kubernetes的eviction机制

eviction，即驱赶的意思，意思是当节点出现异常时，kubernetes将有相应的机制驱赶该节点上的Pod。eviction在openstack的nova组件中也存在。

目前kubernetes中存在两种eviction机制，分别由kube-controller-manager和kubelet实现

1. kube-controller-manager实现的eviction

kube-controller-manager主要由多个控制器构成，而eviction的功能主要由node controller这个控制器实现。

kube-controller-manager提供了以下启动参数控制eviction

• pod-eviction-timeout：即当节点宕机该事件间隔后，开始eviction机制，驱赶宕机节点上的Pod，默认为5min
• node-eviction-rate: 驱赶速率，即驱赶Node的速率，由令牌桶流控算法实现，默认为0.1，即每秒驱赶0.1个节点，注意这里不是驱赶Pod的速率，而是驱赶节点的速率。相当于每隔10s，清空一个节点
• secondary-node-eviction-rate: 二级驱赶速率，当集群中宕机节点过多时，相应的驱赶速率也降低，默认为0.01
• unhealthy-zone-threshold：不健康zone阈值，会影响什么时候开启二级驱赶速率，默认为0.55，即当该zone中节点宕机数目超过55%，而认为该zone不健康
• large-cluster-size-threshold:大集群法制，当该zone的节点多余该阈值时，则认为该zone是一个大集群。大集群节点宕机数目超过55%时，则将驱赶速率降为0.0.1，假如是小集群，则将速率直接降为0

node-controller代码主要位于pkg/controller/node目录下

1.1 zone

为了控制eviction，kubernete将节点划分为不同的zone，主要通过给节点加label实现

• failure-domain.beta.kubernetes.io/zone
• failure-domain.beta.kubernetes.io/region

zone名称由上述的zone和region标签组合而成，两个节点zone和region分别相同，则位于同一个zone，否则不同zone。假如二者都为空，就位于default zone

zone有四种不同的状态

• stateInitial
• stateNormal
• stateFullDisruption
• statePartialDisruption

初始化状态比较好理解，假如节点刚刚加入集群，它所在的zone刚刚被发现，则该zone的状态是initial，这是一个非常短暂的时间，其余的状态，由以下函数决定

func (nc *NodeController) ComputeZoneState(nodeReadyConditions []*v1.NodeCondition) (int, zoneState) {
    readyNodes := 0
    notReadyNodes := 0
    for i := range nodeReadyConditions {
        if nodeReadyConditions[i] != nil && nodeReadyConditions[i].Status == v1.ConditionTrue {
            readyNodes++
        } else {
            notReadyNodes++
        }
    }       
    switch {    
    case readyNodes == 0 && notReadyNodes > 0:
        return notReadyNodes, stateFullDisruption
    case notReadyNodes > 2 && float32(notReadyNodes)/float32(notReadyNodes+readyNodes) >= nc.unhealthyZoneThreshold:
        return notReadyNodes, statePartialDisruption
    default:
        return notReadyNodes, stateNormal
    }
}

注意这里统计的某个zone下面的节点状态，而不是所有。当该zone下面ready的节点为0，而notReady节点大于0时，即认为所有节点都宕机了，所以状态为stateFullDisruption；当notReady节点大于两个，而且notReady节点占比超过unhealthyZoneThreshold，即0.55时，认为是statePartialDisruption，其他情况则认为stateNormal

这四种状态会如何影响eviction速度呢？看如下函数

func (nc *NodeController) setLimiterInZone(zone string, zoneSize int, state zoneState) {
    switch state {
    case stateNormal:
        if nc.useTaintBasedEvictions {
            nc.zoneNotReadyOrUnreachableTainer[zone].SwapLimiter(nc.evictionLimiterQPS)
        } else {
            nc.zonePodEvictor[zone].SwapLimiter(nc.evictionLimiterQPS)
        }    
    case statePartialDisruption:
        if nc.useTaintBasedEvictions {
            nc.zoneNotReadyOrUnreachableTainer[zone].SwapLimiter(
                nc.enterPartialDisruptionFunc(zoneSize))
        } else {
            nc.zonePodEvictor[zone].SwapLimiter(
                nc.enterPartialDisruptionFunc(zoneSize))
        }    
    case stateFullDisruption:
        if nc.useTaintBasedEvictions {
            nc.zoneNotReadyOrUnreachableTainer[zone].SwapLimiter(
                nc.enterFullDisruptionFunc(zoneSize))
        } else {
            nc.zonePodEvictor[zone].SwapLimiter(
                nc.enterFullDisruptionFunc(zoneSize))
        }    
    }    
}

