Giraph源码分析（九）Aggregators原理解析

每个Worker在开始计算前，会调用BspServiceWorker类的prepareSuperStep()方法来进行聚集器值的派发和接受其他Workers发送的聚集器值。调用关系如下：

BspServiceWorker类的prepareSuperStep()方法如下：

@Override
public void prepareSuperstep() {
   if (getSuperstep() != INPUT_SUPERSTEP) {
	  /*
	   * aggregatorHandler为WorkerAggregatorHandler类型.可参考上文中MasterAggregatorHandler的类继承关系
	   * workerAggregatorRequestProcessor声明为WorkerAggregatorRequestProcessor（接口）类型，
	   * 实际为NettyWorkerAggregatorRequestProcessor的实例，用于Worker间发送聚集器的值。
	   */
      aggregatorHandler.prepareSuperstep(workerAggregatorRequestProcessor);
   }
}

public void prepareSuperstep(WorkerAggregatorRequestProcessor requestProcessor) {
    AllAggregatorServerData allAggregatorData =
        serviceWorker.getServerData().getAllAggregatorData();
    /**
     * 等待直到Master发送给该Worker的聚集器都已接受完，
     * 返回值为Master发送给该Worker的所有Data（聚集器）
     */
    Iterable dataToDistribute =
        allAggregatorData.getDataFromMasterWhenReady(
            serviceWorker.getMasterInfo());
  
    // 把从Master收到的Data（聚集器）发送给其他所有Workers
    requestProcessor.distributeAggregators(dataToDistribute);

    // 等待直到接受完其他Workers发送给该Workers的聚集器
    allAggregatorData.fillNextSuperstepMapsWhenReady(
        getOtherWorkerIdsSet(), previousAggregatedValueMap,
        currentAggregatorMap);
    // 只是清空allAggregatorServerData的List masterData对象
    // 为下一个超级步接受Master发送的聚集器做准备
    allAggregatorData.reset();
}

  /**
   * Require more permits. This will increase the number of times permits
   * were required. Doesn't wait for permits to become available.
   *
   * @param permits Number of permits to require
   * @param taskId Task id which required permits
   */
  public synchronized void requirePermits(long permits, int taskId) {
    arrivedTaskIds.add(taskId);
    waitingOnPermits += permits;
    notifyAll();
  }

 /**
   * Send aggregators to their owners and in the end to the master
   *
   * @param requestProcessor Request processor for aggregators
   */
  public void finishSuperstep(
      WorkerAggregatorRequestProcessor requestProcessor) {
    OwnerAggregatorServerData ownerAggregatorData =
        serviceWorker.getServerData().getOwnerAggregatorData();
    // First send partial aggregated values to their owners and determine
    // which aggregators belong to this worker
    for (Map.Entry> entry :
        currentAggregatorMap.entrySet()) {
        boolean sent = requestProcessor.sendAggregatedValue(entry.getKey(),
            entry.getValue().getAggregatedValue());
        if (!sent) {
          // If it's my aggregator, add it directly
          ownerAggregatorData.aggregate(entry.getKey(),
              entry.getValue().getAggregatedValue());
        }
    }
    // Flush
    requestProcessor.flush();
    // Wait to receive partial aggregated values from all other workers
    Iterable> myAggregators =
        ownerAggregatorData.getMyAggregatorValuesWhenReady(
            getOtherWorkerIdsSet());

    // Send final aggregated values to master
    AggregatedValueOutputStream aggregatorOutput =
        new AggregatedValueOutputStream();
    for (Map.Entry entry : myAggregators) {
        int currentSize = aggregatorOutput.addAggregator(entry.getKey(),
            entry.getValue());
        if (currentSize > maxBytesPerAggregatorRequest) {
          requestProcessor.sendAggregatedValuesToMaster(
              aggregatorOutput.flush());
        }   
    }
    requestProcessor.sendAggregatedValuesToMaster(aggregatorOutput.flush());
    // Wait for master to receive aggregated values before proceeding
    serviceWorker.getWorkerClient().waitAllRequests();
    ownerAggregatorData.reset();
  }

