Multiple selectors in multiple threads

Question

Is it beneficial to distribute incoming connections among n threads, each with its own independent NIO Selector, where n is, say, the number of cores in the server? Suppose I'm writing a server, which should handle a number of client connections. I could have something like:

selector.select();
Iterator<SelectionKey> i = selector.selectedKeys().iterator();

while (i.hasNext()) {
   SelectionKey key = i.next();
   i.remove();

   if (!key.isValid())
      continue;

   if (key.isAcceptable()) {
      // get one of the n selectors (I'd have one per core)
      Selector chosenSelector = getRandomSelector();

      // delegate the new connection to the chosen selector
      SocketChannel newChannel = key.channel.accept();
      newChannel.configureBlocking(false);
      newChannel.register(chosenSelector, SelectionKey.OP_READ);
   }
}

Do you guys think this makes sense? I mean, running n threads, each with a different selector? Or should I just stick to having one single selector thread that handles OP_READ for all connections? Or maybe something else?

Answer 1

No it is not beneficial, as the relation of code that needs to be processed vs. the time it takes for the IO operations is negligible. Especially if you consider the extra time you would need for the synchronization of fragmented data. It is however beneficial to have the processing of the received data done in separate threads.

So basically: have a single-threaded selector loop that copies the data from one buffer into a task-buffer for further processing in a separate thread, then launch a Runnable with that task-buffer in an Executor to process that copied data.

Answer 2

This is exactly what Netty does. It uses N threads calling Selector.select() where N = (num available CPUs * 2) It is multiplied by 2 because of hyperthreading. This is the parent EventLoop which is considered the "acceptor eventloop" for accepting socket connections. Then there is the child EventLoop of thread workers

Suggest you have a look at NioEventLoopGroup-class