func createAndInitKubelet(......) {
    k, err = kubelet.NewMainKubelet(
        ......
    )
    if err != nil {
        return nil, err
    }

    k.BirthCry()

    k.StartGarbageCollection()

    return k, nil
}

func (kl *Kubelet) StartGarbageCollection() {
    loggedContainerGCFailure := false
    
    // 1、启动容器垃圾回收服务
    go wait.Until(func() {
        if err := kl.containerGC.GarbageCollect(); err != nil {
            loggedContainerGCFailure = true
        } else {
            var vLevel klog.Level = 4
            if loggedContainerGCFailure {
                vLevel = 1
                loggedContainerGCFailure = false
            }

            klog.V(vLevel).Infof("Container garbage collection succeeded")
        }
    }, ContainerGCPeriod, wait.NeverStop)
    
    // 2、检查 ImageGCHighThresholdPercent 参数的值
    if kl.kubeletConfiguration.ImageGCHighThresholdPercent == 100 {
        return
    }

    // 3、启动镜像垃圾回收服务
    prevImageGCFailed := false
    go wait.Until(func() {
        if err := kl.imageManager.GarbageCollect(); err != nil {
            ......
            prevImageGCFailed = true
        } else {
            var vLevel klog.Level = 4
            if prevImageGCFailed {
                vLevel = 1
                prevImageGCFailed = false
            }
        }
    }, ImageGCPeriod, wait.NeverStop)
}

func NewMainKubelet(){
    ......
    // MinAge、MaxPerPodContainer、MaxContainers 分别上文章开头提到的与容器垃圾回收有关的
    // 三个参数
    containerGCPolicy := kubecontainer.ContainerGCPolicy{
        MinAge:             minimumGCAge.Duration,
        MaxPerPodContainer: int(maxPerPodContainerCount),
        MaxContainers:      int(maxContainerCount),
    }
    
    // 初始化 containerGC 模块
    containerGC, err := kubecontainer.NewContainerGC(klet.containerRuntime, containerGCPolicy, klet.sourcesReady)
    if err != nil {
        return nil, err
    }
    ......
}

func NewContainerGC(runtime Runtime, policy ContainerGCPolicy, sourcesReadyProvider SourcesReadyProvider) (ContainerGC, error) {
    if policy.MinAge < 0 {
        return nil, fmt.Errorf("invalid minimum garbage collection age: %v", policy.MinAge)
    }

    return &realContainerGC{
        runtime:              runtime,
        policy:               policy,
        sourcesReadyProvider: sourcesReadyProvider,
    }, nil
}

func (cgc *realContainerGC) GarbageCollect() error {
    return cgc.runtime.GarbageCollect(cgc.policy, cgc.sourcesReadyProvider.AllReady(), false)
}

func (cgc *containerGC) GarbageCollect(gcPolicy kubecontainer.ContainerGCPolicy, allSourcesReady bool, evictTerminatedPods bool) error {
    errors := []error{}
    // 1、回收 pod 中的 container
    if err := cgc.evictContainers(gcPolicy, allSourcesReady, evictTerminatedPods); err != nil {
        errors = append(errors, err)
    }

    // 2、回收 pod 中的 sandboxes
    if err := cgc.evictSandboxes(evictTerminatedPods); err != nil {
        errors = append(errors, err)
    }

    // 3、回收 pod 以及 container 的 log dir
    if err := cgc.evictPodLogsDirectories(allSourcesReady); err != nil {
        errors = append(errors, err)
    }
    return utilerrors.NewAggregate(errors)
}

func (cgc *containerGC) evictContainers(gcPolicy kubecontainer.ContainerGCPolicy, allSourcesReady bool, evictTerminatedPods bool) error {
    // 1、获取可被回收的容器列表
    evictUnits, err := cgc.evictableContainers(gcPolicy.MinAge)
    if err != nil {
        return err
    }

    // 2、回收 Deleted 状态以及 Terminated 状态的 pod，此处 allSourcesReady 指 kubelet 
    //    支持的三种 podSource 是否都可用
    if allSourcesReady {
        for key, unit := range evictUnits {
            if cgc.podStateProvider.IsPodDeleted(key.uid) || (cgc.podStateProvider.IsPodTerminated(key.uid) && evictTerminatedPods) {
                cgc.removeOldestN(unit, len(unit)) 
                delete(evictUnits, key)
            }
        }
    }
    
    // 3、为非 Deleted 状态以及 Terminated 状态的 pod 保留 MaxPerPodContainer 个已经退出的容器
    if gcPolicy.MaxPerPodContainer >= 0 {
        cgc.enforceMaxContainersPerEvictUnit(evictUnits, gcPolicy.MaxPerPodContainer)
    }

