http://johanlouwers.blogspot.de/2010/02/hard-limit-for-maximum-open-file.html
这篇文章区分介绍了user-specific/system-wide的fd limits, 以及hard limit和soft limit.
具体设置：
http://www.cyberciti.biz/faq/linux-increase-the-maximum-number-of-open-files/
具体设置及查看当前打开数：
http://www.randombugs.com/linux/tuning-file-descriptors-limits-on-linux.html