Ubuntu server memory use doesn't add up

Question

I have a small VPS server that I use as both a micro and development webhost and as a platform for whenever I want to do something somewhere that isn't my laptop. It's somewhat underspec with only 128M ram available (256 burst), but it works.

To deal with this, I have turned down a bunch of settings, etc. However, it still ends up reaching my cap whenever I try to do stuff.

I have done a bunch of research into this, but can't find anything other than people who mistook buffers and cache for actual process memory use.

I keep feeling like something doesn't add up right, so I finally tried

sh-3.2$ free
             total       used       free     shared    buffers     cached
Mem:        131072     131072          0          0          0          0
-/+ buffers/cache:     131072          0
Swap:            0          0          0
sh-3.2$ ps aux | awk '{sum += $4} END {print sum}'
67.4
sh-3.2$

So while all of my processes add up to 2/3 of my total, I'm somehow still entirely out of ram. htop agrees with both of them (bar at the top says 128M/128M; numbers agree).

Is it possible that free isn't keeping track correctly or something due to the VPS? Or is there actually something mysterious using 30% of my available ram?

Answer 1

You are summing the resident set for the processes. This excludes any virtual space non-resident but allocated. (This can't be allocated from swap as you don't have any.) As noted above, your calculation also excludes the kernel and its data structures.

Shared code and unused stack space also figure into virtual size. When you map a library you may only page in a small part of the library (resident set), but need to allocate space for the entire library (virtual set). Only read-only space for a shared library is double counted.

Answer 2

How much memory is your kernel using?

grep Memory /var/log/syslog and look at the reserved field.