本節開始主要分析kubernetes原始碼部分，版本基於當前最新的1.13.4。
啟動分析
Kubernetes基礎元件的入口均在cmd目錄下，kube-schduler入口在scheduler.go下，如圖
command的形式，引用的是spf13類庫
command的形式，呼叫Execute方法執行定義的Run方法
runCommand方法，通過完成配置的初始化，呼叫Run方法，進一步啟動。
Run方法分析
Run方法主要做了以下工作：
1、判斷是否需要新增VolumeScheduling新特性；
2、初始化排程引數的相關結構體；
3、配置準備事件廣播；
4、健康檢查相關配置；
5、Metrics相關配置；
6、啟動所有的Informer(kubernetes主要就是通過Informer和Workqueue機制監聽事件的變化)；
7、判斷是否需要LeaderElection，決定最終的啟動。
排程介面
最終的排程介面進入的是pkg下的scheduler.go檔案，通過啟動單獨的協程處理排程工作。
scheduleOne方法分析
scheduleOne，顧名思義，每次排程一個Pod，整體檔案如下
// scheduleOne does the entire scheduling workflow for a single pod. It is serialized on the scheduling algorithm's host fitting.
func (sched *Scheduler) scheduleOne() {
// 1.從佇列中取出待排程的Pod
pod := sched.config.NextPod()
// pod could be nil when schedulerQueue is closed
if pod == nil {
return
}
if pod.DeletionTimestamp != nil {
sched.config.Recorder.Eventf(pod, v1.EventTypeWarning, "FailedScheduling", "skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
klog.V(3).Infof("Skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
return
}
klog.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()
// 2.獲取待排程Pod匹配的主機名
suggestedHost, err := sched.schedule(pod)
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 {
preemptionStartTime := time.Now()
sched.preempt(pod, fitError)
metrics.PreemptionAttempts.Inc()
metrics.SchedulingAlgorithmPremptionEvaluationDuration.Observe(metrics.SinceInMicroseconds(preemptionStartTime))
metrics.SchedulingLatency.WithLabelValues(metrics.PreemptionEvaluation).Observe(metrics.SinceInSeconds(preemptionStartTime))
// Pod did not fit anywhere, so it is counted as a failure. If preemption
// succeeds, the pod should get counted as a success the next time we try to
// schedule it. (hopefully)
metrics.PodScheduleFailures.Inc()
} else {
klog.Errorf("error selecting node for pod: %v", err)
metrics.PodScheduleErrors.Inc()
}
return
}
metrics.SchedulingAlgorithmLatency.Observe(metrics.SinceInMicroseconds(start))
// 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.
// 3.Pod與Node快取，保證排程一直進行，不用等待每次繫結完成（繫結是一個耗時的過程）
assumedPod := pod.DeepCopy()
// Assume volumes first before assuming the pod.
//
// If all volumes are completely bound, then allBound is true and binding will be skipped.
//
// Otherwise, binding of volumes is started after the pod is assumed, but before pod binding.
//
// This function modifies 'assumedPod' if volume binding is required.
// 4.判斷是否需要VolumeScheduling特性
allBound, err := sched.assumeVolumes(assumedPod, suggestedHost)
if err != nil {
klog.Errorf("error assuming volumes: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
// assume modifies `assumedPod` by setting NodeName=suggestedHost
// 5.Pod對應的NodeName寫上主機名，存入快取
err = sched.assume(assumedPod, suggestedHost)
if err != nil {
klog.Errorf("error assuming pod: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
// bind the pod to its host asynchronously (we can do this b/c of the assumption step above).
