Unaccounted memory usage in Red Hat Enterprise Linux.

Issue

• The system has 40 GiB of memory and the total memory used by the processes running on the system is 20 GiB, but Free command is showing 39 GiB of memory has used in the system.

# cat sos_commands/memory/free_-m 
             total       used       free     shared    buffers     cached
Mem:         40076      38908       1167      18268          1        603
-/+ buffers/cache:      38304       1772
Swap:        65151       5431      59720

• Need to find out where the rest 19 GiB of memory being used in the system.