Why is memory load hard to measure?
 

Hi!

I keep hearing that different programs (like free, top, exmap etc.) give a different output of how much RAM is used for which purpose.

Why is it so hard to measure used memory?

Thanks in advance :)

Mostly because linux shares libraries across many programs. So it's tough to know exactly how much memory is used by a single app. free -lm is close enough for me (look at the line "-/+ buffers/cache" -- the free column is how much memory is free for applications to use).

Oh, OK. Thank you very much for the answer :D

Though what I still do not understand, is why some prefer for example ExMap instead of free. I mean, if it is impossible to know for certain how much of memory a certain process uses, *can* there at all be any difference between how you try to measure it?

Debate has been long and volumious - even amongst the kernel devs.
What does "used memory" mean (to you) ???.

I don't necessarily agree with the way exmap accounts the (shared) pages, but it was a good attempt with the data available at the time.
/proc/<pid>/smaps is the latest iteration of attempts to account for the memory allocation.