4. 大同步后，Master调用MasterAggregatorHandler类的prepareSusperStep(masterClient)方法，收集聚集器的值。方法内容如下：

  public void prepareSuperstep(MasterClient masterClient) {

    // 收集上次超级步的聚集值，为master compute 做准备
    for (AggregatorWrapper aggregator : aggregatorMap.values()) {
	// 如果是 Persistent Aggregator，则累加
	if (aggregator.isPersistent()) {
        aggregator.aggregateCurrent(aggregator.getPreviousAggregatedValue());
      }
      aggregator.setPreviousAggregatedValue(
          aggregator.getCurrentAggregatedValue());
      aggregator.resetCurrentAggregator();
      progressable.progress();
    }
  }

备注：Job迭代结束条件有三，满足其一就行：
1) 达到最大迭代次数
2) 没有活跃顶点且没有消息在传递
3) 终止MasterCompute计算

总结：为解决在多个Aggregator条件下，Master成为系统瓶颈的问题。采取了把所有Aggregator派发给某一部分Workers，由这些Workers完成全局的聚集值的计算与发送，Master只需要与这些Workers进行简单数据通信即可，大大降低了Master的工作量。

追加：下面用图示方法说明上述执行过程。

实验条件：

1). 一个Master，四个Worker

2). 两个Aggregators，记为A1和A2。

1. Master把Aggregators发送给Workers，收到Aggregator的Worker就作为该Aggregator的Owner。下图中Master把A1发送给Worker1，A2发送给Worker3.那么Worker1就作为A1的Owner，Worker3就是A2的Owner。该步骤在MasterAggregatorHandler类的finishSuperStep(MasterClient masterClient) 方法中完成，使用的是SendAggregatorsToOwnerRequest 通信协议。注：每个Owner Worker 可能有多个聚集器。

图1 Master分发Aggregator

2. Workers接受Master发送的Aggregator，然后把Aggregator发送给其他Workers。Worker1要把A1分别发送给Worker2、Worker3和Worker4；Worker3要把A2分别发送给Worker1、Worker2和Worker4。该步骤在WorkerAggregatorHandler类的prepareSuperstep( WorkerAggregatorRequestProcessor requestProcessor)方法中完成，使用的是SendAggregatorsToMasterRequest 通信协议。此步骤完成后，每个Worker上都有了聚集器A1和A2（具体为上一个超步的全局最终聚集值）。

3. 每个Worker调用Vertex.compute()方法开始计算，收集本地的Aggregator聚集值。对聚集体A1来说，Worker1、Worker2、Worker3、Worker4的本地聚集值依次记为：

A1₁ 、A1₂、 A1_3、A1_{4；对聚集器A2来说，Worker1、Worker2、Worker3、Worker4的本地聚集值依次记为：}

A2₁ 、A2₂、 A2₃、A2_{4 。计算完成后，每个Worker就要把本地的聚集值发送给聚集器的Owner，聚集器的Owner在接受的时候会合并聚集。那么A11 、A12、 A13、A14要发送给Worker1进行全局聚集得到A1^’，A21 、A22、 A23、A24 要发送给Worker3进行全局聚集得到A2^{’ 。}}

_{^{公式如下：}}

此部分采用的是SendWorkerAggregatorsRequest通信协议。Worker1和Worker3要把汇总的A1和A2的新值：A1’ 和A2’发送给Master，供下一次超级步的MasterCompute.compute()方法使用采用的是SendAggregatorsToMasterRequest通信协议。此部分在WorkerAggregatorHandler类的finishSuperstep( WorkerAggregatorRequestProcessor requestProcessor)方法中完成。过程如下图所示：

4. Master收到Worker1发送的A1’ 和Woker3发送的A2’后，此步骤在MasterAggregatorHandler类的prepareSusperStep(masterClient)方法中完成。然后调用MasterCompute.compute()方法，此方法可能会修改聚集器的值，如得到A1’’和A2’’。在masterCompute.compute()方法内若根据聚集器的值调用了MasterCompute类的haltCompute()方法来终止MaterCompute，则表明要结束整个Job。那么Master就会通知所有Workers要结束整个作业；在该方法内若没有调用MasterCompute类的haltCompute()方法，则回到步骤1继续进行迭代，继续把A1’’发送给Worker1，A2’’发送给Worker3。

完！

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