Optimizing SQUID in Linux, although the 1st is very old, it still has some tips to learn.

http://www.linux-faqs.com/squid.php

http://2bits.com/articles/increasing-drupals-speed-squid-caching-reverse-proxy.html

http://hotops.blogspot.com/2007/06/high-performance-reverse-proxy-squid.html

Ulimit details. always in /etc/security/limits.conf at least in CentOS&OpenSuse

http://wiki.linuxquestions.org/wiki/Ulimit