其中enterPartialDisruptionFunc就是函数ReducedQPSFunc
func (nc *NodeController) ReducedQPSFunc(nodeNum int) float32 {
    if int32(nodeNum) > nc.largeClusterThreshold {
        return nc.secondaryEvictionLimiterQPS
    }
    return 0
} 

而enterFullDisruptionFunc是函数HealthyQPSFunc
func (nc *NodeController) HealthyQPSFunc(nodeNum int) float32 {
    return nc.evictionLimiterQPS 
}   

即加入zone状态是normal，那么速率为0.1，假如zone状态是FullDisruption，速率也是0.1；假如zone是PartialDisruption，假如是大集群，速率为0.0.1，小集群则直接降为0

1.2 两种不同的eviction方法

目前node controller存在两种不同的eviction方法，即通过taint或者传统方法

if nc.useTaintBasedEvictions {

    go wait.Until(nc.doTaintingPass, nodeEvictionPeriod, wait.NeverStop)
} else {
    go wait.Until(nc.doEvictionPass, nodeEvictionPeriod, wait.NeverStop)
}

其中nodeEvictionPeriod为100ms，即每隔100ms就会执行doEvictionPass或doTaintingPass

1.2.1 传统eviction方法

zonePodEvictor类型为map[string]*RateLimitedTimedQueue，即每个zone都有一个队列，队列带了流控算法，里面存储的是unready的节点，节点上的pod需要被eviction

func (nc *NodeController) doEvictionPass() {
    nc.evictorLock.Lock()
    defer nc.evictorLock.Unlock()
    for k := range nc.zonePodEvictor {
        nc.zonePodEvictor[k].Try(func(value TimedValue) (bool, time.Duration) {
            node, err := nc.nodeLister.Get(value.Value)
            ...
            nodeUid, _ := value.UID.(string)
            remaining, err := deletePods(nc.kubeClient, nc.recorder, value.Value, nodeUid, nc.daemonSetStore)
            ...
            if remaining {
                glog.Infof("Pods awaiting deletion due to NodeController eviction")
            }
            return true, 0
        })  
    }
}

deletePods是驱逐节点的主要函数

func deletePods(kubeClient clientset.Interface, recorder record.EventRecorder, nodeName, nodeUID string, daemonStore extensionslisters.DaemonSetLister) (bool, error) {
    remaining := false
    selector := fields.OneTermEqualSelector(api.PodHostField, nodeName).String()
    options := metav1.ListOptions{FieldSelector: selector}
    pods, err := kubeClient.Core().Pods(metav1.NamespaceAll).List(options)
    var updateErrList []error
    
    ...

    for _, pod := range pods.Items {
        // Defensive check, also needed for tests.
        if pod.Spec.NodeName != nodeName {
            continue
        }

        // 设置Pod终止理由
        if _, err = setPodTerminationReason(kubeClient, &pod, nodeName); err != nil {
            if errors.IsConflict(err) {
                updateErrList = append(updateErrList,
                    fmt.Errorf("update status failed for pod %q: %v", format.Pod(&pod), err))
                continue
            }
        }
        // 该Pod正在被删除，忽略
        if pod.DeletionGracePeriodSeconds != nil {
            remaining = true
            continue
        }
        // 假如该节点是又daemonset管理，则忽略
        _, err := daemonStore.GetPodDaemonSets(&pod)
        if err == nil {
            continue
        }
        if err := kubeClient.Core().Pods(pod.Namespace).Delete(pod.Name, nil); err != nil {
            return false, err
        }
        remaining = true
    }
    ...
    return remaining, nil
}

底层其实就是delete节点上的Pod，假如Pod是由daemonset管理，则忽略，因为即使删除了，daemonset还是会在该节点上重建

1.2.2 taint机制 taint机制还处于试验状态，默认不开启，假如要开始，则要在所有组件上设置–feature-gates TaintNodesByCondition=true

当节点状态为unready时，打上node.alpha.kubernetes.io/notReady的taint 当节点状态为unknown时，打上node.alpha.kubernetes.io/unreachable的taint

