By default Linux operating system use more RAM memory for disk cache, because of reading data from cache if more faster than reading data from hard drive,  we can clear cached on your system and this is a non-destruction operation,

Check your used cached space with the commands free or top

#  free -m


   total       used       free     shared    buffers     cached

Mem:          5816       5371        445          0        246       4413

-/+ buffers/cache:        711       5105

Swap:         4095          0       4095

or
#   top -c


Mem:   5956188k total,  5502448k used,   453740k free,   252276k buffers

Swap:  4194296k total,        0k used,  4194296k free,  4519988k cached

To free your cached use this /proc/sys/vm/drop_caches, which can help
free a lot of memory space.

To free your pagecache:
 
# echo 1 > /proc/sys/vm/drop_caches

To free your dentries and inodes:
 
# echo 2 > /proc/sys/vm/drop_caches

To free your pagecache, dentries and inodes:
 
echo 3 > /proc/sys/vm/drop_caches

Now, check your cached and buffers status.
 
# free -m


    total       used       free     shared    buffers     cached

Mem:          5816        507       5308          0          0         39

-/+ buffers/cache:        467       5349

Swap:         4095          0       4095
         
Do you want to know RAM is free to use for your applications look on  "-/+ buffers/cache" its shows Megabytes: 5349 MB.

-/+ buffers/cache:        467       5349