讲到了DAGScheduler.submitMissingTasks中最终调用了taskScheduler.submitTasks来提交任务。
这篇我们就从taskScheduler.submitTasks开始讲，深入理解TaskScheduler的运行过程。
提交Task
调用栈如下：
TaskSchedulerImpl.submitTasks
CoarseGrainedSchedulerBackend.reviveOffers
CoarseGrainedSchedulerBackend.DriverEndpoint.makeOffers
TaskSchedulerImpl.resourceOffers
TaskSchedulerImpl.resourceOfferSingleTaskSet
CoarseGrainedSchedulerBackend.DriverEndpoint.launchTasks
TaskSchedulerImpl.submitTasks
TaskSchedulerImpl是TaskScheduler的子类，重写了submitTasks：
override def submitTasks(taskSet: TaskSet) { val tasks = taskSet.tasks
logInfo("Adding task set " + taskSet.id + " with " + tasks.length + " tasks") this.synchronized { //生成TaskSetManager
val manager = createTaskSetManager(taskSet, maxTaskFailures) val stage = taskSet.stageId val stageTaskSets =
taskSetsByStageIdAndAttempt.getOrElseUpdate(stage, new HashMap[Int, TaskSetManager])
stageTaskSets(taskSet.stageAttemptId) = manager val conflictingTaskSet = stageTaskSets.exists { case (_, ts) =>
ts.taskSet != taskSet && !ts.isZombie
} if (conflictingTaskSet) { throw new IllegalStateException(s"more than one active taskSet for stage $stage:" + s" ${stageTaskSets.toSeq.map{_._2.taskSet.id}.mkString(",")}")
} //将manager等信息放入调度器
schedulableBuilder.addTaskSetManager(manager, manager.taskSet.properties) if (!isLocal && !hasReceivedTask) {
starvationTimer.scheduleAtFixedRate(new TimerTask() { override def run() { if (!hasLaunchedTask) {
logWarning("Initial job has not accepted any resources; " + "check your cluster UI to ensure that workers are registered " + "and have sufficient resources")
} else { this.cancel()
}
}
}, STARVATION_TIMEOUT_MS, STARVATION_TIMEOUT_MS)
}
hasReceivedTask = true
} //分配资源
backend.reviveOffers()
}CoarseGrainedSchedulerBackend.reviveOffers
下面我们来讲讲上一节代码中最后一句：
backend.reviveOffers()
我们先回过头来看TaskScheduler是如何启动的：
override def start() {
backend.start() if (!isLocal && conf.getBoolean("spark.speculation", false)) {
logInfo("Starting speculative execution thread")
speculationScheduler.scheduleAtFixedRate(new Runnable { override def run(): Unit = Utils.tryOrStopSparkContext(sc) {
checkSpeculatableTasks()
}
}, SPECULATION_INTERVAL_MS, SPECULATION_INTERVAL_MS, TimeUnit.MILLISECONDS)
}
}我们可以看到TaskScheduler.start会调用backend.start()。
backend是一个SchedulerBackend接口。SchedulerBackend接口由CoarseGrainedSchedulerBackend类实现。我们看下CoarseGrainedSchedulerBackend的start：
override def start() { val properties = new ArrayBuffer[(String, String)] for ((key, value) <- scheduler.sc.conf.getAll) { if (key.startsWith("spark.")) {
properties += ((key, value))
}
}
driverEndpoint = createDriverEndpointRef(properties)
}我们可以看到CoarseGrainedSchedulerBackend的start会生成driverEndpoint，它是一个rpc的终端，一个RpcEndpoint接口，它由ThreadSafeRpcEndpoint接口实现，而ThreadSafeRpcEndpoint由CoarseGrainedSchedulerBackend的内部类DriverEndpoint实现。
CoarseGrainedSchedulerBackend的reviveOffers就是发送给这个rpc的终端ReviveOffers信号。
override def reviveOffers() {
driverEndpoint.send(ReviveOffers)
}CoarseGrainedSchedulerBackend.DriverEndpoint.makeOffers
DriverEndpoint有两种发送信息的函数。一个是send，发送信息后不需要对方回复。一个是ask，发送信息后需要对方回复。
对应着，也有两种接收信息的函数。一个是receive，接收后不回复对方：
override def receive: PartialFunction[Any, Unit] = { case StatusUpdate(executorId, taskId, state, data) =>
scheduler.statusUpdate(taskId, state, data.value) if (TaskState.isFinished(state)) {
executorDataMap.get(executorId) match { case Some(executorInfo) =>
executorInfo.freeCores += scheduler.CPUS_PER_TASK
makeOffers(executorId) case None =>
logWarning(s"Ignored task status update ($taskId state $state) " + s"from unknown executor with ID $executorId")
}
} case ReviveOffers =>
makeOffers() case KillTask(taskId, executorId, interruptThread) =>
executorDataMap.get(executorId) match { case Some(executorInfo) =>
executorInfo.executorEndpoint.send(KillTask(taskId, executorId, interruptThread)) case None =>
logWarning(s"Attempted to kill task $taskId for unknown executor $executorId.")
