i often run software where if I use the default /etc/security/limits.conf file,
you are limited to stack space of 8192 KB.
I therefore modify /etc/security/limits.conf
and in there i do
* hard stack unlimited
* soft stack unlimited

i noticed in another thread on the forum someone asked about limiting memory during a copy (cp) command and made me wonder.

some of my systems have > 100GB of RAM.
often if i do a big copy of data from one volume to another... something like:
cp /data/rondata/* /backup/
where it's thousands of files and 500+ GB of data I'll notice the amount of ram in the system gets used up. If I do a top and watch memory used/free I'll see it slowly get eaten up by the copy command. is this normal and caused by my stack unlimited setting, or by something else?

You'd need to check the code of cp to tell whether it's stack space (automatic variables) or something else.
It might be just opening files and using mmap() .. a fast technique.

In your example probably the main use of RAM is disk caching.
http://www.linuxatemyram.com/