public void start() {
    // at present, this won't be accessed by multiple threads,
    // still, it is safer approach to synchronize it
    synchronized (ParallelGatewaySenderQueue.class) {
      if (removalThread == null) {
        removalThread = new BatchRemovalThread(sender.getCache(), this);
        removalThread.start();
      }
    }
  }
Start the background batch removal thread.
geode-core/src/main/java/org/apache/geode/internal/cache/wan/parallel/ParallelGatewaySenderQueue.java