}
}另外一个是receiveAndReply，接收后回复对方：
override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = { case RegisterExecutor(executorId, executorRef, hostname, cores, logUrls) => if (executorDataMap.contains(executorId)) {
executorRef.send(RegisterExecutorFailed("Duplicate executor ID: " + executorId))
context.reply(true)
} else {
val executorAddress = if (executorRef.address != null) {
executorRef.address
} else {
context.senderAddress
}
logInfo(s"Registered executor $executorRef ($executorAddress) with ID $executorId")
addressToExecutorId(executorAddress) = executorId
totalCoreCount.addAndGet(cores)
totalRegisteredExecutors.addAndGet(1) val data = new ExecutorData(executorRef, executorRef.address, hostname,
cores, cores, logUrls) CoarseGrainedSchedulerBackend.this.synchronized {
executorDataMap.put(executorId, data) if (currentExecutorIdCounter < executorId.toInt) {
currentExecutorIdCounter = executorId.toInt
} if (numPendingExecutors > 0) {
numPendingExecutors -= 1
logDebug(s"Decremented number of pending executors ($numPendingExecutors left)")
}
}
executorRef.send(RegisteredExecutor)
context.reply(true)
listenerBus.post( SparkListenerExecutorAdded(System.currentTimeMillis(), executorId, data))
makeOffers()
} case StopDriver =>
context.reply(true)
stop() case StopExecutors =>
logInfo("Asking each executor to shut down") for ((_, executorData) <- executorDataMap) {
executorData.executorEndpoint.send(StopExecutor)
}
context.reply(true) case RemoveExecutor(executorId, reason) =>
executorDataMap.get(executorId).foreach(_.executorEndpoint.send(StopExecutor))
removeExecutor(executorId, reason)
context.reply(true) case RetrieveSparkAppConfig => val reply = SparkAppConfig(sparkProperties, SparkEnv.get.securityManager.getIOEncryptionKey())
context.reply(reply)
}
private def makeOffers() {
val activeExecutors = executorDataMap.filterKeys(executorIsAlive) val workOffers = activeExecutors.map { case (id, executorData) => new WorkerOffer(id, executorData.executorHost, executorData.freeCores)
}.toIndexedSeq
launchTasks(scheduler.resourceOffers(workOffers))
}我们可以看到之前在CoarseGrainedSchedulerBackend的reviveOffers发送的ReviveOffers信号会在receive中被接收，从而调用makeOffers：
case ReviveOffers => makeOffers()
makeOffers做的工作为：
private def makeOffers() { //过滤掉被杀死的Executor
val activeExecutors = executorDataMap.filterKeys(executorIsAlive) //根据activeExecutors生成workOffers，
//即executor所能提供的资源信息。
val workOffers = activeExecutors.map { case (id, executorData) => new WorkerOffer(id, executorData.executorHost, executorData.freeCores)
}.toIndexedSeq //scheduler.resourceOffers分配资源，
//并launchTasks发送任务
launchTasks(scheduler.resourceOffers(workOffers))
}launchTasks主要的实现是向executor发送LaunchTask信号：
executorData.executorEndpoint.send(LaunchTask(new SerializableBuffer(serializedTask)))
TaskSchedulerImpl.resourceOffers
下面我们来深入上节scheduler.resourceOffers分配资源的函数：
def resourceOffers(offers: IndexedSeq[WorkerOffer]): Seq[Seq[TaskDescription]] = synchronized { //标记每个活的节点并记录它的主机名
//并且追踪是否有新的executor加入
var newExecAvail = false
for (o <- offers) { if (!hostToExecutors.contains(o.host)) {
hostToExecutors(o.host) = new HashSet[String]()
} if (!executorIdToRunningTaskIds.contains(o.executorId)) {
hostToExecutors(o.host) += o.executorId
executorAdded(o.executorId, o.host)
executorIdToHost(o.executorId) = o.host
executorIdToRunningTaskIds(o.executorId) = HashSet[Long]()
