code: private long getTaskCumulativeRssmem(TaskAttemptID tid) {
  	    ProcessTreeInfo ptInfo = processTreeInfoMap.get(tid);
  	    ProcfsBasedProcessTree pTree = ptInfo.getProcessTree();
  	    return pTree == null ? 0 : pTree.getCumulativeVmem();
    }
docs: Return the cumulative rss memory used by a task
  @param tid the task attempt ID of the task
  @return rss memory usage in bytes. 0 if the process tree is not available