performance - multithreaded or multiprocess applications

Processes and threads on linux are very similar to each other - the main difference is that the whole virtual memory is shared as well as certain things like signal handling differ.

This makes for cheaper context switches between threads (no need for costly MMU reloads etc.) but doesn't necessarily cause much difference in speed (especially outside of thread creation).

For designing a highly network intensive application, basically the only solution is to use an evented architecture (otherwise you'll bog down the system with huge amount of processes/threads and spend more time on their management than actually running work code), where you react to I/O on sockets and based on which sockets exhibit activity do apropriate operations.

A famous writeup about the problems faced in such situations is "The C10k problem", available from http://www.kegel.com/c10k.html - it describes different I/O approaches, so despite being a bit dated, it's a very good introduction.

Be careful before jumping deeply into reactor-like designs, though - they can get unwieldy and complex, so see if you can't use library/language that provides a nicer abstraction over it (Erlang is my personal favourite in this, languages with coroutines like Go can be useful too).

If your threads are doing the job independent from one another, under linux, there is simply no reason to not going with multiple processes instead. Multiple processes would increase your memory usage as each process has its own private memory space, but on the other hand sharing the memory space between independent threads is the worse decision. Context switching between threads vs processes is usually done better for processes rather than threads although its a little bit architecture and code dependent. Processes are safe to not get serialized with locks and mutex es. Processes are easier to manage and interact with in Linux. here is a good document you might find interesting (http://elinux.org/images/1/1c/Ben-Yossef-GoodBadUgly.pdf).