Linux Operating System that powered by Linux kernel have an advanced disk cache algorithm that used to store frequently access data from Hard disk.
This is to explain why some Linux server’s RAM still getting high memory usage even you upgrade your RAM.
To find out how much memory using by the Linux-based server, you may use the linux command “free” to track it. To get the detailed/actual memory usage, you may run the following command from your Linux box.
You should get some result as below,
total used free shared buffers cached Mem: 15905 2879 13026 0 384 900 -/+ buffers/cache: 1593 14311 Swap: 3999 0 3999
With the result above, you may see that the used memory is 2879MB and free memory is 13026MB. However, the cached memory is 1593MB while the actual free memory will be 14311MB.