newExecAvail = true
} for (rack <- getRackForHost(o.host)) {
hostsByRack.getOrElseUpdate(rack, new HashSet[String]()) += o.host
}
} // 为了避免将Task集中分配到某些机器，随机的打散它们
val shuffledOffers = Random.shuffle(offers) // 建立每个worker的TaskDescription数组
val tasks = shuffledOffers.map(o => new ArrayBuffer[TaskDescription](o.cores)) //记录各个worker的available Cpus
val availableCpus = shuffledOffers.map(o => o.cores).toArray //获取按照调度策略排序好的TaskSetManager
val sortedTaskSets = rootPool.getSortedTaskSetQueue for (taskSet <- sortedTaskSets) {
logDebug("parentName: %s, name: %s, runningTasks: %s".format(
taskSet.parent.name, taskSet.name, taskSet.runningTasks)) //如果有新的executor加入
//则需要从新计算TaskSetManager的就近原则
if (newExecAvail) {
taskSet.executorAdded()
}
} // 得到调度序列中的每个TaskSet,
// 然后按节点的locality级别增序分配资源
// Locality优先序列为: PROCESS_LOCAL, NODE_LOCAL, NO_PREF, RACK_LOCAL, ANY
for (taskSet <- sortedTaskSets) { var launchedAnyTask = false
var launchedTaskAtCurrentMaxLocality = false
//按照就近原则分配
for (currentMaxLocality <- taskSet.myLocalityLevels) {
do { //resourceOfferSingleTaskSet为单个TaskSet分配资源，
//若该LocalityLevel的节点下不能再为之分配资源了，
//则返回false
launchedTaskAtCurrentMaxLocality = resourceOfferSingleTaskSet(
taskSet, currentMaxLocality, shuffledOffers, availableCpus, tasks)
launchedAnyTask |= launchedTaskAtCurrentMaxLocality
} while (launchedTaskAtCurrentMaxLocality)
} if (!launchedAnyTask) {
taskSet.abortIfCompletelyBlacklisted(hostToExecutors)
}
} if (tasks.size > 0) {
hasLaunchedTask = true
} return tasks
}这里涉及到两个排序，首先调度器会对TaskSet进行排序：
val sortedTaskSets = rootPool.getSortedTaskSetQueue
取出每个TaskSet后，我们又会根据从近到远的Locality Level 的来对各个Task进行资源的分配。
TaskSchedulerImpl.resourceOfferSingleTaskSet
接下来我们来看下为单个TaskSet分配资源的具体实现：
private def resourceOfferSingleTaskSet(
taskSet: TaskSetManager,
maxLocality: TaskLocality,
shuffledOffers: Seq[WorkerOffer],
availableCpus: Array[Int],
tasks: IndexedSeq[ArrayBuffer[TaskDescription]]) : Boolean = { var launchedTask = false
//遍历各个executor
for (i <- 0 until shuffledOffers.size) { val execId = shuffledOffers(i).executorId val host = shuffledOffers(i).host if (availableCpus(i) >= CPUS_PER_TASK) { try { //获取taskSet中，相对于该execId, host所能接收的最大距离maxLocality的task
//maxLocality的值在TaskSchedulerImpl.resourceOffers中从近到远的遍历
for (task <- taskSet.resourceOffer(execId, host, maxLocality)) {
tasks(i) += task val tid = task.taskId
taskIdToTaskSetManager(tid) = taskSet
taskIdToExecutorId(tid) = execId
executorIdToRunningTaskIds(execId).add(tid)
availableCpus(i) -= CPUS_PER_TASK
assert(availableCpus(i) >= 0)
launchedTask = true
}
} catch { case e: TaskNotSerializableException =>
logError(s"Resource offer failed, task set ${taskSet.name} was not serializable") return launchedTask
}
}
} return launchedTask
}CoarseGrainedSchedulerBackend.DriverEndpoint.launchTasks
我们回到CoarseGrainedSchedulerBackend.DriverEndpoint.makeOffers，看最后一步，发送任务的函数launchTasks：
private def launchTasks(tasks: Seq[Seq[TaskDescription]]) { for (task <- tasks.flatten) { val serializedTask = ser.serialize(task) //若序列话Task大小达到Rpc限制，
//则停止
if (serializedTask.limit >= maxRpcMessageSize) {
scheduler.taskIdToTaskSetManager.get(task.taskId).foreach { taskSetMgr => try { var msg = "Serialized task %s:%d was %d bytes, which exceeds max allowed: " + "spark.rpc.message.maxSize (%d bytes). Consider increasing " + "spark.rpc.message.maxSize or using broadcast variables for large values."
