Two important parameters for Linux performance optimization (refer)

Vfs_cache_pressure:
This file indicates that the kernel is recycled for directory and inode
Cache memory preference; default value: 100 indicates that the kernel uses directory and inode according to pagecache and swapcache
Cache stays at a reasonable percentage; lowering this value below 100 will lead to kernel preference for directory and inode
Cache; an increase of more than 100 will lead to the kernel tends to recycle directory and inode cache.
Default: 100

Min_free_kbytes:
This file indicates the minimum number of idle memory (Kbytes) reserved by the Linux VM ).
Default setting: 724 (MB physical memory)

Change command:
Sysctl-w vm. vfs_cache_pressure = 200
Sysctl-w vm. min_free_kbytes = 1024

See:
Http://bbs.chinaunix.net/viewthread.php? Tid = 849389
Http://blog.sina.com.cn/s/blog_4b4de658010006m7.html