memory difference on top and htop

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I got a new VPS as my database server.
I installed only mysql and started. After some time(even after shutting down mysql
service), I see only %3-4 of the memory used in htop but according to top I only have
30MB free memory. It has a total of 4GB RAM. I don't know which one to trust. Can
someone explain me the difference of top and htop memory usage and what may be causing
the high usage at top
stats?

thanks.

Answer

It's just the difference of whether you consider memory that contains
discardable data as used or not. The memory is used in the sense that it contains
information that may be useful. But it's free in the sense that the information can
simply be discarded if the memory is needed.

For
example, say you run a program. The executable file that holds the program itself is
still in memory. But that data is not needed at the time. If the program runs again,
however, the information can be used from memory so it doesn't have to be loaded from
disk again. But if that memory is needed for some other purpose, the file data can be
thrown away since it's on the disk. Should this be considered free memory or
not?

The reason so much memory is used is
because free memory is wasted memory. Only memory that is being used can improve
performance. The OS tries to keep as little memory free as possible, only enough to
handle special situations such as bursts of data received from the network. It's not
like if you use only 3GB today you can use 5GB tomorrow. If you only use 3GB today, you
just wasted 1GB.

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