how to clear caches in linux

http://www.linuxinsight.com/proc_sys_vm_drop_caches.html
To free pagecache, dentries and inodes:

• echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation, and dirty objects are not freeable, the user should run "sync" first in order to make sure all cached objects are freed.