// 6.請求apiserver，非同步處理最終的繫結，寫入到etcd
go func() {
// Bind volumes first before Pod
if !allBound {
err := sched.bindVolumes(assumedPod)
if err != nil {
klog.Errorf("error binding volumes: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
}
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 {
klog.Errorf("error binding pod: %v", err)
metrics.PodScheduleErrors.Inc()
} else {
metrics.PodScheduleSuccesses.Inc()
}
}()
}
複製程式碼主要做了以下工作：
1、從佇列中取出待排程的Pod
2、根據排程演算法（預選+優選）獲取待排程Pod匹配的主機，如果未獲取到合適的主機，判斷是否需要preempt，即Pod的搶佔策略，為Pod分配節點
3、將當前Pod快取起來，假定已經繫結成功（主要是為了將scheduling與binding過程分開）
4、判斷是否需要VolumeScheduling特性繼續新增Pod資訊
5、Pod對應的NodeName寫上主機名（排程的本質就是將為空的NodeName寫上相應的Node的值）
6、啟動新的binding協程，請求apiserver，非同步處理最終的繫結，將結果寫入到etcd中
排程演算法
最終的排程在generic_scheduler.go的Schedule方法。排程主要分兩步，預選和優選。
預選
預選演算法呼叫的介面是findNodesThatFit，主要程式碼如下：
// Filters the nodes to find the ones that fit based on the given predicate functions
// Each node is passed through the predicate functions to determine if it is a fit
func (g *genericScheduler) findNodesThatFit(pod *v1.Pod, nodes []*v1.Node) ([]*v1.Node, FailedPredicateMap, error) {
var filtered []*v1.Node
failedPredicateMap := FailedPredicateMap{}
// 該if表示，如果沒有配置預選的演算法，則直接將所有的Node寫入匹配陣列
if len(g.predicates) == 0 {
filtered = nodes
} else {
allNodes := int32(g.cache.NodeTree().NumNodes)
// numFeasibleNodesToFind保證一次性不用返回過多的Node數量，避免陣列過大
numNodesToFind := g.numFeasibleNodesToFind(allNodes)
// Create filtered list with enough space to avoid growing it
// and allow assigning.
filtered = make([]*v1.Node, numNodesToFind)
errs := errors.MessageCountMap{}
var (
predicateResultLock sync.Mutex
filteredLen int32
equivClass *equivalence.Class
)
ctx, cancel := context.WithCancel(context.Background())
// We can use the same metadata producer for all nodes.
meta := g.predicateMetaProducer(pod, g.cachedNodeInfoMap)
if g.equivalenceCache != nil {
// getEquivalenceClassInfo will return immediately if no equivalence pod found
equivClass = equivalence.NewClass(pod)
}
// checkNode處理預選策略
checkNode := func(i int) {
var nodeCache *equivalence.NodeCache
// 每次獲取Node資訊
nodeName := g.cache.NodeTree().Next()
if g.equivalenceCache != nil {
nodeCache = g.equivalenceCache.LoadNodeCache(nodeName)
}
fits, failedPredicates, err := podFitsOnNode(
pod,
meta,
g.cachedNodeInfoMap[nodeName],
g.predicates,
nodeCache,
g.schedulingQueue,
g.alwaysCheckAllPredicates,
equivClass,
)
if err != nil {
predicateResultLock.Lock()
errs[err.Error()]++
predicateResultLock.Unlock()
return
}
if fits {
// 保證獲取的Node數量在numNodesToFind內
length := atomic.AddInt32(&filteredLen, 1)
if length > numNodesToFind {
// 通知ParallelizeUntil任務結束
cancel()
atomic.AddInt32(&filteredLen, -1)
} else {
filtered[length-1] = g.cachedNodeInfoMap[nodeName].Node()
}
} else {
predicateResultLock.Lock()
failedPredicateMap[nodeName] = failedPredicates
predicateResultLock.Unlock()
}
}
// Stops searching for more nodes once the configured number of feasible nodes
// are found.
