// If the node is decommissioning, send an update to have the total
    // resource equal to the used resource, so no available resource to
    // schedule.
    if (nm.getState() == NodeState.DECOMMISSIONING) {
      this.rmContext
          .getDispatcher()
          .getEventHandler()
          .handle(
              new RMNodeResourceUpdateEvent(nm.getNodeID(), ResourceOption
                  .newInstance(getSchedulerNode(nm.getNodeID())
                      .getUsedResource(), 0)));
    }

    if (continuousSchedulingEnabled) {
      if (!completedContainers.isEmpty()) {  //心跳调度
        attemptScheduling(node);
      }
    } else {
      attemptScheduling(node);  //持续调度
    }

    // Updating node resource utilization
    node.setAggregatedContainersUtilization(
        nm.getAggregatedContainersUtilization());
    node.setNodeUtilization(nm.getNodeUtilization());

/**
 * Thread which attempts scheduling resources continuously,
 * asynchronous to the node heartbeats.
 */
private class ContinuousSchedulingThread extends Thread {

  @Override
  public void run() {
    while (!Thread.currentThread().isInterrupted()) {
      try {
        continuousSchedulingAttempt();
        Thread.sleep(getContinuousSchedulingSleepMs());
      } catch (InterruptedException e) {
        LOG.warn("Continuous scheduling thread interrupted. Exiting.", e);
        return;
      }
    }
  }
}

void continuousSchedulingAttempt() throws InterruptedException {
    long start = getClock().getTime();
    List<NodeId> nodeIdList = new ArrayList<NodeId>(nodes.keySet());
    // Sort the nodes by space available on them, so that we offer
    // containers on emptier nodes first, facilitating an even spread. This
    // requires holding the scheduler lock, so that the space available on a
    // node doesn't change during the sort.
    synchronized (this) {
      Collections.sort(nodeIdList, nodeAvailableResourceComparator);
    }

    // iterate all nodes
    for (NodeId nodeId : nodeIdList) {
      FSSchedulerNode node = getFSSchedulerNode(nodeId);
      try {
        if (node != null && Resources.fitsIn(minimumAllocation,
            node.getAvailableResource())) {
          attemptScheduling(node);
        }
      } catch (Throwable ex) {
        LOG.error("Error while attempting scheduling for node " + node +
            ": " + ex.toString(), ex);
        if ((ex instanceof YarnRuntimeException) &&
            (ex.getCause() instanceof InterruptedException)) {
          // AsyncDispatcher translates InterruptedException to
          // YarnRuntimeException with cause InterruptedException.
          // Need to throw InterruptedException to stop schedulingThread.
          throw (InterruptedException)ex.getCause();
        }
      }
    }

    boolean validReservation = false;
    FSAppAttempt reservedAppSchedulable = node.getReservedAppSchedulable();
    if (reservedAppSchedulable != null) {
      validReservation = reservedAppSchedulable.assignReservedContainer(node);
    }
    if (!validReservation) {
      // No reservation, schedule at queue which is farthest below fair share
      int assignedContainers = 0;
      Resource assignedResource = Resources.clone(Resources.none());
      Resource maxResourcesToAssign =
          Resources.multiply(node.getAvailableResource(), 0.5f);
      while (node.getReservedContainer() == null) {
        boolean assignedContainer = false;
        Resource assignment = queueMgr.getRootQueue().assignContainer(node);
        if (!assignment.equals(Resources.none())) {
          assignedContainers++;
          assignedContainer = true;
          Resources.addTo(assignedResource, assignment);
        }
        if (!assignedContainer) { break; }
        if (!shouldContinueAssigning(assignedContainers,
            maxResourcesToAssign, assignedResource)) {
          break;
        }
      }

  @Override
  public Resource assignContainer(FSSchedulerNode node) { 对于每一个服务器，对资源树进行一次递归搜索
    Resource assigned = Resources.none();

    // If this queue is over its limit, reject
    if (!assignContainerPreCheck(node)) {
      return assigned;
    }

    // Hold the write lock when sorting childQueues
    writeLock.lock();
    try {
      Collections.sort(childQueues, policy.getComparator());
    } finally {
      writeLock.unlock();
    }

/*
     * We are releasing the lock between the sort and iteration of the
     * "sorted" list. There could be changes to the list here:
     * 1. Add a child queue to the end of the list, this doesn't affect
     * container assignment.
     * 2. Remove a child queue, this is probably good to take care of so we
     * don't assign to a queue that is going to be removed shortly.
     */
    readLock.lock();
    try {
      for (FSQueue child : childQueues) {
        assigned = child.assignContainer(node);
        if (!Resources.equals(assigned, Resources.none())) {
          break;
        }
      }
    } finally {
      readLock.unlock();
    }
    return assigned;

