static void setFlushInterval(long millis) {
        // Don't accept shorter interval than 1 s.
        long interval = millis < 1000 ? 1000 : millis;
        flushInterval = interval;
        if (interval < flushInterval) {
            synchronized (JVM.FILE_DELTA_CHANGE) {
                JVM.FILE_DELTA_CHANGE.notifyAll();
            }
        }
    }
interval < flushInterval is always false