一。前述
上次讲完MapReduce的输入后，这次开始讲MapReduce的输出。注意MapReduce的原语很重要：
“相同”的key为一组，调用一次reduce方法，方法内迭代这一组数据进行计算！！！！！
二。代码
继续看MapTask任务。
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);源码解析一 } 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。构造OutPut对象：
NewOutputCollector(org.apache.hadoop.mapreduce.JobContext jobContext, JobConf job, TaskUmbilicalProtocol umbilical, TaskReporter reporter ) throws IOException, ClassNotFoundException { collector = createSortingCollector(job, reporter);//对应解析源码1.2 partitions = jobContext.getNumReduceTasks();//分区数等于Reduce数，分区数大于分组的概念。 if (partitions > 1) { partitioner = (org.apache.hadoop.mapreduce.Partitioner<K,V>) ReflectionUtils.newInstance(jobContext.getPartitionerClass(), job);//对应源码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;//用户不设置时默认框架一个reduce，并且分区号为0 } }; } }
@Override
public void write(K key, V value) throws IOException, InterruptedException {
collector.collect(key, value,
partitioner.getPartition(key, value, partitions));//上下文对象构造写出的值，放在collect缓存区中。
}
解析1.1
public Class<? extends Partitioner<?,?>> getPartitionerClass() throws ClassNotFoundException { return (Class<? extends Partitioner<?,?>>) conf.getClass(PARTITIONER_CLASS_ATTR, HashPartitioner.class);//当用户设置取用户的，没设置默认HashPartitioner 对应解析源码1.1.1
解析源码1.2createSortingCollector类的具体实现
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);//解析源码对应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"); }
解析源码1.2.1 缓冲区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);//缓冲区溢写阈值， final int sortmb = job.getInt(JobContext.IO_SORT_MB, 100);//缓冲区默认单位是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从缓冲区往磁盘写文件的时候需要排序，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();//排序所使用的比较器 见源码解析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端的组合
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);//小文件最少是3时，会合并小文件。
spillThread.setDaemon(true);//线程是另外一个线程负责写的 见解析源码1.2.1.2
spillThread.setName("SpillThread");
spillLock.lock();
总结：Mappper输出到缓冲区默认是100M，写到0.8时，会溢写！！！！这块可以调优。通过来回折半来调比如第一次调整50% 然后再80%中减小 70% 然后60%来回折半。
Combine一定要注意，比如求平均值
解析1,2.1.1排序比较器的实现
public RawComparator getOutputKeyComparator() { Class<? extends RawComparator> theClass = getClass( JobContext.KEY_COMPARATOR, null, RawComparator.class);字典排序 默认 if (theClass != null) return ReflectionUtils.newInstance(theClass, this); return WritableComparator.get(getMapOutputKeyClass().asSubclass(WritableComparable.class), this);//如果用户没有设置排序比较器，就是Key类型自己的比较器，所以Key必须实现序列化，反序列化，比较器。 }
总结：框架默认使用Key的比较器，字典排序 默认，用户也可以覆盖Key的比较器，自定义。！！！
解析源码1.2.1.2 溢写线程做的事
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();//排序溢写 } 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; } } }
总结：Map往缓冲区写入东西，线程把缓冲区中的内容做溢写，开始排序，溢写使用快排！！！Combine也在内存中，buffer也在内存，这些计算逻辑都在内存中，排序算法也在内存中，因为Map方法在内存中，这是第一次Combine,从Buffer产生一堆小文件的时候，然后一堆小文件在合并的时候还会执行一次Combine，这次有条件限制（小文件数量大于3）。
解析源码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;!!!重要取分区的写法！！
总结1.以上源码来源于 output = new NewOutputCollector(taskContext, job, umbilical, reporter)；所以可得出在输出构造的时候需要构造一个分区器。要么是0的，要么是用户设置的，要么是默认的。
总结2.在输出构造中，有缓冲区的设置。
总结3，以上方法都是OutPut的初始化。
总结4.Map输出的K，V变成K，V,P然后写入到环形缓冲区，内存缓存区80%，然后溢写排序，（先按分区排序，然后再按Key的组排序），然后生成小文件，然后合并，用的归并算法，此时小文件已经是内部有序的，所以使用归并算法，一次io即可。
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