public synchronized void setRefreshAtFixedRate(boolean refreshAtFixedRate) {
    this.refreshAtFixedRate = refreshAtFixedRate;
    rescheduleTimer();
  }
set true if refresh for timer has to be fixed rate see scheduleAtFixedRate method of
{@link TimerTask}

TODO never called
geode-core/src/main/java/org/apache/geode/internal/admin/StatAlertsManager.java