// 並行處理多個Node的checkNode工作
workqueue.ParallelizeUntil(ctx, 16, int(allNodes), checkNode)
filtered = filtered[:filteredLen]
if len(errs) > 0 {
return []*v1.Node{}, FailedPredicateMap{}, errors.CreateAggregateFromMessageCountMap(errs)
}
}
if len(filtered) > 0 && len(g.extenders) != 0 {
for _, extender := range g.extenders {
if !extender.IsInterested(pod) {
continue
}
filteredList, failedMap, err := extender.Filter(pod, filtered, g.cachedNodeInfoMap)
if err != nil {
if extender.IsIgnorable() {
klog.Warningf("Skipping extender %v as it returned error %v and has ignorable flag set",
extender, err)
continue
} else {
return []*v1.Node{}, FailedPredicateMap{}, err
}
}
for failedNodeName, failedMsg := range failedMap {
if _, found := failedPredicateMap[failedNodeName]; !found {
failedPredicateMap[failedNodeName] = []algorithm.PredicateFailureReason{}
}
failedPredicateMap[failedNodeName] = append(failedPredicateMap[failedNodeName], predicates.NewFailureReason(failedMsg))
}
filtered = filteredList
if len(filtered) == 0 {
break
}
}
}
return filtered, failedPredicateMap, nil
}
複製程式碼findNodesThatFit主要做了幾個操作
1、判斷是否配置了預選演算法，如果沒有，直接返回Node列表資訊；
2、如果配置了預選演算法，則同時對多個Node（最多一次16個）呼叫checkNode方法，判斷Pod是否可以排程在該Node上；
3、預選篩選之後，如果配置了排程的擴充套件演算法，需要繼續對篩選後的Pod與Node進行再一次的篩選，獲取最終匹配的Node列表。
這裡有一個注意的地方，獲取匹配的Node節點數量時，通過numFeasibleNodesToFind函式限制了每次獲取的節點數，最大值為100。這樣當匹配到相應的Node數時，checkNode方法不再呼叫。
這裡個人覺著有些問題，當Node數量足夠多的時候（大於100），由於numFeasibleNodesToFind限制了Node數量，導致並不能掃描到所有的Node，這樣可能導致最合適的Node沒有被掃描到，匹配到的只是較優先的Node，則最終排程到的Node也不是最合適的Node，只是相較於比較合適。
最終實現排程判斷的介面是podFitsOnNode。
podFitsOnNode最難理解的就是for迴圈了兩次，根據註釋，大致意思如下：
1、第一次迴圈，將所有的優先順序比較高或者相等的nominatedPods加入到Node中，更新meta和nodeInfo。nominatedPods是指已經分配到Node內但是還沒有真正執行起來的Pods。這樣做可以保證優先順序高的Pods不會因為現在的Pod的加入而導致排程失敗；
2、第二次排程，不將nominatedPods加入到Node內。這樣的原因是因為考慮到像Pod affinity策略的話，如果當前的Pod依賴的是nominatedPods，這樣就會有問題。因為，nominatedPods不能保證一定可以排程到相應的Node上。
// podFitsOnNode checks whether a node given by NodeInfo satisfies the given predicate functions.
// For given pod, podFitsOnNode will check if any equivalent pod exists and try to reuse its cached
// predicate results as possible.
// This function is called from two different places: Schedule and Preempt.
// When it is called from Schedule, we want to test whether the pod is schedulable
// on the node with all the existing pods on the node plus higher and equal priority
// pods nominated to run on the node.
// When it is called from Preempt, we should remove the victims of preemption and
// add the nominated pods. Removal of the victims is done by SelectVictimsOnNode().
// It removes victims from meta and NodeInfo before calling this function.
// ---
// podFitsOnNode根據給定的NodeInfo判斷是否匹配相應的預選函式
// 對於一個給定的Pod，podFitsOnNode會檢查之前是否有等價的Pod，這樣就可以直接複用等價Pod的預選結果
// 該函式會有兩個地方呼叫：Schedule和Preempt
// 當Schedule（正常排程）的時候，判斷Node上所有已經存在的Pod和將被指定將要排程到這個Node上的其他所有高優先順序Pod外，當前的Pod是否可以排程
// 當Preempt（搶佔式）的時候，待定。。。
func podFitsOnNode(
pod *v1.Pod,
meta algorithm.PredicateMetadata,
info *schedulercache.NodeInfo,
predicateFuncs map[string]algorithm.FitPredicate,
nodeCache *equivalence.NodeCache,
queue internalqueue.SchedulingQueue,
alwaysCheckAllPredicates bool,
equivClass *equivalence.Class,
) (bool, []algorithm.PredicateFailureReason, error) {
var (
eCacheAvailable bool
failedPredicates []algorithm.PredicateFailureReason
)
podsAdded := false
// We run predicates twice in some cases. If the node has greater or equal priority
// nominated pods, we run them when those pods are added to meta and nodeInfo.
// If all predicates succeed in this pass, we run them again when these
// nominated pods are not added. This second pass is necessary because some
// predicates such as inter-pod affinity may not pass without the nominated pods.
// If there are no nominated pods for the node or if the first run of the
// predicates fail, we don't run the second pass.
// We consider only equal or higher priority pods in the first pass, because
// those are the current "pod" must yield to them and not take a space opened
// for running them. It is ok if the current "pod" take resources freed for
// lower priority pods.
