Go2Linux posted a guide about cleaning the cache buffer pages from Linux memory

Cache in Linux memory is where the Kernel stores the information it may need later, as memory is incredible faster than disk, it is great that the Linux Kernel takes care about that.
Anyway you can also manipulate how the cache behaves, there usually no need to do that, as Linux Operating system is very efficient in managing your computer memory, and will automatically free the RAM and drop the cache if some application needs memory.

  How to clear or drop the cache buffer pages from Linux memory