Implement network (probably, tcp) server in a simple, single-threaded way;
make it multitreaded:
2.1. use some shared resources: e.g. request counter with mutex
2.2 use raw clone() syscall with CLONE_THREAD + CLONE_SIGHAND + CLONE_VM params;
2.3 use pthread as it's the POSIX-compatible threading library on all modern nix;
Play around the CLONE_xxx parameters: use separated signals tables, shared and non-shared fds, etc.
Check how sharing a single connection fd across multiple threads works and affects preformance.