Question about free/used memory on Linux

["Ravinandan Arakali (rarakali)" <rarakali@cisco.com>]

Hi kernel gurus,
I am trying to find out the memory that's used on my linux box.
I find that there are quite a few confusing metrics. How do
I find out the "true" used memory ?

1. For eg. "free -m" shows free memory (excluding buffers/caches) 
as 308 MB while I can see(from "df" output) that the the tmpfs 
partitions take up about 400 MB. So, does "free -m" not consider 
the tmpfs partitions ?

2. I try to add up RSS field of all processes reported by
"ps aux" command. But is it true that this would be misleading
in that, shared memory used by, say 2 processes would show
up twice here although there's only one copy in memory. Also
does this consider the fact that there's only one copy
of shared libraries ?

3. I guess "free -m" and "top" commands use /proc/meminfo 
and hence all these outputs are same ?

Thanks,
Ravi
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