Where's my RAM if it's not in cache/buffers or used by processes?

1. free -m is accurate. I'm not familiar with ps_mem, but suspect it's reporting only certain kinds of memory usage by the programs. Try using top -a, see if it gives you more information.

2. The memory is likely used by programs' "resource" space. Restarting them will clear some of this out, but I'd be surprised if it wasn't back to the same place in an hour.

3. Why do you want to "reclaim" this memory. You're not out of RAM. "Free" memory is wasted memory. You'd be better off using 100%, even if only to get the most marginal of performance gains (which is why Linux uses it for caching aggressively).

4. See also: Why is Linux reporting "free" memory strangely?

   Sounds like you've got the same misunderstands of memory utilization as many people. This Q&A has a great explanation of what's going, on along with links to more information. But in short, unless OOM is killing processes let the OS do it's thing. Serious, kernel programmers have dedicated large portions of their lives to getting this right, I'd dare not question their expertise unless there's an obvious problem.

try to check out:

cat /proc/meminfo

could give you the extra info you need :)