// Requiring that the new pod is schedulable in both circumstances ensures that
// we are making a conservative decision: predicates like resources and inter-pod
// anti-affinity are more likely to fail when the nominated pods are treated
// as running, while predicates like pod affinity are more likely to fail when
// the nominated pods are treated as not running. We can't just assume the
// nominated pods are running because they are not running right now and in fact,
// they may end up getting scheduled to a different node.
// 兩次迴圈的原因主要就是因為NominatedPods排程的不一定就是此Node，還有Pod的親和性等問題
for i := 0; i < 2; i++ {
metaToUse := meta
nodeInfoToUse := info
if i == 0 {
// 第一次排程，根據NominatedPods更新meta和nodeInfo資訊，pod根據更新後的資訊去預選
// 第二次排程，meta和nodeInfo資訊不變，保證pod不完全依賴於NominatedPods（主要考慮到pod親和性之類的）
podsAdded, metaToUse, nodeInfoToUse = addNominatedPods(pod, meta, info, queue)
} else if !podsAdded || len(failedPredicates) != 0 {
break
}
// Bypass eCache if node has any nominated pods.
// TODO(bsalamat): consider using eCache and adding proper eCache invalidations
// when pods are nominated or their nominations change.
eCacheAvailable = equivClass != nil && nodeCache != nil && !podsAdded
for predicateID, predicateKey := range predicates.Ordering() {
var (
fit bool
reasons []algorithm.PredicateFailureReason
err error
)
//TODO (yastij) : compute average predicate restrictiveness to export it as Prometheus metric
if predicate, exist := predicateFuncs[predicateKey]; exist {
if eCacheAvailable {
fit, reasons, err = nodeCache.RunPredicate(predicate, predicateKey, predicateID, pod, metaToUse, nodeInfoToUse, equivClass)
} else {
fit, reasons, err = predicate(pod, metaToUse, nodeInfoToUse)
}
if err != nil {
return false, []algorithm.PredicateFailureReason{}, err
}
if !fit {
// eCache is available and valid, and predicates result is unfit, record the fail reasons
failedPredicates = append(failedPredicates, reasons...)
// if alwaysCheckAllPredicates is false, short circuit all predicates when one predicate fails.
if !alwaysCheckAllPredicates {
klog.V(5).Infoln("since alwaysCheckAllPredicates has not been set, the predicate " +
"evaluation is short circuited and there are chances " +
"of other predicates failing as well.")
break
}
}
}
}
}
return len(failedPredicates) == 0, failedPredicates, nil
}
複製程式碼之後就是根據預選的排程演算法，一個個判斷是否都滿足。這裡有個小優化，如果當前的Pod在之前有一個等價的Pod，則直接從快取返回相應上一次的結果。如果成功則不用繼續呼叫預選演算法。但是，對於快取部分，我個人有些疑問，可能對於上一個Pod快取的結果是成功的，但是本次排程，Node資訊發生變化了，快取結果是成功的，但是實際上可能並不一定會成功。
預選排程演算法
本節主要說的是預設的排程演算法。預設的程式碼在pkg/scheduler/algorithmprovider/defaults/defaults.go下，defaultPredicates方法返回的是預設的一系列預選演算法。與預選相關的程式碼都在pkg/scheduler/algorithm/predicates/predicates.go下
排程方法基本一致，引數為
(pod *v1.Pod, meta algorithm.PredicateMetadata, nodeInfo *schedulercache.NodeInfo)，返回值為(bool, []algorithm.PredicateFailureReason, error)。優選
預選完成之後會得到一個Node的陣列。如果預選合適的節點數大於1，則需要呼叫優選演算法根據評分獲取最優的節點。
優選演算法呼叫的介面是PrioritizeNodes，使用與預選類似的多工同步呼叫方式，採用MapReduce的思想，Map根據不同的優選演算法獲取對某一Node的值，根據Reduce統計最終的結果。
優選排程演算法
優選排程演算法預設程式碼在pkg/scheduler/algorithmprovider/defaults/defaults.go下，defaultPriorities方法返回的是預設的一系列優選演算法，通過工廠模式處理相應的優選演算法，程式碼如下
func defaultPriorities() sets.String {
return sets.NewString(
// spreads pods by minimizing the number of pods (belonging to the same service or replication controller) on the same node.
factory.RegisterPriorityConfigFactory(
"SelectorSpreadPriority",
factory.PriorityConfigFactory{
MapReduceFunction: func(args factory.PluginFactoryArgs) (algorithm.PriorityMapFunction, algorithm.PriorityReduceFunction) {
return priorities.NewSelectorSpreadPriority(args.ServiceLister, args.ControllerLister, args.ReplicaSetLister, args.StatefulSetLister)
},
Weight: 1,
},
),
// pods should be placed in the same topological domain (e.g. same node, same rack, same zone, same power domain, etc.)