msg = msg.format(task.taskId, task.index, serializedTask.limit, maxRpcMessageSize)
taskSetMgr.abort(msg)
} catch { case e: Exception => logError("Exception in error callback", e)
}
}
} else { // 减少改task所对应的executor信息的core数量
val executorData = executorDataMap(task.executorId)
executorData.freeCores -= scheduler.CPUS_PER_TASK
logDebug(s"Launching task ${task.taskId} on executor id: ${task.executorId} hostname: " + s"${executorData.executorHost}.") //向executorEndpoint 发送LaunchTask 信号
executorData.executorEndpoint.send(LaunchTask(new SerializableBuffer(serializedTask)))
}
}
}executorEndpoint接收到LaunchTask信号（包含SerializableBuffer(serializedTask) ）后，会开始执行任务。
调度任务
Pool.getSortedTaskSetQueue
上一章我们讲到TaskSchedulerImpl.resourceOffers中会调用：
val sortedTaskSets = rootPool.getSortedTaskSetQueue
获取按照调度策略排序好的TaskSetManager。接下来我们深入讲解这行代码。
rootPool是一个Pool对象。Pool定义为：一个可调度的实体，代表着Pool的集合或者TaskSet的集合，即Schedulable为一个接口，由Pool类和TaskSetManager类实现
getSortedTaskSetQueue：
override def getSortedTaskSetQueue: ArrayBuffer[TaskSetManager] = { //生成TaskSetManager数组
var sortedTaskSetQueue = new ArrayBuffer[TaskSetManager] //对调度实体进行排序
val sortedSchedulableQueue =
schedulableQueue.asScala.toSeq.sortWith(taskSetSchedulingAlgorithm.comparator) for (schedulable <- sortedSchedulableQueue) { //从调度实体中取得TaskSetManager数组
sortedTaskSetQueue ++= schedulable.getSortedTaskSetQueue
}
sortedTaskSetQueue
}其中调度算法taskSetSchedulingAlgorithm，会在Pool被生成时候根据SchedulingMode被设定为FairSchedulingAlgorithm或者FIFOSchedulingAlgorithm
var taskSetSchedulingAlgorithm: SchedulingAlgorithm = {
schedulingMode match { case SchedulingMode.FAIR => new FairSchedulingAlgorithm() case SchedulingMode.FIFO => new FIFOSchedulingAlgorithm() case _ => val msg = "Unsupported scheduling mode: $schedulingMode. Use FAIR or FIFO instead."
