There is zero of free memory in the output of free command. Why?

Question

• No free memory is presented as a result of the free command:

  # free -mh
  total used free shared buffers cached
  Mem: 1.0G 1.0G 0B 599M 0B 649M
  -/+ buffers/cache: 374M 649M
  Swap: 1.0G 21M 1.0G

• Why does the server use 100% of available memory?

Answer

Even though in the output above the amount of free memory is equal to 0, caches and buffers will be freed automatically if memory gets scarce

The amount of “real” free memory is equal to the sum of cached plus buffers memory. This value is displayed as the second value in the third line of free command (“-/+ buffers/cache” line):

# free -mh

total used free shared buffers cached
Mem: 1.0G 1.0G 0B 599M 0B 649M
-/+ buffers/cache: 374M 649M
Swap: 1.0G 21M 1.0G

In the example above, the “real” free memory is equal to 649 MB