@Override
    protected void flushInMemory(MutableSegment currentActiveMutableSegment) {
      super.flushInMemory(currentActiveMutableSegment);
      if (Thread.currentThread().getName().equals(LARGE_CELL_THREAD_NAME)) {
        if (largeCellPreUpdateCounter.get() <= 1) {
          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