Giraph源碼分析（八）—— 統計每個SuperStep中參與計算的頂點數目

作者|白松

目的：科研中，需要分析在每次迭代過程中參與計算的頂點數目，來進一步優化系統。比如，在SSSP的compute()方法最后一行，都會把當前頂點voteToHalt，即變為InActive狀態。所以每次迭代完成后，所有頂點都是InActive狀態。在大同步后，收到消息的頂點會被激活，變為Active狀態，然后調用頂點的compute()方法。本文的目的就是統計每次迭代過程中，參與計算的頂點數目。下面附上SSSP的compute()方法：

@Override
  public void compute(Iterable messages) {
    if (getSuperstep() == 0) {
      setValue(new DoubleWritable(Double.MAX_VALUE));
    }
    double minDist = isSource() ? 0d : Double.MAX_VALUE;
    for (DoubleWritable message : messages) {
      minDist = Math.min(minDist, message.get());
    }
    if (minDist < getValue().get()) {
      setValue(new DoubleWritable(minDist));
      for (Edge edge : getEdges()) {
        double distance = minDist + edge.getValue().get();
        sendMessage(edge.getTargetVertexId(), new DoubleWritable(distance));
      }
    }
    //把頂點置為InActive狀態
    voteToHalt();
  }

附：giraph中算法的終止條件是：沒有活躍頂點且worker間沒有消息傳遞。

hama-0.6.0中算法的終止條件只是：判斷是否有活躍頂點。不是真正的pregel思想，半成品。

修改過程如下：

1. org.apache.giraph.partition. PartitionStats 類

添加變量和方法，用來統計每個Partition在每個超步中參與計算的頂點數目。添加的變量和方法如下：

/** computed vertices in this partition */
private long computedVertexCount=0;

/**
* Increment the computed vertex count by one.
*/
public void incrComputedVertexCount() {
    ++ computedVertexCount;
}

/**
 * @return the computedVertexCount
 */
public long getComputedVertexCount() {
    return computedVertexCount;
}

修改readFields()和write()方法，每個方法追加最后一句。當每個Partition計算完成后，會把自己的computedVertexCount發送給Master，Mater再讀取匯總。

@Override
public void readFields(DataInput input) throws IOException {
    partitionId = input.readInt();
    vertexCount = input.readLong();
    finishedVertexCount = input.readLong();
    edgeCount = input.readLong();
    messagesSentCount = input.readLong();
    //添加下條語句
    computedVertexCount=input.readLong();
}

@Override
public void write(DataOutput output) throws IOException {
    output.writeInt(partitionId);
    output.writeLong(vertexCount);
    output.writeLong(finishedVertexCount);
    output.writeLong(edgeCount);
    output.writeLong(messagesSentCount);
    //添加下條語句
    output.writeLong(computedVertexCount);
}

2. org.apache.giraph.graph. GlobalStats 類

   添加變量和方法，用來統計每個超步中參與計算的頂點總數目，包含每個Worker上的所有Partitions。

 /** computed vertices in this partition 
  *  Add by BaiSong 
  */
  private long computedVertexCount=0;
     /**
     * @return the computedVertexCount
     */
    public long getComputedVertexCount() {
        return computedVertexCount;
    }

修改addPartitionStats(PartitionStats partitionStats)方法，增加統計computedVertexCount功能。

/**
  * Add the stats of a partition to the global stats.
  *
  * @param partitionStats Partition stats to be added.
  */
  public void addPartitionStats(PartitionStats partitionStats) {
    this.vertexCount += partitionStats.getVertexCount();
    this.finishedVertexCount += partitionStats.getFinishedVertexCount();
    this.edgeCount += partitionStats.getEdgeCount();
    //Add by BaiSong，添加下條語句
    this.computedVertexCount+=partitionStats.getComputedVertexCount();
 }

當然為了Debug方便，也可以修改該類的toString()方法（可選），修改后的如下：

public String toString() {
        return "(vtx=" + vertexCount + ", computedVertexCount="
                + computedVertexCount + ",finVtx=" + finishedVertexCount
                + ",edges=" + edgeCount + ",msgCount=" + messageCount
                + ",haltComputation=" + haltComputation + ")";
    }

3. org.apache.giraph.graph. ComputeCallable<I,V,E,M>

添加統計功能。在computePartition()方法中，添加下面一句。

if (!vertex.isHalted()) {
        context.progress();
        TimerContext computeOneTimerContext = computeOneTimer.time();
        try {
            vertex.compute(messages);
        //添加下面一句，當頂點調用完compute()方法后，就把該Partition的computedVertexCount加1
            partitionStats.incrComputedVertexCount();
        } finally {
           computeOneTimerContext.stop();
        }
……

4. 添加Counters統計，和我的博客Giraph源碼分析（七）—— 添加消息統計功能 類似，此處不再詳述。添加的類為：org.apache.giraph.counters.GiraphComputedVertex，下面附上該類的源碼：

package org.apache.giraph.counters;

import java.util.Iterator;
import java.util.Map;

import org.apache.hadoop.mapreduce.Mapper.Context;
import com.google.common.collect.Maps;

/**
 * Hadoop Counters in group "Giraph Messages" for counting every superstep
 * message count.
 */

public class GiraphComputedVertex extends HadoopCountersBase {
    /** Counter group name for the giraph Messages */
    public static final String GROUP_NAME = "Giraph Computed Vertex";

    /** Singleton instance for everyone to use */
    private static GiraphComputedVertex INSTANCE;

    /** superstep time in msec */
    private final Map superstepVertexCount;

    private GiraphComputedVertex(Context context) {
        super(context, GROUP_NAME);
        superstepVertexCount = Maps.newHashMap();
    }

    /**
     * Instantiate with Hadoop Context.
     * 
     * @param context
     *            Hadoop Context to use.
     */
    public static void init(Context context) {
        INSTANCE = new GiraphComputedVertex(context);
    }

    /**
     * Get singleton instance.
     * 
     * @return singleton GiraphTimers instance.
     */
    public static GiraphComputedVertex getInstance() {
        return INSTANCE;
    }

    /**
     * Get counter for superstep messages
     * 
     * @param superstep
     * @return
     */
    public GiraphHadoopCounter getSuperstepVertexCount(long superstep) {
        GiraphHadoopCounter counter = superstepVertexCount.get(superstep);
        if (counter == null) {
            String counterPrefix = "Superstep: " + superstep+" ";
            counter = getCounter(counterPrefix);
            superstepVertexCount.put(superstep, counter);
        }
        return counter;
    }

    @Override
    public Iterator iterator() {
        return superstepVertexCount.values().iterator();
    }
}

5. 實驗結果，運行程序后。會在終端輸出每次迭代參與計算的頂點總數目。 測試SSSP（SimpleShortestPathsVertex類），輸入圖中共有9個頂點和12條邊。輸出結果如下：

上圖測試中，共有6次迭代。紅色框中，顯示出了每次迭代過沖參與計算的頂點數目，依次是：9,4,4,3,4,0

解釋：在第0個超步，每個頂點都是活躍的，所有共有9個頂點參與計算。在第5個超步，共有0個頂點參與計算，那么就不會向外發送消息，加上每個頂點都是不活躍的，所以算法迭代終止。

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