Preemption抢占式调度的方法是什么
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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}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:293func (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:191func (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-mechanicsfunc (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:756func 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：
NodeSelectorNotMatch,
PodNotMatchHostName,
TaintsTolerationsNotMatch,
NodeLabelPresenceViolated,
NodeNotReady,
NodeNetworkUnavailable,
NodeUnschedulable,
NodeUnknownCondition
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。
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:583func 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:659func 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:501func 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。
感谢各位的阅读，以上就是"Preemption抢占式调度的方法是什么"的内容了，经过本文的学习后，相信大家对Preemption抢占式调度的方法是什么这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是，小编将为大家推送更多相关知识点的文章，欢迎关注！
