这里主要说下在task初始化时的distribute purge相关的类：TrackerDistributedCacheManager
4.6.0的distribute purge行为由4个参数控制。 
local.cache.size #默认10737418240
mapreduce.tasktracker.cache.local.numberdirectories #默认10000
mapreduce.tasktracker.cache.local.keep.pct #默认0.95
mapreduce.tasktracker.distributedcache.checkperiod #默认1分钟
在声明一个TrackerDistributedCacheManager类的实例时，会初始化一个CleanupThread类的实例。 
this.cleanupThread=newCleanupThread(conf);
并启动 
publicvoidstartCleanupThread() {
this.cleanupThread.start();
  }
（由TaskTracker类的initialize方法调用
// Initialize DistributedCache
this.distributedCacheManager=newTrackerDistributedCacheManager(
this.fConf,taskController);
this.distributedCacheManager.startCleanupThread();
）
CleanupThread是TrackerDistributedCacheManager类的内部类：
其原理是启动一个thread，定期去触发 BaseDirManager类的checkAndCleanup方法，不会阻塞当前进程。间隔时间由mapreduce.tasktracker.distributedcache.checkperiod控制。 
public void run() {
  while ( running) {
    try {
      Thread. sleep(cleanUpCheckPeriod);
      baseDirManager.checkAndCleanup();  //调用checkAndCleanup方法
    } catch (IOException e) {
      LOG.error("Exception in DistributedCache CleanupThread." , e);
    } catch(InterruptedException e) {
      LOG.info("Cleanup..." ,e);
      //To force us to exit cleanly
      running = false;
    } catch (Throwable t) {
      exitTaskTracker(t);
    }
  }
}
BaseDirManager也是TrackerDistributedCacheManager的内部类，控制distribute cache的删除操作和删除后状态数据的更新：
其中checkAndCleanup方法的实现： 
    Collection toBeDeletedCache = new LinkedList();
    HashMap toBeCleanedBaseDir = new HashMap();
.........
         for (Map.Entry baseDir : properties.entrySet()) { //循环遍历mapred.local.dir目录大小和文件量
          CacheDir baseDirCounts = baseDir.getValue();
          LOG.debug(baseDir.getKey() + ": allowedCacheSize=" + allowedCacheSize +
              ",baseDirCounts.size=" + baseDirCounts.size +
              ",allowedCacheSubdirs=" + allowedCacheSubdirs +
              ",baseDirCounts.subdirs=" + baseDirCounts.subdirs);
          if (allowedCacheSize < baseDirCounts.size ||
              allowedCacheSubdirs < baseDirCounts.subdirs) { //触发purge的条件（local.cache.size小于某一个目录大小，mapreduce.tasktracker.cache.local.numberdirectories小于某一个文件下的文件数量）
            CacheDir tcc = new CacheDir();
            tcc.size = baseDirCounts.size - allowedCacheSizeCleanupGoal;
            tcc.subdirs = baseDirCounts.subdirs - allowedCacheSubdirsCleanupGoal;
            toBeCleanedBaseDir.put(baseDir.getKey(), tcc); //生成需要删除的目录的HashMap
          }
        }
实际的删除动作： 
// do the deletion, after releasing the global lock
for (CacheStatus cacheStatus : toBeDeletedCache) {  //循环遍历需要删除的cache
  cacheStatus. lock.lock();  //获取删除对象的锁
  try {
    Path localizedDir = cacheStatus.getLocalizedUniqueDir();
    if (cacheStatus.user == null) {
      TrackerDistributedCacheManager. LOG.info("Deleted path " + localizedDir);
      try {
        localFs.delete(localizedDir, true);  //public的情况调用FileSystem的delete方法
      } catch (IOException e) {
        TrackerDistributedCacheManager. LOG.warn("Could not delete distributed cache empty directory "
                 + localizedDir, e);
      }
    } else {  
      TrackerDistributedCacheManager. LOG.info("Deleted path " + localizedDir + " as " + cacheStatus.user );
      String base = cacheStatus.getBaseDir().toString();
      String userDir = TaskTracker.getUserDir(cacheStatus. user);
      int skip = base.length() + 1 + userDir.length() + 1;
      String relative = localizedDir.toString().substring(skip);
      taskController.deleteAsUser(cacheStatus.user , relative);  //private的情况调用TaskController的deleteAsUser的方法
    }
    deleteCacheInfoUpdate(cacheStatus);
  } finally {
    cacheStatus. lock.unlock();
  }
}