and wait (your prompt won't return immediately, but other processes will continue to run). It will slowly unload the caches (2.5 min for my 24GB cache on 2 pair of raid 1'd 2TB WD blacks on a 2Ghz 4 year old cpu on a box w/64GB) - beware, you will suddenly have no caches, and any processes reading data will be pulling off raw disk so you'll probably see IO wait jump up for a while til cache is repopulated.
Clear PageCache only: # sync; echo 1 > /proc/sys/vm/drop_caches Clear dentries and inodes: # sync; echo 2 > /proc/sys/vm/drop_caches Clear PageCache, dentries and inodes: # sync; echo 3 > /proc/sys/vm/drop_caches
echo 3 > /proc/sys/vm/drop_caches