一。前述
上次講完MapReduce的輸入後，這次開始講MapReduce的輸出。注意MapReduce的原語很重要：
“相同”的key為一組，呼叫一次reduce方法，方法內迭代這一組資料進行計算！！！！！
二。程式碼
繼續看MapTask任務。
private <INKEY,INVALUE,OUTKEY,OUTVALUE> void runNewMapper(final JobConf job, final TaskSplitIndex splitIndex, final TaskUmbilicalProtocol umbilical, TaskReporter reporter ) throws IOException, ClassNotFoundException, InterruptedException { // make a task context so we can get the classes org.apache.hadoop.mapreduce.TaskAttemptContext taskContext = new org.apache.hadoop.mapreduce.task.TaskAttemptContextImpl(job, getTaskID(), reporter); // make a mapper org.apache.hadoop.mapreduce.Mapper<INKEY,INVALUE,OUTKEY,OUTVALUE> mapper = (org.apache.hadoop.mapreduce.Mapper<INKEY,INVALUE,OUTKEY,OUTVALUE>) ReflectionUtils.newInstance(taskContext.getMapperClass(), job); // make the input format org.apache.hadoop.mapreduce.InputFormat<INKEY,INVALUE> inputFormat = (org.apache.hadoop.mapreduce.InputFormat<INKEY,INVALUE>) ReflectionUtils.newInstance(taskContext.getInputFormatClass(), job); // rebuild the input split org.apache.hadoop.mapreduce.InputSplit split = null; split = getSplitDetails(new Path(splitIndex.getSplitLocation()), splitIndex.getStartOffset()); LOG.info("Processing split: " + split); org.apache.hadoop.mapreduce.RecordReader<INKEY,INVALUE> input = new NewTrackingRecordReader<INKEY,INVALUE> (split, inputFormat, reporter, taskContext); job.setBoolean(JobContext.SKIP_RECORDS, isSkipping()); org.apache.hadoop.mapreduce.RecordWriter output = null; // get an output object if (job.getNumReduceTasks() == 0) { output = new NewDirectOutputCollector(taskContext, job, umbilical, reporter); } else { output = new NewOutputCollector(taskContext, job, umbilical, reporter);原始碼解析一 } org.apache.hadoop.mapreduce.MapContext<INKEY, INVALUE, OUTKEY, OUTVALUE> mapContext = new MapContextImpl<INKEY, INVALUE, OUTKEY, OUTVALUE>(job, getTaskID(), input, output, committer, reporter, split); org.apache.hadoop.mapreduce.Mapper<INKEY,INVALUE,OUTKEY,OUTVALUE>.Context mapperContext = new WrappedMapper<INKEY, INVALUE, OUTKEY, OUTVALUE>().getMapContext( mapContext); try { input.initialize(split, mapperContext); mapper.run(mapperContext); mapPhase.complete(); setPhase(TaskStatus.Phase.SORT); statusUpdate(umbilical); input.close(); input = null; output.close(mapperContext); output = null; } finally { closeQuietly(input); closeQuietly(output, mapperContext); } }
解析一。構造OutPut物件：
NewOutputCollector(org.apache.hadoop.mapreduce.JobContext jobContext, JobConf job, TaskUmbilicalProtocol umbilical, TaskReporter reporter ) throws IOException, ClassNotFoundException { collector = createSortingCollector(job, reporter);//對應解析原始碼1.2 partitions = jobContext.getNumReduceTasks();//分割槽數等於Reduce數，分割槽數大於分組的概念。 if (partitions > 1) { partitioner = (org.apache.hadoop.mapreduce.Partitioner<K,V>) ReflectionUtils.newInstance(jobContext.getPartitionerClass(), job);//對應原始碼1.1 } else { partitioner = new org.apache.hadoop.mapreduce.Partitioner<K,V>() { @Override public int getPartition(K key, V value, int numPartitions) { return partitions - 1;//使用者不設定時預設框架一個reduce，並且分割槽號為0 } }; } }
@Override
public void write(K key, V value) throws IOException, InterruptedException {
collector.collect(key, value,
partitioner.getPartition(key, value, partitions));//上下文物件構造寫出的值，放在collect快取區中。
