[CentOS] Finding memory usage

Fri Jul 27 15:10:23 UTC 2018
Bowie Bailey <Bowie_Bailey at BUC.com>

I have a CentOS 7 server that is running out of memory and I can't
figure out why.

Running "free -h" gives me this:
              total        used        free      shared  buff/cache  
Mem:           3.4G        2.4G        123M        5.9M       
928M        626M
Swap:          1.9G        294M        1.6G

The problem is that I can't find 2.4G of usage.  If I look at resident
memory usage using "top", the top 5 processes are using a total of
390M.  The next highest process is using 8M.  For simplicity, if I
assume the other 168 processes are all using 8M (which is WAY too high),
that still only gives a total of 1.7G.  The tmpfs filesystems are only
using 18M, so that shouldn't be an issue.

Yesterday, the available memory was down around 300M when I checked it. 
After checking some things and stopping all of the major processes,
available memory was still low.  I gave up and rebooted the machine,
which brought available memory back up to 2.8G with everything running.

How can I track what is using the memory when the usage doesn't show up
in top?