Hadoop的mapred JobTracker端源码概览

http://jiwenke.iteye.com/blog/335093

上一节看到TaskTracker启动新任务的过程，这里接着看看在JobTracker中是怎样响应和调度的，在hadoop中，我们看到采用的是pull的方式拿到任务。 

HeartbeatResponse heartbeatResponse = jobClient.heartbeat(status,
                                                            justStarted, askForNewTask,
                                                              heartbeatResponseId);

这里是TaskTracker想JobTracker发送heartbeat的地方 - 使用的是RPC,这样我们你就来到JobTracker了: 

public synchronized HeartbeatResponse heartbeat(TaskTrackerStatus status,
                                                  boolean initialContact, boolean acceptNewTasks, short responseId)
    throws IOException {
.............
    //如果是接受新任务的话，让JotTracker去进行调度，这里会调用taskScheduler的assignTasks
    if (acceptNewTasks) {
      TaskTrackerStatus taskTrackerStatus = getTaskTracker(trackerName);
      if (taskTrackerStatus == null) {
        LOG.warn("Unknown task tracker polling; ignoring: " + trackerName);
      } else {
        List<Task> tasks = getSetupAndCleanupTasks(taskTrackerStatus);
    //这里是准备assignTask的地方，由配置的调度器来决定怎样调度
        if (tasks == null ) {
         tasks = taskScheduler.assignTasks(taskTrackerStatus);
        }
        if (tasks != null) {
          for (Task task : tasks) {
            expireLaunchingTasks.addNewTask(task.getTaskID());
            LOG.debug(trackerName + " -> LaunchTask: " + task.getTaskID());
            actions.add(new LaunchTaskAction(task));
          }
        }
      }
    }

这个taskScheduler采用的是默认的    

taskScheduler = (TaskScheduler) ReflectionUtils.newInstance(schedulerClass,conf);

这是在配置文件中指定的，"mapred.jobtracker.taskScheduler"，常常是JobQueueTaskScheduler是hadoop的实现,FIFO类型的调度器，让我们看看这个调度器是怎样assignTasks的： 

public synchronized List<Task> assignTasks(TaskTrackerStatus taskTracker)
      throws IOException {

    ClusterStatus clusterStatus = taskTrackerManager.getClusterStatus();
    int numTaskTrackers = clusterStatus.getTaskTrackers();

    Collection<JobInProgress> jobQueue =
      jobQueueJobInProgressListener.getJobQueue();

    //
    // Get map + reduce counts for the current tracker.
    //
    int maxCurrentMapTasks = taskTracker.getMaxMapTasks();
    int maxCurrentReduceTasks = taskTracker.getMaxReduceTasks();
    int numMaps = taskTracker.countMapTasks();
    int numReduces = taskTracker.countReduceTasks();

    //
    // Compute average map and reduce task numbers across pool
    //
    int remainingReduceLoad = 0;
    int remainingMapLoad = 0;
    synchronized (jobQueue) {
      for (JobInProgress job : jobQueue) {
        if (job.getStatus().getRunState() == JobStatus.RUNNING) {
          int totalMapTasks = job.desiredMaps();
          int totalReduceTasks = job.desiredReduces();
          remainingMapLoad += (totalMapTasks - job.finishedMaps());
          remainingReduceLoad += (totalReduceTasks - job.finishedReduces());
        }
      }
    }

    // find out the maximum number of maps or reduces that we are willing
    // to run on any node.
    int maxMapLoad = 0;
    int maxReduceLoad = 0;
    if (numTaskTrackers > 0) {
      maxMapLoad = Math.min(maxCurrentMapTasks,
                            (int) Math.ceil((double) remainingMapLoad /
                                            numTaskTrackers));
      maxReduceLoad = Math.min(maxCurrentReduceTasks,
                               (int) Math.ceil((double) remainingReduceLoad
                                               / numTaskTrackers));
    }

    int totalMaps = clusterStatus.getMapTasks();
    int totalMapTaskCapacity = clusterStatus.getMaxMapTasks();
    int totalReduces = clusterStatus.getReduceTasks();
    int totalReduceTaskCapacity = clusterStatus.getMaxReduceTasks();

    //
    // In the below steps, we allocate first a map task (if appropriate),
    // and then a reduce task if appropriate.  We go through all jobs
    // in order of job arrival; jobs only get serviced if their
    // predecessors are serviced, too.
    //

    //
    // We hand a task to the current taskTracker if the given machine
    // has a workload that's less than the maximum load of that kind of
    // task.
    //

    if (numMaps < maxMapLoad) {

      int totalNeededMaps = 0;
      synchronized (jobQueue) {
        for (JobInProgress job : jobQueue) {
          if (job.getStatus().getRunState() != JobStatus.RUNNING) {
            continue;
          }
      //这里是取得Task的地方，需要到job中去取
          Task t = job.obtainNewMapTask(taskTracker, numTaskTrackers,
              taskTrackerManager.getNumberOfUniqueHosts());
          if (t != null) {
            return Collections.singletonList(t);
          }

          //
          // Beyond the highest-priority task, reserve a little
          // room for failures and speculative executions; don't
          // schedule tasks to the hilt.
          //
          totalNeededMaps += job.desiredMaps();
          int padding = 0;
          if (numTaskTrackers > MIN_CLUSTER_SIZE_FOR_PADDING) {
            padding = Math.min(maxCurrentMapTasks,
                               (int)(totalNeededMaps * padFraction));
          }
          if (totalMaps + padding >= totalMapTaskCapacity) {
            break;
          }
        }
      }
    }

    //
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      2010-11-14 11:23
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