Mongo Async driver when used in netty mode, gives an option to define EventLoopGroup in the constructor for NettyStreamFactoryFactory.

However it is not possible to use EpollEventLoopGroup since in NettyStream class in openAsync() bootstrap channel will always be NioSocketChannel.

     public void openAsync(final AsyncCompletionHandler<Void> handler) {
           Bootstrap bootstrap = new Bootstrap();
           bootstrap.group(workerGroup);
          bootstrap.channel(NioSocketChannel.class);

https://github.com/mongodb/mongo-java-driver/blob/3.2.x/driver-core/src/main/com/mongodb/connection/netty/NettyStream.java