打上taint后，必然有相应的控制器去处理

// Run starts NoExecuteTaintManager which will run in loop until `stopCh` is closed.
func (tc *NoExecuteTaintManager) Run(stopCh <-chan struct{}) {   
    go func(stopCh <-chan struct{}) {
        for {            
            item, shutdown := tc.nodeUpdateQueue.Get()
            if shutdown {
                break    
            }            
            nodeUpdate := item.(*nodeUpdateItem) 
            select {     
            case <-stopCh:            
                break    
            case tc.nodeUpdateChannel <- nodeUpdate:
            }
        }
    }(stopCh)            

    go func(stopCh <-chan struct{}) {
        for {
            item, shutdown := tc.podUpdateQueue.Get()
            if shutdown {
                break
            }
            podUpdate := item.(*podUpdateItem)
            select {     
            case <-stopCh:            
                break
            case tc.podUpdateChannel <- podUpdate:
            }
        }
    }(stopCh)
    
    for {                
        select {         
        case <-stopCh:   
            break        
        case nodeUpdate := <-tc.nodeUpdateChannel:
            tc.handleNodeUpdate(nodeUpdate)
        case podUpdate := <-tc.podUpdateChannel: 
            // If we found a Pod update we need to empty Node queue first.
        priority:
            for {        
                select {
                case nodeUpdate := <-tc.nodeUpdateChannel:
                    tc.handleNodeUpdate(nodeUpdate)
                default:
                    break priority            
                }
            }
            // After Node queue is emptied we process podUpdate.
            tc.handlePodUpdate(podUpdate)
        }
    }
}

func deletePodHandler(c clientset.Interface, emitEventFunc func(types.NamespacedName)) func(args *WorkArgs) error {
    return func(args *WorkArgs) error {
        ns := args.NamespacedName.Namespace
        name := args.NamespacedName.Name
        glog.V(0).Infof("NoExecuteTaintManager is deleting Pod: %v", args.NamespacedName.String())
        if emitEventFunc != nil { 
            emitEventFunc(args.NamespacedName)
        }
        var err error    
        for i := 0; i < retries; i++ {
            err = c.Core().Pods(ns).Delete(name, &metav1.DeleteOptions{})
            if err == nil {           
                break
            }
            time.Sleep(10 * time.Millisecond)
        }
        return err
    }                    
}

本质上还是讲带eviction节点上的pod加入到删除队列上

2. kubelet的eviction机制

kube-controller-manager的eviction机制是粗粒度的，即驱赶一个节点上的所有pod，而kubelet则是细粒度的，它驱赶的是节点上的某些Pod，驱赶哪些Pod与之前讲过的Pod的Qos机制有关。

kubelet的eviction机制，只有当节点内存和磁盘资源紧张时，才会开启，他的目的就是为了回收node节点的资源。之前提过，kubelet还有oom-killer可以回收资源，那为什么还需要eviction呢？这是因为oom-killer将Pod杀掉后，假如Pod的RestartPolicy设置为Always，则kubelet隔段时间后，仍然会在该节点上启动该Pod。而kublet eviction则会将该Pod从该节点上删除。

kubelet提供了以下参数控制eviction

• eviction-hard：一系列的阈值，比如memory.available<1Gi，即当节点可用内存低于1Gi时，会立即触发一次pod eviction
• eviction-max-pod-grace-period：eviction-soft时，终止Pod的grace时间
• eviction-minimum-reclaim：表示每一次eviction必须至少回收多少资源
• eviction-pressure-transition-period：默认为5分钟，脱离pressure condition的时间，超过阈值时，节点会被设置为memory pressure或者disk pressure，然后开启pod eviction
• eviction-soft：与hard相对应，也是一系列法制，比如memory.available<1.5Gi。但它不会立即执行pod eviction，而会等待eviction-soft-grace-period时间，假如该时间过后，依然还是达到了eviction-soft，则触发一次pod eviction
• eviction-soft-grace-period：默认为90秒

2.1 核心代码

kubelet eviction的核心代码就是如下，里面的synchronize就是核心函数

func (m *managerImpl) Start(diskInfoProvider DiskInfoProvider, podFunc ActivePodsFunc, podCleanedUpFunc PodCleanedUpFunc, nodeProvider NodeProvider, monitoringInterval time.Duration) {
    // start the eviction manager monitoring
    go func() {
        for {
            if evictedPods := m.synchronize(diskInfoProvider, podFunc, nodeProvider); evictedPods != nil {
                glog.Infof("eviction manager: pods %s evicted, waiting for pod to be cleaned up", format.Pods(evictedPods))
                m.waitForPodsCleanup(podCleanedUpFunc, evictedPods)
            } else {
                time.Sleep(monitoringInterval)
            }       
        }       
    }()     
}

