хоть и ОЧЕНЬ старая статья, но все же, как мне кажется, не потерявшая свою актуальность:
http://bulk.fefe.de/scalability/Conclusion: Linux 2.6 is the clear winner, scaling O(1) in every respect. The clear loser is OpenBSD; I have never seen bad performance of this magnitude. Even Windows would probably outperform OpenBSD. NetBSD performance leaves a lot to be desired as well. This mmap graph is the only part of the whole benchmark suite where FreeBSD did not perform top notch. If the FreeBSD people fix this one dark spot, they will share the top space with Linux 2.6.