throw new IllegalArgumentException(msg)
}
}TaskSchedulerImpl.initialize
Pool被生成是什么时候被生成的呢？我们来看下TaskSchedulerImpl的初始化就能发现：
def initialize(backend: SchedulerBackend) { this.backend = backend // 创建一个名字为空的rootPool
rootPool = new Pool("", schedulingMode, 0, 0)
schedulableBuilder = {
schedulingMode match { //TaskSchedulerImpl在初始化时，
//根据SchedulingMode来创建不同的schedulableBuilder
case SchedulingMode.FIFO => new FIFOSchedulableBuilder(rootPool) case SchedulingMode.FAIR => new FairSchedulableBuilder(rootPool, conf) case _ => throw new IllegalArgumentException(s"Unsupported spark.scheduler.mode: $schedulingMode")
}
}
schedulableBuilder.buildPools()
}FIFO 调度
FIFOSchedulableBuilder.addTaskSetManager
接下来，我们回过头看TaskSchedulerImpl.submitTasks中的schedulableBuilder.addTaskSetManager。
schedulableBuilder.addTaskSetManager(manager, manager.taskSet.properties)
我们深入讲一下addTaskSetManager：
这里写图片描述
schedulableBuilder是一个SchedulableBuilder接口，SchedulableBuilder接口由两个类FIFOSchedulableBuilder和FairSchedulableBuilder实现。
我们这里先讲解FIFOSchedulableBuilder，FIFOSchedulableBuilder的addTaskSetManager：
override def addTaskSetManager(manager: Schedulable, properties: Properties) {
rootPool.addSchedulable(manager)
}再看addSchedulable：
override def addSchedulable(schedulable: Schedulable) {
require(schedulable != null)
schedulableQueue.add(schedulable)
schedulableNameToSchedulable.put(schedulable.name, schedulable)
schedulable.parent = this
}实际上是将manager加入到schedulableQueue（这里是FIFO的queue），将manger的name加入到一个名为schedulableNameToSchedulable的 ConcurrentHashMap[String, Schedulable]中，并将manager的parent设置为rootPool。
FIFOSchedulableBuilder.buildPools()
上述后一行代码：
schedulableBuilder.buildPools()
buildPools会因不同的调度器而异。如果是FIFOSchedulableBuilder，那么就为空：
override def buildPools() { // nothing
}这是因为rootPool里面不包含其他的Pool，而是像上述所讲的直接将manager的parent设置为rootPool。实际上，这是一种2层的树形结构，第0层为rootPool，第二层叶子节点为各个manager：
这里写图片描述
FIFOSchedulingAlgorithm
一切就绪后，我们可以来看FIFO的核心调度算法了：
private[spark] class FIFOSchedulingAlgorithm extends SchedulingAlgorithm { override def comparator(s1: Schedulable, s2: Schedulable): Boolean = { //priority实际上是 Job ID
val priority1 = s1.priority val priority2 = s2.priority //先比较Job ID
var res = math.signum(priority1 - priority2) if (res == 0) { //若Job ID相同，
//则比较 Stage ID
val stageId1 = s1.stageId val stageId2 = s2.stageId
res = math.signum(stageId1 - stageId2)
}
res < 0
}
}FAIR 调度
FairSchedulableBuilder.addTaskSetManager
FairSchedulableBuilder的addTaskSetManager会比FIFOSchedulableBuilder的复杂：
override def addTaskSetManager(manager: Schedulable, properties: Properties) { //先生成一个默认的parentPool
var poolName = DEFAULT_POOL_NAME
var parentPool = rootPool.getSchedulableByName(poolName) //若有配置信息，
//则根据配置信息得到poolName
if (properties != null) { //FAIR_SCHEDULER_PROPERTIES = "spark.scheduler.pool"
poolName = properties.getProperty(FAIR_SCHEDULER_PROPERTIES, DEFAULT_POOL_NAME)
parentPool = rootPool.getSchedulableByName(poolName) //若rootPool中没有这个pool
if (parentPool == null) { //我们会根据用户在app上的配置生成新的pool，
//而不是根据xml 文件
parentPool = new Pool(poolName, DEFAULT_SCHEDULING_MODE, DEFAULT_MINIMUM_SHARE, DEFAULT_WEIGHT)
rootPool.addSchedulable(parentPool)
logInfo("Created pool %s, schedulingMode: %s, minShare: %d, weight: %d".format(
poolName, DEFAULT_SCHEDULING_MODE, DEFAULT_MINIMUM_SHARE, DEFAULT_WEIGHT))
}
} //将这个manager加入到这个pool
parentPool.addSchedulable(manager)
logInfo("Added task set " + manager.name + " tasks to pool " + poolName)
}
}FairSchedulableBuilder.buildPools()