2.2 何时检测触发eviction的条件

目前主要由两种机制检测触发eviction的条件

• 第一种就是定时触发，前面的synchronize位于一个for循环，其中monitoringInterval为10s，也就是每隔10s会去检测出发条件
• 通过cgroup订阅而触发，也就是假如内存低于阈值，cgroup就会通知kubelet去执行synchronize，内核层通知应用层，通过eventfd实现

if m.config.KernelMemcgNotification && !m.notifiersInitialized { 
        glog.Infof("eviction manager attempting to integrate with kernel memcg notification api")
        m.notifiersInitialized = true
        // start soft memory notification
        err = startMemoryThresholdNotifier(m.config.Thresholds, observations, false, func(desc string) {
            glog.Infof("soft memory eviction threshold crossed at %s", desc)
            // TODO wait grace period for soft memory limit
            m.synchronize(diskInfoProvider, podFunc, nodeProvider)
        })
        if err != nil {  
            glog.Warningf("eviction manager: failed to create hard memory threshold notifier: %v", err)
        }
        // start hard memory notification
        err = startMemoryThresholdNotifier(m.config.Thresholds, observations, true, func(desc string) { 
            glog.Infof("hard memory eviction threshold crossed at %s", desc)
            m.synchronize(diskInfoProvider, podFunc, nodeProvider)
        })
        if err != nil {
            glog.Warningf("eviction manager: failed to create soft memory threshold notifier: %v", err)
        }
    } 

2.3 资源的回收

kubelet的eviction主要会回收两种资源，内存和磁盘

• 磁盘回收：主要通过删除已经终止的容器和未使用的镜像
• 内存回收：主要通过终止正在运行的Pod

2.4 Qos对Eviction的影响

eviction manager会获取该节点上所有的容器，然后根据一定的算法对Pod进行排序，这里看看针对内存如何排序

// rankMemoryPressure orders the input pods for eviction in response to memory pressure.
func rankMemoryPressure(pods []*v1.Pod, stats statsFunc) {
    orderedBy(qosComparator, memory(stats)).Sort(pods)
}

// qosComparator compares pods by QoS (BestEffort < Burstable < Guaranteed)
func qosComparator(p1, p2 *v1.Pod) int {
    qosP1 := v1qos.GetPodQOS(p1)
    qosP2 := v1qos.GetPodQOS(p2)
    // its a tie
    if qosP1 == qosP2 {
        return 0
    }   
    // if p1 is best effort, we know p2 is burstable or guaranteed
    if qosP1 == v1.PodQOSBestEffort {
        return -1
    }
    // we know p1 and p2 are not besteffort, so if p1 is burstable, p2 must be guaranteed
    if qosP1 == v1.PodQOSBurstable {
        if qosP2 == v1.PodQOSGuaranteed {  
            return -1
        }
        return 1
    }
    // ok, p1 must be guaranteed.
    return 1
}

从qosComparator函数可以看出，Pod排列顺序为

PodQOSBestEffort < PodQOSBurstable < PodQOSGuaranteed

即首先会回收BestEffort的Pod，然后回收Burstable，最后才会回收Guranteed

2.5 Eviction的本质

// we kill at most a single pod during each eviction interval
    for i := range activePods {
        pod := activePods[i]      
        // If the pod is marked as critical and static, and support for critical pod annotations is enabled,
        // do not evict such pods. Static pods are not re-admitted after evictions.
        // https://github.com/kubernetes/kubernetes/issues/40573 has more details.
        if utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation) &&
            kubelettypes.IsCriticalPod(pod) && kubepod.IsStaticPod(pod) {
            continue     
        }
        status := v1.PodStatus{   
            Phase:   v1.PodFailed,    
            Message: fmt.Sprintf(message, resourceToReclaim),
            Reason:  reason,          
        }
        // record that we are evicting the pod
        m.recorder.Eventf(pod, v1.EventTypeWarning, reason, fmt.Sprintf(message, resourceToReclaim))
        gracePeriodOverride := int64(0)
        if softEviction {
            gracePeriodOverride = m.config.MaxPodGracePeriodSeconds
        }
        // this is a blocking call and should only return when the pod and its containers are killed.
        err := m.killPodFunc(pod, status, &gracePeriodOverride)
        if err != nil {  
            glog.Warningf("eviction manager: error while evicting pod %s: %v", format.Pod(pod), err)
        }                
        return []*v1.Pod{pod}     
    }

每次最多回收1个Pod

假如是hardEviction，则PodDeleteGracePeriod设置为0，即立即删除，否则设置为MaxPodGracePeriodSeconds。然后调用killPodFunc删除Pod

3. 注意点

值得注意的是，当kubernetes驱赶Pod的时候，kubernetes并不会重新创建Pod，假如要重新创建Pod，需要借助replicationcontroller、relicaset和deployment等机制。也就是说假如，你直接创建一个Pod，当它被kubernetes驱赶时，该Pod直接被删除了，不会重建。而利用replicationcontroller等机制，由于少了一个Pod，这些控制器就会重新创建一个Pod

文章转自：http://licyhust.com/%E5%AE%B9%E5%99%A8%E6%8A%80%E6%9C%AF/2017/10/24/eviction/

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