// as some other pods, or, conversely, should not be placed in the same topological domain as some other pods.
factory.RegisterPriorityConfigFactory(
"InterPodAffinityPriority",
factory.PriorityConfigFactory{
Function: func(args factory.PluginFactoryArgs) algorithm.PriorityFunction {
return priorities.NewInterPodAffinityPriority(args.NodeInfo, args.NodeLister, args.PodLister, args.HardPodAffinitySymmetricWeight)
},
Weight: 1,
},
),
// Prioritize nodes by least requested utilization.
factory.RegisterPriorityFunction2("LeastRequestedPriority", priorities.LeastRequestedPriorityMap, nil, 1),
// Prioritizes nodes to help achieve balanced resource usage
factory.RegisterPriorityFunction2("BalancedResourceAllocation", priorities.BalancedResourceAllocationMap, nil, 1),
// Set this weight large enough to override all other priority functions.
// TODO: Figure out a better way to do this, maybe at same time as fixing #24720.
factory.RegisterPriorityFunction2("NodePreferAvoidPodsPriority", priorities.CalculateNodePreferAvoidPodsPriorityMap, nil, 10000),
// Prioritizes nodes that have labels matching NodeAffinity
factory.RegisterPriorityFunction2("NodeAffinityPriority", priorities.CalculateNodeAffinityPriorityMap, priorities.CalculateNodeAffinityPriorityReduce, 1),
// Prioritizes nodes that marked with taint which pod can tolerate.
factory.RegisterPriorityFunction2("TaintTolerationPriority", priorities.ComputeTaintTolerationPriorityMap, priorities.ComputeTaintTolerationPriorityReduce, 1),
// ImageLocalityPriority prioritizes nodes that have images requested by the pod present.
factory.RegisterPriorityFunction2("ImageLocalityPriority", priorities.ImageLocalityPriorityMap, nil, 1),
)
}
複製程式碼用到的優選演算法通過程式碼結構基本可以看出
通過優選之後，呼叫
selectHost方法獲取分數最高的Node。如果多個Node分數相同，則使用輪詢的方式得到最終的Node。
搶佔排程
當通過正常的排程流程如果沒有找到合適的節點（主要是預選沒有合適的節點），會判斷需不需要進行搶佔排程，具體的程式碼在pkg/scheduler/scheduler.go檔案下，用到的方法preempt，具體如下：
// preempt tries to create room for a pod that has failed to schedule, by preempting lower priority pods if possible.
// If it succeeds, it adds the name of the node where preemption has happened to the pod annotations.
// It returns the node name and an error if any.
// ---
// preempt儘可能的通過去搶佔低優先順序的Pod的空間，為排程失敗的Pod創造空間
// 如果成功了，就會去新增在Pod註解中宣告的Node名稱
// 返回Node名稱和錯誤（如果有錯誤的話）
func (sched *Scheduler) preempt(preemptor *v1.Pod, scheduleErr error) (string, error) {
// 1.判斷是否開啟Pod優先順序，排程器是否配置了DisablePreemption，兩者中任一滿足即停止搶佔
if !util.PodPriorityEnabled() || sched.config.DisablePreemption {
klog.V(3).Infof("Pod priority feature is not enabled or preemption is disabled by scheduler configuration." +
" No preemption is performed.")
