[YARN] MRAppMaster心跳原理 - 快车库

博客原文
hackshell

最近集群遇到一个问题,就是集群在跑任务的时候，AM会超时10min而被KILL，但任务重跑则成功，问题是随机的出现的，所以初步怀疑是因为AM心跳汇报出现问题或则RM因为繁忙hang住，AM因为某些机制导致等待10min不汇报心跳，所以我们还是先了解，AM是如何向RM汇报心跳的。

在MRAppMaster中，ContainerAllocatorRouter负责向RM申请资源（发送心跳）

RMContainerAllocator其最终父类是RMCommunicator，它实现了RMHeartbeatHandler接口

public interface RMHeartbeatHandler {
  long getLastHeartbeatTime(); // 获取上一次心跳的时间
  void runOnNextHeartbeat(Runnable callback); // 回调注册到callback队列的callback函数
}

每一次心跳回来，都会执行一次注册在heartbeatCallbacks中的回调函数：

allocatorThread = new Thread(new Runnable() {
      @Override
      public void run() {
        while (!stopped.get() && !Thread.currentThread().isInterrupted()) {
              ......
              heartbeat();            
              lastHeartbeatTime = context.getClock().getTime();// 记录上一次心跳时间
              executeHeartbeatCallbacks(); // 执行回调函数
              ....
});

RMCommunicator类中：

private void executeHeartbeatCallbacks() {
    Runnable callback = null;
    while ((callback = heartbeatCallbacks.poll()) != null) {
      callback.run();
    }
  }

在RMCommunicator启动时，首先会向RM注册，把自己的host和port告诉RM，然后在启动一条线程（startAllocatorThread）定期的调用RMContainerAllocator中实现的heartbeat方法（向RM申请资源，定期汇报信息，告诉RM自己还活着）。

AM初始化同时也会初始化RMCommunicator：

protected void serviceStart() throws Exception {
  scheduler= createSchedulerProxy(); // 获取RM的代理
  register(); // 注册
  startAllocatorThread(); // 心跳线程
....
}

AM的ContainerAllocatorRouter事件处理流程如下图：

注册流程：

调用RMCommunicator远程调用ApplicationMasterService的registerApplicationMaster方法，设置维护responseId，然后把它加入AMLivelinessMonitor中，并使用map记录时间，用来监控AM是否因为长时间没有心跳而超时，如果AM长时间没有心跳信息更新，RM就会通知NodeManager把AM移除。

心跳线程：

在发送心跳的过程中，即也是获取资源的过程

@Override
  protected synchronized void heartbeat() throws Exception {
    scheduleStats.updateAndLogIfChanged("Before Scheduling: ");
    List<Container> allocatedContainers = getResources();// 重要的方法
    if (allocatedContainers.size() > 0) {
      scheduledRequests.assign(allocatedContainers);
    }
   ......
  }

获取资源的过程：

private List<Container> getResources() throws Exception {
     ...
     response = makeRemoteRequest(); // 和RM进行交互
     ...
     // 优先处理RM发送过来的命令
     if (response.getAMCommand() != null) {
         switch(response.getAMCommand()) {
                case AM_RESYNC:
                case AM_SHUTDOWN:
                     eventHandler.handle(new JobEvent(this.getJob().getID(),
                                     JobEventType.JOB_AM_REBOOT));
                     throw new YarnRuntimeException("Resource Manager doesn't recognize AttemptId: " +
                             this.getContext().getApplicationID());
                default:
                     ....
      }
     // 等等一系列处理
}
}

构建请求：

protected AllocateResponse makeRemoteRequest() throws IOException {
    AllocateRequest allocateRequest =
        AllocateRequest.newInstance(lastResponseID,
          super.getApplicationProgress(), new ArrayList<ResourceRequest>(ask),
          new ArrayList<ContainerId>(release), blacklistRequest);
    AllocateResponse allocateResponse;
    allocateResponse = scheduler.allocate(allocateRequest); // RPC调用ApplicationMasterService的allocate方法
    .....
}

每一次心跳的调用都会刷新AMLivelinessMonitor的时间，代表AM还活着

而且我们通过代码可以看出，资源请求被封装为一个ask，即一个ResourceRequest的ArrayList的资源列表例如：

priority:20 host:host9 capability:<memory:2048, vCores:1>
priority:20 host:host2 capability:<memory:2048, vCores:1>
priority:20 host:host10 capability:<memory:2048, vCores:1>
priority:20 host:/rack/rack3203 capability:<memory:2048, vCores:1>
priority:20 host:/rack/rack3202 capability:<memory:2048, vCores:1>
priority:20 host:* capability:<memory:2048, vCores:1>

然而，ask是如何被构造的呢？

RMContainerAllocator中的addMap,addReduce,assign方法中对ask的数据内容进行了修改

addContainerReq --> addResourceRequest --> addResourceRequestToAsk;

通过在代码自己添加日志可以看出，资源会被分为local，rack，和any级别去申请资源

最终变为一个ask list发送到RM上：

 ask Capability:<memory:2048, vCores:1> ResourceName:* NumContainers:384 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:/rack/rack3201 NumContainers:227 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:/rack/rack3202 NumContainers:231 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:/rack/rack3203 NumContainers:152 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:/rack/rack3204 NumContainers:158 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:host1 NumContainers:46 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:host5 NumContainers:52 Priority:20 RelaxLocality:true
 ask Capability:<memory:2048, vCores:1> ResourceName:host6 NumContainers:38 Priority:20 RelaxLocality:true

类似日志为：

getResources() for application_1438330253091_0004: ask=29 release= 0 newContainers=0 finishedContainers=0 resourcelimit=<memory:0, vCores:0> knownNMs=24