}
解析1.1
public Class<? extends Partitioner<?,?>> getPartitionerClass() throws ClassNotFoundException { return (Class<? extends Partitioner<?,?>>) conf.getClass(PARTITIONER_CLASS_ATTR, HashPartitioner.class);//當使用者設定取使用者的，沒設定預設HashPartitioner 對應解析原始碼1.1.1
解析原始碼1.2createSortingCollector類的具體實現
private <KEY, VALUE> MapOutputCollector<KEY, VALUE> createSortingCollector(JobConf job, TaskReporter reporter) throws IOException, ClassNotFoundException { MapOutputCollector.Context context = new MapOutputCollector.Context(this, job, reporter); Class<?>[] collectorClasses = job.getClasses( JobContext.MAP_OUTPUT_COLLECTOR_CLASS_ATTR, MapOutputBuffer.class); int remainingCollectors = collectorClasses.length; for (Class clazz : collectorClasses) { try { if (!MapOutputCollector.class.isAssignableFrom(clazz)) { throw new IOException("Invalid output collector class: " + clazz.getName() + " (does not implement MapOutputCollector)"); } Class<? extends MapOutputCollector> subclazz = clazz.asSubclass(MapOutputCollector.class); LOG.debug("Trying map output collector class: " + subclazz.getName()); MapOutputCollector<KEY, VALUE> collector = ReflectionUtils.newInstance(subclazz, job); collector.init(context);//解析原始碼對應1.2.1 LOG.info("Map output collector class = " + collector.getClass().getName()); return collector; } catch (Exception e) { String msg = "Unable to initialize MapOutputCollector " + clazz.getName(); if (--remainingCollectors > 0) { msg += " (" + remainingCollectors + " more collector(s) to try)"; } LOG.warn(msg, e); } } throw new IOException("Unable to initialize any output collector"); }
解析原始碼1.2.1 緩衝區collect的初始化
public void init(MapOutputCollector.Context context ) throws IOException, ClassNotFoundException { job = context.getJobConf(); reporter = context.getReporter(); mapTask = context.getMapTask(); mapOutputFile = mapTask.getMapOutputFile(); sortPhase = mapTask.getSortPhase(); spilledRecordsCounter = reporter.getCounter(TaskCounter.SPILLED_RECORDS); partitions = job.getNumReduceTasks(); rfs = ((LocalFileSystem)FileSystem.getLocal(job)).getRaw(); //sanity checks final float spillper = job.getFloat(JobContext.MAP_SORT_SPILL_PERCENT, (float)0.8);//緩衝區溢寫閾值， final int sortmb = job.getInt(JobContext.IO_SORT_MB, 100);//緩衝區預設單位是100M indexCacheMemoryLimit = job.getInt(JobContext.INDEX_CACHE_MEMORY_LIMIT, INDEX_CACHE_MEMORY_LIMIT_DEFAULT); if (spillper > (float)1.0 || spillper <= (float)0.0) { throw new IOException("Invalid \"" + JobContext.MAP_SORT_SPILL_PERCENT + "\": " + spillper); } if ((sortmb & 0x7FF) != sortmb) { throw new IOException( "Invalid \"" + JobContext.IO_SORT_MB + "\": " + sortmb); } sorter = ReflectionUtils.newInstance(job.getClass("map.sort.class", QuickSort.class, IndexedSorter.class), job);//Map從緩衝區往磁碟寫檔案的時候需要排序，用的快排。 // buffers and accounting int maxMemUsage = sortmb << 20; maxMemUsage -= maxMemUsage % METASIZE; kvbuffer = new byte[maxMemUsage]; bufvoid = kvbuffer.length; kvmeta = ByteBuffer.wrap(kvbuffer) .order(ByteOrder.nativeOrder()) .asIntBuffer(); setEquator(0); bufstart = bufend = bufindex = equator; kvstart = kvend = kvindex; maxRec = kvmeta.capacity() / NMETA; softLimit = (int)(kvbuffer.length * spillper); bufferRemaining = softLimit; LOG.info(JobContext.IO_SORT_MB + ": " + sortmb); LOG.info("soft limit at " + softLimit); LOG.info("bufstart = " + bufstart + "; bufvoid = " + bufvoid); LOG.info("kvstart = " + kvstart + "; length = " + maxRec);
