Let’s have a closer look at the output of ‘top’ command. Usually it looks like this:
Mem: 1155072k total, 981032k used, 174040k free, 40396k buffers
Most people think that in this case we have 981032k used memory, and only 174040k free . But that’s not correct, this is global misunderstanding of memory stats and ways that Linux systems work with memory (RAM).
In every Linux-based environment some application runs and uses some memory until application finishes its job. However, some data left by this application is still present in memory just in case another application will need it, so it will work way faster.
This memory block is marked as ‘can be reused’. Using ‘top’ command you will see it as ‘used’, so the correct way to check ‘real’ amount of free RAM is to execute ‘free -m’ command. It’ll give something like:
root@host [~]# free -m
total used free shared buffers cached
Mem: 1128 956 171 0 38 720
-/+ buffers/cache: 197 930
Swap: 1023 0 1023
The line you need to look at is “-/+ buffers/cache”, it shows actually used and free memory blocks. The above example shows 930Mb of free RAM.