return "", nil
}
// 2.獲取待搶佔Pod的資訊
preemptor, err := sched.config.PodPreemptor.GetUpdatedPod(preemptor)
if err != nil {
klog.Errorf("Error getting the updated preemptor pod object: %v", err)
return "", err
}
// 3.根據配置的演算法獲取搶佔的節點
// 獲取到的四個引數
// 1.搶佔獲取到的Node
// 2.需要被刪除掉的低優先順序的Pod列表
// 3.需要刪除掉的nominatedPods列表
// 4.錯誤資訊
node, victims, nominatedPodsToClear, err := sched.config.Algorithm.Preempt(preemptor, sched.config.NodeLister, scheduleErr)
metrics.PreemptionVictims.Set(float64(len(victims)))
if err != nil {
klog.Errorf("Error preempting victims to make room for %v/%v.", preemptor.Namespace, preemptor.Name)
return "", err
}
var nodeName = ""
if node != nil {
// 1.將Pod和Node結合，更新相應的資訊(Pod的nodeName有值)，並且構造apiserver的呼叫
// 2.所有的將要被刪除的Pod一一被刪除
// 只有兩者都滿足了，才能保證搶佔成功
nodeName = node.Name
// Update the scheduling queue with the nominated pod information. Without
// this, there would be a race condition between the next scheduling cycle
// and the time the scheduler receives a Pod Update for the nominated pod.
sched.config.SchedulingQueue.UpdateNominatedPodForNode(preemptor, nodeName)
// Make a call to update nominated node name of the pod on the API server.
err = sched.config.PodPreemptor.SetNominatedNodeName(preemptor, nodeName)
if err != nil {
klog.Errorf("Error in preemption process. Cannot update pod %v/%v annotations: %v", preemptor.Namespace, preemptor.Name, err)
sched.config.SchedulingQueue.DeleteNominatedPodIfExists(preemptor)
return "", err
}
for _, victim := range victims {
if err := sched.config.PodPreemptor.DeletePod(victim); err != nil {
klog.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, nodeName)
}
}
// Clearing nominated pods should happen outside of "if node != nil". Node could
// be nil when a pod with nominated node name is eligible to preempt again,
// but preemption logic does not find any node for it. In that case Preempt()
// function of generic_scheduler.go returns the pod itself for removal of the annotation.
// 4.刪除nominatedPods，不要求一定成功，對整體結果不影響
for _, p := range nominatedPodsToClear {
rErr := sched.config.PodPreemptor.RemoveNominatedNodeName(p)
if rErr != nil {
klog.Errorf("Cannot remove nominated node annotation of pod: %v", rErr)
// We do not return as this error is not critical.
}
}
return nodeName, err
}
複製程式碼整體程式碼結構比較清晰，有如下幾個步驟：
1、判斷是否需要進行搶佔排程，主要有兩個判斷項（PodPriority是否開啟、排程器是否配置DisablePreemption），兩者缺一不可；
2、獲取待搶佔排程Pod配置的資訊；
3、通過配置演算法的搶佔策略獲取搶佔排程的結果（最核心的步驟）；
4、收尾工作（更新Pod的資訊、刪除低優先順序的Pod、刪除一些資源如nominatedPods）
整個過程最核心的是排程演算法獲取排程結果的介面，同預選優選一樣，預設的排程實現均在generic_scheduler.go檔案，方法是Preempt。
Preempt方法返回四個引數，分別是
1)Preempt得到的Node；
2)被搶佔的Pod的列表（待刪除）；
3)將要被清除的nominatedPods（待清除）；
4)可能返回的error訊息
Preempt方法主要執行以下幾個步驟：
1、從預選失敗的節點中獲取可以用來做搶佔排程的節點，通過一個switch語句排除不可以用來做搶佔排程的節點
2、獲取PDB（Pod中斷預算）列表，用來做後續的判斷標準；
3、通過呼叫
selectNodesForPreemption方法，判斷哪些Node可以進行搶佔排程。通過ParallelizeUntil方法同步對所有的Node進行判斷，判斷路徑為checkNode-->selectVictimsOnNode-->podFitsOnNode，最終同預選方法類似，使用了podFitsOnNode方法。不同於普通預選，搶佔排程會先對Pod優先順序判斷，然後在移除掉優先順序較低的Pod之後再呼叫podFitsOnNode方法，以此達到搶佔的效果。selectNodesForPreemption方法返回的引數是一個map型別的值，key為Node資訊，value為該Node如果作為排程節點，將要清除的一些資訊，包括Pods和PDB資訊
4、獲取到搶佔排程可以實現的Nodes資源後，繼續通過擴充套件的演算法進行過濾；
5、選中最終的搶佔排程的Node，呼叫
pickOneNodeForPreemption方法，主要基於5個原則：a)PDB violations（違規）值最小的Node；
b)挑選具有最低優先順序受害者的節點，即被清除的Node上的Pods，它的優先順序是最低的；
c)通過所有受害者Pods（將被刪除的低優先順序Pods）的優先順序總和做區分；
d)如果多個Node優先順序總和仍然相等，則選擇具有最小受害者數量的Node；
e)如果多個Node優先順序總和仍然相等，則選擇第一個這樣的Node（隨機排序）;
6、選中最終的Node之後，記錄該Node上優先順序較低的NominatedPods，這些Pod還未排程，需要將其排程關係進行刪除，重新應用。程式碼如下：
// 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 1) the node, 2) the list of preempted pods if such a node is found,
// 3) A list of pods whose nominated node name should be cleared, and 4) any
// possible error.