FairSchedulableBuilder.buildPools需要根据$SPARK_HOME/conf/fairscheduler.xml文件来构建调度树。配置文件大致如下：
<allocations> <pool name="production"> <schedulingMode>FAIR</schedulingMode> <weight>1</weight> <minShare>2</minShare> </pool> <pool name="test"> <schedulingMode>FIFO</schedulingMode> <weight>2</weight> <minShare>3</minShare> </pool></allocations>
buildFairSchedulerPool：
private def buildFairSchedulerPool(is: InputStream) { //加载xml 文件
val xml = XML.load(is) //遍历
for (poolNode <- (xml \\ POOLS_PROPERTY)) { val poolName = (poolNode \ POOL_NAME_PROPERTY).text var schedulingMode = DEFAULT_SCHEDULING_MODE
var minShare = DEFAULT_MINIMUM_SHARE
var weight = DEFAULT_WEIGHT
val xmlSchedulingMode = (poolNode \ SCHEDULING_MODE_PROPERTY).text if (xmlSchedulingMode != "") { try {
schedulingMode = SchedulingMode.withName(xmlSchedulingMode)
} catch { case e: NoSuchElementException =>
logWarning(s"Unsupported schedulingMode: $xmlSchedulingMode, " + s"using the default schedulingMode: $schedulingMode")
}
} val xmlMinShare = (poolNode \ MINIMUM_SHARES_PROPERTY).text if (xmlMinShare != "") {
minShare = xmlMinShare.toInt
} val xmlWeight = (poolNode \ WEIGHT_PROPERTY).text if (xmlWeight != "") {
weight = xmlWeight.toInt
} //根据xml的配置，
//最终生成一个新的Pool
val pool = new Pool(poolName, schedulingMode, minShare, weight) //将这个Pool加入到rootPool中
rootPool.addSchedulable(pool)
logInfo("Created pool %s, schedulingMode: %s, minShare: %d, weight: %d".format(
poolName, schedulingMode, minShare, weight))
}
}可想而知，FAIR 调度并不是简单的公平调度。我们会先根据xml配置文件生成很多pool加入rootPool中，而每个app会根据配置“spark.scheduler.pool”的poolName，将TaskSetManager加入到某个pool中。其实，rootPool还会对Pool也进程一次调度。
所以，在FAIR调度策略中包含了两层调度。第一层的rootPool内的多个Pool，第二层是Pool内的多个TaskSetManager。fairscheduler.xml文件中， weight（任务权重）和minShare（最小任务数）是来设置第一层调度的，该调度使用的是FAIR算法。而第二层调度由schedulingMode设置。
这里写图片描述
但对于Standalone模式下的单个app，FAIR调度的多个Pool显得鸡肋，因为app只能选择一个Pool。但是我们可以在代码级别硬编码的去分配：
sc.setLocalProperty("spark.scheduler.pool", "Pool_1")FAIRSchedulingAlgorithm
接下来，我们就来讲解FAIR算法：
private[spark] class FairSchedulingAlgorithm extends SchedulingAlgorithm { override def comparator(s1: Schedulable, s2: Schedulable): Boolean = { val minShare1 = s1.minShare val minShare2 = s2.minShare val runningTasks1 = s1.runningTasks val runningTasks2 = s2.runningTasks //若s1运行的任务数小于s1的最小任务数
val s1Needy = runningTasks1 < minShare1 //若s2运行的任务数小于s2的最小任务数
val s2Needy = runningTasks2 < minShare2 //minShareRatio = 运行的任务数/最小任务数
//代表着负载程度，越小，负载越小
val minShareRatio1 = runningTasks1.toDouble / math.max(minShare1, 1.0) val minShareRatio2 = runningTasks2.toDouble / math.max(minShare2, 1.0) //taskToWeightRatio = 运行的任务数/权重
//权重越大，越优先
//即taskToWeightRatio 越小 越优先
val taskToWeightRatio1 = runningTasks1.toDouble / s1.weight.toDouble val taskToWeightRatio2 = runningTasks2.toDouble / s2.weight.toDouble var compare = 0
//若s1运行的任务小于s1的最小任务数，而s2不然
//则s1优先
if (s1Needy && !s2Needy) { return true
}
//若s2运行的任务小于s2的最小任务数，而s1不然
//则s2优先
else if (!s1Needy && s2Needy) { return false
}
//若s1 s2 运行的任务都小于自己的的最小任务数
//比较minShareRatio，哪个小，哪个优先
else if (s1Needy && s2Needy) {
compare = minShareRatio1.compareTo(minShareRatio2)
}
//若s1 s2 运行的任务都不小于自己的的最小任务数
//比较taskToWeightRatio，哪个小，哪个优先
else {
compare = taskToWeightRatio1.compareTo(taskToWeightRatio2)
} if (compare < 0) { true
} else if (compare > 0) { false
} else {
s1.name < s2.name
}
}
}至此，TaskScheduler在发送任务给executor前的工作就全部完成了。
作者：小爷Souljoy
链接：https://www.jianshu.com/p/423bb19f7246
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