  @Override
  public int compare(Schedulable s1, Schedulable s2) {
    double minShareRatio1, minShareRatio2;
    double useToWeightRatio1, useToWeightRatio2;
    Resource minShare1 = Resources.min(RESOURCE_CALCULATOR, null,
        s1.getMinShare(), s1.getDemand());
    Resource minShare2 = Resources.min(RESOURCE_CALCULATOR, null,
        s2.getMinShare(), s2.getDemand());
    boolean s1Needy = Resources.lessThan(RESOURCE_CALCULATOR, null,
        s1.getResourceUsage(), minShare1);
    boolean s2Needy = Resources.lessThan(RESOURCE_CALCULATOR, null,
        s2.getResourceUsage(), minShare2);
    minShareRatio1 = (double) s1.getResourceUsage().getMemorySize()
        / Resources.max(RESOURCE_CALCULATOR, null, minShare1, ONE).getMemorySize();
    minShareRatio2 = (double) s2.getResourceUsage().getMemorySize()
        / Resources.max(RESOURCE_CALCULATOR, null, minShare2, ONE).getMemorySize();
    useToWeightRatio1 = s1.getResourceUsage().getMemorySize() /
        s1.getWeights().getWeight(ResourceType.MEMORY);
    useToWeightRatio2 = s2.getResourceUsage().getMemorySize() /
        s2.getWeights().getWeight(ResourceType.MEMORY);
    int res = 0;
    if (s1Needy && !s2Needy)
      res = -1;
    else if (s2Needy && !s1Needy)
      res = 1;
    else if (s1Needy && s2Needy)
      res = (int) Math.signum(minShareRatio1 - minShareRatio2);
    else
      // Neither schedulable is needy
      res = (int) Math.signum(useToWeightRatio1 - useToWeightRatio2);
    if (res == 0) {
      // Apps are tied in fairness ratio. Break the tie by submit time and job
      // name to get a deterministic ordering, which is useful for unit tests.
      res = (int) Math.signum(s1.getStartTime() - s2.getStartTime());
      if (res == 0)
        res = s1.getName().compareTo(s2.getName());
    }
    return res;
  }
}

@Override
    public int compare(Schedulable s1, Schedulable s2) {
      int res = compareDemand(s1, s2);

      // Pre-compute resource usages to avoid duplicate calculation
      Resource resourceUsage1 = s1.getResourceUsage();
      Resource resourceUsage2 = s2.getResourceUsage();

      if (res == 0) {
        res = compareMinShareUsage(s1, s2, resourceUsage1, resourceUsage2);
      }

      if (res == 0) {
        res = compareFairShareUsage(s1, s2, resourceUsage1, resourceUsage2);
      }

      // Break the tie by submit time
      if (res == 0) {
        res = (int) Math.signum(s1.getStartTime() - s2.getStartTime());
      }

      // Break the tie by job name
      if (res == 0) {
        res = s1.getName().compareTo(s2.getName());
      }

      return res;
    }


    private int compareDemand(Schedulable s1, Schedulable s2) {
      int res = 0;
      Resource demand1 = s1.getDemand();
      Resource demand2 = s2.getDemand();
      if (demand1.equals(Resources.none()) && Resources.greaterThan(
              RESOURCE_CALCULATOR, null, demand2, Resources.none())) {
        res = 1;
      } else if (demand2.equals(Resources.none()) && Resources.greaterThan(
              RESOURCE_CALCULATOR, null, demand1, Resources.none())) {
        res = -1;
      }
      return res;
    }


    private int compareMinShareUsage(Schedulable s1, Schedulable s2,
                                     Resource resourceUsage1, Resource resourceUsage2) {
      int res;
      Resource minShare1 = Resources.min(RESOURCE_CALCULATOR, null,
              s1.getMinShare(), s1.getDemand());
      Resource minShare2 = Resources.min(RESOURCE_CALCULATOR, null,
              s2.getMinShare(), s2.getDemand());
      boolean s1Needy = Resources.lessThan(RESOURCE_CALCULATOR, null,
              resourceUsage1, minShare1);
      boolean s2Needy = Resources.lessThan(RESOURCE_CALCULATOR, null,
              resourceUsage2, minShare2);

      if (s1Needy && !s2Needy) {
        res = -1;
      } else if (s2Needy && !s1Needy) {
        res = 1;
      } else if (s1Needy && s2Needy) {
        double minShareRatio1 = (double) resourceUsage1.getMemorySize() /
                Resources.max(RESOURCE_CALCULATOR, null, minShare1, ONE)
                        .getMemorySize();
        double minShareRatio2 = (double) resourceUsage2.getMemorySize() /
                Resources.max(RESOURCE_CALCULATOR, null, minShare2, ONE)
                        .getMemorySize();
        res = (int) Math.signum(minShareRatio1 - minShareRatio2);
      } else {
        res = 0;
      }

      return res;
    }


    /**
     * To simplify computation, use weights instead of fair shares to calculate
     * fair share usage.
     */
    private int compareFairShareUsage(Schedulable s1, Schedulable s2,
                                      Resource resourceUsage1, Resource resourceUsage2) {
      double weight1 = s1.getWeights().getWeight(ResourceType.MEMORY);
      double weight2 = s2.getWeights().getWeight(ResourceType.MEMORY);
      double useToWeightRatio1;
      double useToWeightRatio2;
      if (weight1 > 0.0 && weight2 > 0.0) {
        useToWeightRatio1 = resourceUsage1.getMemorySize() / weight1;
        useToWeightRatio2 = resourceUsage2.getMemorySize() / weight2;
      } else { // Either weight1 or weight2 equals to 0
        if (weight1 == weight2) {
          // If they have same weight, just compare usage
          useToWeightRatio1 = resourceUsage1.getMemorySize();
          useToWeightRatio2 = resourceUsage2.getMemorySize();
        } else {
          // By setting useToWeightRatios to negative weights, we give the
          // zero-weight one less priority, so the non-zero weight one will
          // be given slots.
          useToWeightRatio1 = -weight1;
          useToWeightRatio2 = -weight2;
        }
      }

      return (int) Math.signum(useToWeightRatio1 - useToWeightRatio2);
    }

  }