I remember finding a C++ (or maybe plain C) library which does thread pooling, socket polling and other stuff, needed to write a high-performance server on Linux. (And this library was not Boost itself — but it could be that it used Boost.)
As usual, I can't remember the name of the library. Any guesses?
Update: that was not ACE.
Spserver uses libevent and so should have good performance on all platforms.
You could use Boost.ASIO directly and thread pool:
Multiple threads may call io_service::run() to set up a pool of threads from which completion handlers may be invoked ... all threads that have joined an io_service's pool are considered equivalent, and the io_service may distribute work across them in an arbitrary fashion.
