@Override
  public OverflowQueueWithDMStats<Runnable> getSerialQueue(InternalDistributedMember sender) {
    if (MULTI_SERIAL_EXECUTORS) {
      return serialQueuedExecutorPool.getSerialQueue(sender);
    } else {
      return serialQueue;
    }
  }
returns the serialThread's queue if throttling is being used, null if not
geode-core/src/main/java/org/apache/geode/distributed/internal/ClusterOperationExecutors.java