    // 4、若 kubelet 启动时指定了 --maximum-dead-containers（默认为 -1 即不限制）参数，
    //   此时需要为 node 保留退出的容器数不能超过 --maximum-dead-containers 个
    if gcPolicy.MaxContainers >= 0 && evictUnits.NumContainers() > gcPolicy.MaxContainers {
        numContainersPerEvictUnit := gcPolicy.MaxContainers / evictUnits.NumEvictUnits()
        if numContainersPerEvictUnit < 1 {
            numContainersPerEvictUnit = 1
        }
        cgc.enforceMaxContainersPerEvictUnit(evictUnits, numContainersPerEvictUnit)

        numContainers := evictUnits.NumContainers()
        if numContainers > gcPolicy.MaxContainers {
            flattened := make([]containerGCInfo, 0, numContainers)
            for key := range evictUnits {
                flattened = append(flattened, evictUnits[key]...)
            }
            sort.Sort(byCreated(flattened))

            cgc.removeOldestN(flattened, numContainers-gcPolicy.MaxContainers)
        }
    }
    return nil
}

func (cgc *containerGC) evictSandboxes(evictTerminatedPods bool) error {
    // 1、获取 node 上所有的 container
    containers, err := cgc.manager.getKubeletContainers(true)
    if err != nil {
        return err
    }
    // 2、获取 node 上所有的 sandboxes
    sandboxes, err := cgc.manager.getKubeletSandboxes(true)
    if err != nil {
        return err
    }

    // 3、收集所有 container 的 PodSandboxId 
    sandboxIDs := sets.NewString()
    for _, container := range containers {
        sandboxIDs.Insert(container.PodSandboxId)
    }

    // 4、构建 sandboxes 与 pod 的对应关系并将其保存在 sandboxesByPodUID 中
    sandboxesByPod := make(sandboxesByPodUID)
    for _, sandbox := range sandboxes {
        podUID := types.UID(sandbox.Metadata.Uid)
        sandboxInfo := sandboxGCInfo{
            id:         sandbox.Id,
            createTime: time.Unix(0, sandbox.CreatedAt),
        }
        
        if sandbox.State == runtimeapi.PodSandboxState_SANDBOX_READY {
            sandboxInfo.active = true
        }

        if sandboxIDs.Has(sandbox.Id) {
            sandboxInfo.active = true
        }

        sandboxesByPod[podUID] = append(sandboxesByPod[podUID], sandboxInfo)
    }

    // 5、对 sandboxesByPod 进行排序
    for uid := range sandboxesByPod {
        sort.Sort(sandboxByCreated(sandboxesByPod[uid]))
    }

    // 6、遍历 sandboxesByPod，若 sandboxes 所在的 pod 处于 deleted 状态，
    // 则删除该 pod 中所有的 sandboxes 否则只保留退出时间最短的一个 sandboxes
    for podUID, sandboxes := range sandboxesByPod {
        if cgc.podStateProvider.IsPodDeleted(podUID) || (cgc.podStateProvider.IsPodTerminated(podUID) && evictTerminatedPods) {
            cgc.removeOldestNSandboxes(sandboxes, len(sandboxes))
        } else {
            cgc.removeOldestNSandboxes(sandboxes, len(sandboxes)-1)
        }
    }
    return nil
}

func (cgc *containerGC) evictPodLogsDirectories(allSourcesReady bool) error {
    osInterface := cgc.manager.osInterface
    // 1、回收 deleted 状态 pod logs dir
    if allSourcesReady {
        dirs, err := osInterface.ReadDir(podLogsRootDirectory)
        if err != nil {
            return fmt.Errorf("failed to read podLogsRootDirectory %q: %v", podLogsRootDirectory, err)
        }
        for _, dir := range dirs {
            name := dir.Name()
            podUID := parsePodUIDFromLogsDirectory(name)
            if !cgc.podStateProvider.IsPodDeleted(podUID) {
                continue
            }
            err := osInterface.RemoveAll(filepath.Join(podLogsRootDirectory, name))
            if err != nil {
                klog.Errorf("Failed to remove pod logs directory %q: %v", name, err)
            }
        }
    }
    // 2、回收 deleted 状态 container logs 链接目录
    logSymlinks, _ := osInterface.Glob(filepath.Join(legacyContainerLogsDir, fmt.Sprintf("*.%s", legacyLogSuffix)))
    for _, logSymlink := range logSymlinks {
        if _, err := osInterface.Stat(logSymlink); os.IsNotExist(err) {
            err := osInterface.Remove(logSymlink)
            if err != nil {
                klog.Errorf("Failed to remove container log dead symlink %q: %v", logSymlink, err)
            }
        }
    }
    return nil
}