comparator = job.getOutputKeyComparator();//排序所使用的比較器 見原始碼解析1,2.1.1
keyClass = (Class<K>)job.getMapOutputKeyClass();
valClass = (Class<V>)job.getMapOutputValueClass();
serializationFactory = new SerializationFactory(job);
keySerializer = serializationFactory.getSerializer(keyClass);
keySerializer.open(bb);
valSerializer = serializationFactory.getSerializer(valClass);
valSerializer.open(bb);
// combiner
final Counters.Counter combineInputCounter =
reporter.getCounter(TaskCounter.COMBINE_INPUT_RECORDS);
combinerRunner = CombinerRunner.create(job, getTaskID(), //map端的組合
combineInputCounter,
reporter, null);
if (combinerRunner != null) {
final Counters.Counter combineOutputCounter =
reporter.getCounter(TaskCounter.COMBINE_OUTPUT_RECORDS);
combineCollector= new CombineOutputCollector<K,V>(combineOutputCounter, reporter, job);
} else {
combineCollector = null;
}
spillInProgress = false;
minSpillsForCombine = job.getInt(JobContext.MAP_COMBINE_MIN_SPILLS, 3);//小檔案最少是3時，會合並小檔案。
spillThread.setDaemon(true);//執行緒是另外一個執行緒負責寫的 見解析原始碼1.2.1.2
spillThread.setName("SpillThread");
spillLock.lock();
總結：Mappper輸出到緩衝區預設是100M，寫到0.8時，會溢寫！！！！這塊可以調優。通過來回折半來調比如第一次調整50% 然後再80%中減小 70% 然後60%來回折半。
Combine一定要注意，比如求平均值
解析1,2.1.1排序比較器的實現
public RawComparator getOutputKeyComparator() { Class<? extends RawComparator> theClass = getClass( JobContext.KEY_COMPARATOR, null, RawComparator.class);字典排序 預設 if (theClass != null) return ReflectionUtils.newInstance(theClass, this); return WritableComparator.get(getMapOutputKeyClass().asSubclass(WritableComparable.class), this);//如果使用者沒有設定排序比較器，就是Key型別自己的比較器，所以Key必須實現序列化，反序列化，比較器。 }
總結：框架預設使用Key的比較器，字典排序 預設，使用者也可以覆蓋Key的比較器，自定義。！！！
解析原始碼1.2.1.2 溢寫執行緒做的事
protected class SpillThread extends Thread { @Override public void run() { spillLock.lock(); spillThreadRunning = true; try { while (true) { spillDone.signal(); while (!spillInProgress) { spillReady.await(); } try { spillLock.unlock(); sortAndSpill();//排序溢寫 } catch (Throwable t) { sortSpillException = t; } finally { spillLock.lock(); if (bufend < bufstart) { bufvoid = kvbuffer.length; } kvstart = kvend; bufstart = bufend; spillInProgress = false; } } } catch (InterruptedException e) { Thread.currentThread().interrupt(); } finally { spillLock.unlock(); spillThreadRunning = false; } } }
總結：Map往緩衝區寫入東西，執行緒把緩衝區中的內容做溢寫，開始排序，溢寫使用快排！！！Combine也在記憶體中，buffer也在記憶體，這些計算邏輯都在記憶體中，排序演算法也在記憶體中，因為Map方法在記憶體中，這是第一次Combine,從Buffer產生一堆小檔案的時候，然後一堆小檔案在合併的時候還會執行一次Combine，這次有條件限制（小檔案數量大於3）。
解析原始碼1.1.1
public class HashPartitioner<K, V> extends Partitioner<K, V> { /** Use {@link Object#hashCode()} to partition. */ public int getPartition(K key, V value, int numReduceTasks) { return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;!!! }
return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;!!!重要取分割槽的寫法！！
總結1.以上原始碼來源於 output = new NewOutputCollector(taskContext, job, umbilical, reporter)；所以可得出在輸出構造的時候需要構造一個分割槽器。要麼是0的，要麼是使用者設定的，要麼是預設的。
總結2.在輸出構造中，有緩衝區的設定。
總結3，以上方法都是OutPut的初始化。
總結4.Map輸出的K，V變成K，V,P然後寫入到環形緩衝區，記憶體快取區80%，然後溢寫排序，（先按分割槽排序，然後再按Key的組排序），然後生成小檔案，然後合併，用的歸併演算法，此時小檔案已經是內部有序的，所以使用歸併演算法，一次io即可。
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