@Override
  public void start(long startKey, long endKey, int numThreads) throws IOException {
    super.start(startKey, endKey, numThreads);
    nextKeyToWrite.set(startKey);
    wroteUpToKey.set(startKey - 1);

    if (trackWroteKeys) {
      new Thread(new WroteKeysTracker(),
        "MultiThreadedWriterBase-WroteKeysTracker-" + EnvironmentEdgeManager.currentTime()).start();
      numThreadsWorking.incrementAndGet();
    }
  }
Enable this if used in conjunction with a concurrent reader.
hbase-diagnostics/src/main/java/org/apache/hadoop/hbase/util/MultiThreadedWriterBase.java