@Override
    protected void postUpdate(MutableSegment currentActiveMutableSegment) {
      super.postUpdate(currentActiveMutableSegment);
      if (!flushByteSizeLessThanSmallAndLargeCellSize) {
        try {
          postCyclicBarrier.await();
        } catch (Throwable e) {
          throw new RuntimeException(e);
        }
        return;
      }

      if (Thread.currentThread().getName().equals(SMALL_CELL_THREAD_NAME)) {
        try {
          postCyclicBarrier.await();
        } catch (Throwable e) {
          throw new RuntimeException(e);
        }
      }
    }
After largeCellThread finished flushInMemory method, smallCellThread can add cell to
currentActive . That is to say when largeCellThread called flushInMemory method,
currentActive has no cell.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestHStore.java