1. Clear PageCache only.

sync; echo 1 > /proc/sys/vm/drop_caches

2. Clear dentries and inodes.

sync; echo 2 > /proc/sys/vm/drop_caches

3. Clear PageCache, dentries and inodes.

sync; echo 3 > /proc/sys/vm/drop_caches 

4. "Clear Swap"

swapoff -a 
swapon -a 

Make sure to be be logged in as "root"