Sometimes, you can see that memory is not correctly released once used and some caching can remain.
You can see that by using the free command:
# free -m
total used free shared buffers cached
Mem: 64458 53397 11060 0 1550 49163
-/+ buffers/cache: 2683 61774
Swap: 9998 35 9963
At this time, you can so free this cache and release memory by using this command:
# sync; echo 3 > /proc/sys/vm/drop_caches
Now, you can check again:
# free -m
total used free shared buffers cached
Mem: 64458 1162 63295 0 1 32
-/+ buffers/cache: 1128 63329
Swap: 9998 35 9963
The memory has correctly been released and the cache is almost empty!