// Preempt does not update its snapshot. It uses the same snapshot used in the
// scheduling cycle. This is to avoid a scenario where preempt finds feasible
// nodes without preempting any pod. When there are many pending pods in the
// scheduling queue a nominated pod will go back to the queue and behind
// other pods with the same priority. The nominated pod prevents other pods from
// using the nominated resources and the nominated pod could take a long time
// before it is retried after many other pending pods.
func (g *genericScheduler) Preempt(pod *v1.Pod, nodeLister algorithm.NodeLister, scheduleErr error) (*v1.Node, []*v1.Pod, []*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, nil
}
if !podEligibleToPreemptOthers(pod, g.cachedNodeInfoMap) {
klog.V(5).Infof("Pod %v/%v is not eligible for more preemption.", pod.Namespace, pod.Name)
return nil, nil, nil, nil
}
allNodes, err := nodeLister.List()
if err != nil {
return nil, nil, nil, err
}
if len(allNodes) == 0 {
return nil, nil, nil, ErrNoNodesAvailable
}
// 1.獲取預選排程失敗的節點，但是可能是潛在的搶佔可能成功的節點(所有的搶佔節點都是在潛在節點內部選擇)
potentialNodes := nodesWherePreemptionMightHelp(allNodes, fitError.FailedPredicates)
if len(potentialNodes) == 0 {
klog.V(3).Infof("Preemption will not help schedule pod %v/%v on any node.", pod.Namespace, pod.Name)
// In this case, we should clean-up any existing nominated node name of the pod.
return nil, nil, []*v1.Pod{pod}, nil
}
// 2.獲取PDB（Pod中斷預算）列表
pdbs, err := g.pdbLister.List(labels.Everything())
if err != nil {
return nil, nil, nil, err
}
// 3.獲取所有可以進行Preempt的Node節點的資訊，主要包含該節點哪些Pod需要被搶佔掉
nodeToVictims, err := selectNodesForPreemption(pod, g.cachedNodeInfoMap, potentialNodes, g.predicates,
g.predicateMetaProducer, g.schedulingQueue, pdbs)
if err != nil {
return nil, nil, nil, err
}
// We will only check nodeToVictims with extenders that support preemption.
// Extenders which do not support preemption may later prevent preemptor from being scheduled on the nominated
// node. In that case, scheduler will find a different host for the preemptor in subsequent scheduling cycles.
// 4.擴充套件的Preempt排程判斷
nodeToVictims, err = g.processPreemptionWithExtenders(pod, nodeToVictims)
if err != nil {
return nil, nil, nil, err
}
// 5.選中某一個Node
candidateNode := pickOneNodeForPreemption(nodeToVictims)
if candidateNode == nil {
return nil, nil, nil, err
}
// Lower priority pods nominated to run on this node, may no longer fit on
// this node. So, we should remove their nomination. Removing their
// nomination updates these pods and moves them to the active queue. It
// lets scheduler find another place for them.
