code: public long getCumulativeRssmem(int olderThanAge) {
      if (PAGE_SIZE < 0) {
        return 0;
      }
      long totalPages = 0;
      for (ProcessInfo p : processTree.values()) {
        if ((p != null) && (p.getAge() > olderThanAge)) {
          totalPages += p.getRssmemPage();
        }
      }
      return totalPages * PAGE_SIZE; // convert # pages to byte
    }
docs: Get the cumulative resident set size (rss) memory used by all the processes
  in the process-tree that are older than the passed in age.
  
  @param olderThanAge processes above this age are included in the
  memory addition
  @return cumulative rss memory used by the process-tree in bytes,
  for processes older than this age. return 0 if it cannot be
  calculated