Tio be clear : when some sockets are ready for read (ie, the OP_READ flag has been set, and there is something in the socket to be read), the IoProcessor call to select()) will return and we will have a set of SelectionKey returned. Thsi set contains the set of all the channel that are ready for some processing. The IoProcessor thread will process them one after the other, from top to bottom. That means we don't process multiple sessions in parallel when all those sessions are handled by one singleIoProcessor. You have to be careful in what you do in your application, because any costly processing, or any synchronous access to a remote system will block the other sessions processing.
Now, we always start the server with more than one IoProcessor (typically, Nb core + 1 Ioprocessor). You can also fix a higher number of IoProcessor if you like, but at some point, if your CPU is 100% used, adding more IoProcessor does not help. What kind of performance are you expecting to reach ? (ie, how many requests per second ?)
