code: public Path getInputFile(int mapId, TaskAttemptID reduceTaskId)
      throws IOException {
      // TODO *oom* should use a format here
      return lDirAlloc.getLocalPathToRead(TaskTracker.getIntermediateOutputDir(
                         jobId.toString(), reduceTaskId.toString())
                         + "/map_" + mapId + ".out",
                         conf);
    }
docs: Return a local reduce input file created earlier
  @param mapTaskId a map task id
  @param reduceTaskId a reduce task id