@VisibleForTesting
  public void updateSlowDiskReportAsync(long now) {
    if (isUpdateInProgress.compareAndSet(false, true)) {
      lastUpdateTime = now;
      new Thread(new Runnable() {
        @Override
        public void run() {
          slowDisksReport = getSlowDisks(diskIDLatencyMap,
              maxDisksToReport, now);

          cleanUpOldReports(now);

          isUpdateInProgress.set(false);
        }
      }).start();
    }
  }
@Test
  public void testEmptyReports() {
    tracker.updateSlowDiskReportAsync(timer.monotonicNow());
    assertTrue(getSlowDisksReportForTesting(tracker).isEmpty());
  }