func NewMainKubelet(){
    ......    
    // 初始化时需要指定三个参数，三个参数已经在上文中提到过
    imageGCPolicy := images.ImageGCPolicy{
        MinAge:               kubeCfg.ImageMinimumGCAge.Duration,
        HighThresholdPercent: int(kubeCfg.ImageGCHighThresholdPercent),
        LowThresholdPercent:  int(kubeCfg.ImageGCLowThresholdPercent),
    }
    ......
    imageManager, err := images.NewImageGCManager(klet.containerRuntime, klet.StatsProvider, kubeDeps.Recorder, nodeRef, imageGCPolicy, crOptions.PodSandboxImage)
    if err != nil {
        return nil, fmt.Errorf("failed to initialize image manager: %v", err)
    }
    klet.imageManager = imageManager
    ......
}

func (im *realImageGCManager) GarbageCollect() error {
    // 1、获取容器镜像存储目录挂载点文件系统的磁盘信息
    fsStats, err := im.statsProvider.ImageFsStats()
    if err != nil {
        return err
    }

    var capacity, available int64
    if fsStats.CapacityBytes != nil {
        capacity = int64(*fsStats.CapacityBytes)
    }
    if fsStats.AvailableBytes != nil {
        available = int64(*fsStats.AvailableBytes)
    }

    if available > capacity {
        available = capacity
    }

    if capacity == 0 {
        err := goerrors.New("invalid capacity 0 on image filesystem")
        im.recorder.Eventf(im.nodeRef, v1.EventTypeWarning, events.InvalidDiskCapacity, err.Error())
        return err
    }
    // 2、若使用率大于 HighThresholdPercent，此时需要回收镜像
    usagePercent := 100 - int(available*100/capacity)
    if usagePercent >= im.policy.HighThresholdPercent {
        // 3、计算需要释放的磁盘量
        amountToFree := capacity*int64(100-im.policy.LowThresholdPercent)/100 - available
        
        // 4、调用 im.freeSpace 回收未使用的镜像信息
        freed, err := im.freeSpace(amountToFree, time.Now())
        if err != nil {
            return err
        }

        if freed < amountToFree {
            err := fmt.Errorf("failed to garbage collect required amount of images. Wanted to free %d bytes, but freed %d bytes", amountToFree, freed)
            im.recorder.Eventf(im.nodeRef, v1.EventTypeWarning, events.FreeDiskSpaceFailed, err.Error())
            return err
        }
    }

    return nil
}

func (im *realImageGCManager) freeSpace(bytesToFree int64, freeTime time.Time) (int64, error) {
    // 1、获取已经使用的 images 列表
    imagesInUse, err := im.detectImages(freeTime)
    if err != nil {
        return 0, err
    }

    im.imageRecordsLock.Lock()
    defer im.imageRecordsLock.Unlock()

    // 2、获取所有未使用的 images 信息
    images := make([]evictionInfo, 0, len(im.imageRecords))
    for image, record := range im.imageRecords {
        if isImageUsed(image, imagesInUse) {
            klog.V(5).Infof("Image ID %s is being used", image)
            continue
        }
        images = append(images, evictionInfo{
            id:          image,
            imageRecord: *record,
        })
    }
    // 3、按镜像使用时间进行排序
    sort.Sort(byLastUsedAndDetected(images))
    // 4、回收未使用的镜像
    var deletionErrors []error
    spaceFreed := int64(0)
    for _, image := range images {
        if image.lastUsed.Equal(freeTime) || image.lastUsed.After(freeTime) {
            continue
        }

        if freeTime.Sub(image.firstDetected) < im.policy.MinAge {
            continue
        }

        // 5、调用 im.runtime.RemoveImage 删除镜像
        err := im.runtime.RemoveImage(container.ImageSpec{Image: image.id})
        if err != nil {
            deletionErrors = append(deletionErrors, err)
            continue
        }
        delete(im.imageRecords, image.id)
        spaceFreed += image.size
        if spaceFreed >= bytesToFree {
            break
        }
    }

    if len(deletionErrors) > 0 {
        return spaceFreed, fmt.Errorf("wanted to free %d bytes, but freed %d bytes space with errors in image deletion: %v", bytesToFree, spaceFreed,   errors.NewAggregate(deletionErrors))
    }
    return spaceFreed, nil
}