯文章归档关键词归档开发者手册归档开发者手册 Section 归档