code: private void fifoWeightAdjust(Pool pool) {
      List<JobInProgress> jobs = new ArrayList<JobInProgress>();
      jobs.addAll(pool.getJobs());
      Collections.sort(jobs, fifoComparator);
      double factor = 1.0;
      for (JobInProgress job : jobs) {
        JobInfo info = infos.get(job);
        if (info == null) {
          throw new IllegalStateException("Couldn't find job " + job.jobId +
              " in pool " + pool.getName());
        }
        info.mapWeight *= factor;
        info.reduceWeight *= factor;
        factor *= FIFO_WEIGHT_DECAY_FACTOR;
      }
    }
docs: Boost the weight for the older jobs.