private void wakeupFlushThread() {
    if (wakeupPending.compareAndSet(false, true)) {
      flushQueue.add(WAKEUPFLUSH_INSTANCE);
    }
  }
Returns Return memstore data size or null if <code>r</code> is null
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreFlusher.java