// 6.判斷哪些Pod優先順序較低，後續需要被清除掉，不作為NominatedPods存在
nominatedPods := g.getLowerPriorityNominatedPods(pod, candidateNode.Name)
if nodeInfo, ok := g.cachedNodeInfoMap[candidateNode.Name]; ok {
return nodeInfo.Node(), nodeToVictims[candidateNode].Pods, nominatedPods, err
}
return nil, nil, nil, fmt.Errorf(
"preemption failed: the target node %s has been deleted from scheduler cache",
candidateNode.Name)
}
複製程式碼綜上，搶佔排程主要強調的一點是Pod的優先順序。與普通排程不同的是，搶佔排程對Pod做了明確的優先順序區分，以此來達到搶佔的目的。
選舉
在Scheduler啟動的時候，需要判斷是否需要做選主操作。配置選舉操作很簡單，只需要在配置檔案中新增--leader-elect=true即可。程式碼中，如果檢測到了配置選舉，則首先會參加選舉，只有拿到主節點的scheduler才能執行排程相關工作。
選舉程式碼結構比較簡單，如圖，程式碼位於client-go包中，路徑為client-go/tools/leaderelection/leaderelection.go
le.acquire(ctx)、le.renew(ctx)以及le.config.Callbacks.OnStartedLeading(ctx)。acquire表示是否選主成功，只有成功了之後，才能執行OnStartedLeading和renew。OnStartedLeading是一個回撥方法，執行的就是scheduler的run方法。
renew主要做選主的更新操作。當節點上的scheduler被選主時，還需要不斷的更新資訊，判斷是否主節點功能正常。進入
acquire或者renew方法，有一個共同的呼叫方法是tryAcquireOrRenew，該方法就是整個選舉的核心實現。tryAcquireOrRenew顧名思義，如果沒有獲取到租約，就去獲取leader的租約，否則就去更新租約。主要有三部分操作：1、呼叫
Get操作獲取是否存在ElectionRecord。如果不存在，則呼叫Create方法新建一個新的Endpoint，當前節點為scheduler的主節點，選舉成功；否則，執行更新操作；2、獲取到記錄，表明執行的是更新租約操作，需要驗證當前節點的身份和時間，判斷是否可以執行更新租約操作；
3、更新資訊，執行
Update操作，更新選主資訊。// tryAcquireOrRenew tries to acquire a leader lease if it is not already acquired,
// else it tries to renew the lease if it has already been acquired. Returns true
// on success else returns false.
// ---
// tryAcquireOrRenew，如果沒有獲取到租約，就去獲取leader的租約，否則去更新租約。
func (le *LeaderElector) tryAcquireOrRenew() bool {
now := metav1.Now()
leaderElectionRecord := rl.LeaderElectionRecord{
HolderIdentity: le.config.Lock.Identity(),
LeaseDurationSeconds: int(le.config.LeaseDuration / time.Second),
RenewTime: now,
AcquireTime: now,
}
// 1. obtain or create the ElectionRecord
// 1. 呼叫Endpoint的Get操作，獲取oldLeaderElectionRecord
oldLeaderElectionRecord, err := le.config.Lock.Get()
if err != nil {
if !errors.IsNotFound(err) {
klog.Errorf("error retrieving resource lock %v: %v", le.config.Lock.Describe(), err)
return false
}
// 建立新的Endpoint
if err = le.config.Lock.Create(leaderElectionRecord); err != nil {
klog.Errorf("error initially creating leader election record: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}
// 2. Record obtained, check the Identity & Time
// 2. 獲取到了記錄，檢查下身份和時間資訊，判斷是否合法
if !reflect.DeepEqual(le.observedRecord, *oldLeaderElectionRecord) {
le.observedRecord = *oldLeaderElectionRecord
le.observedTime = le.clock.Now()
}
if le.observedTime.Add(le.config.LeaseDuration).After(now.Time) &&
!le.IsLeader() {
klog.V(4).Infof("lock is held by %v and has not yet expired", oldLeaderElectionRecord.HolderIdentity)
return false
}
// 3. We're going to try to update. The leaderElectionRecord is set to it's default
// here. Let's correct it before updating.
if le.IsLeader() {
leaderElectionRecord.AcquireTime = oldLeaderElectionRecord.AcquireTime
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions
} else {
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions + 1
}
// update the lock itself
if err = le.config.Lock.Update(leaderElectionRecord); err != nil {
klog.Errorf("Failed to update lock: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}
複製程式碼Scheduler的選舉操作比較簡單，主要就是通過判斷記錄在Etcd中的Endpoints是否可以更新來判斷是否可以進行選舉。整個選舉操作依賴於Etcd的特點來保證分散式操作的成功和唯一。在kube-system的namespace下可以檢視相應的endpoint：kube-scheduler。
