How do you accurately determine a process RAM usage

How do you accurately determine a process RAM usage

[P]

I wrote a tool to (try to) report how much RAM a
particular program (apache for e.g.) was using:
http://www.pixelbeat.org/scripts/ps_mem.py

I was then pointed at the following:
http://wiki.apache.org/spamassassin/TopSharedMemoryBug
which describes how copy-on-write pages are
not counted as shared since 2.6.

So how can one determine how much RAM a process is using?
Seems like a fundamental requirement to me.
Could we add a "SharedTotal" column to /proc/$$/statm for e.g. ?

Here's a little python script to demo the problem:

#!/usr/bin/python
import os, sys, time
a="\x00"*(32*1024*1024) #alloc 32MiB
pid=os.fork()
if(not pid):
     time.sleep(1)
     sys.exit()
sizel=map(float,open("/proc/"+str(pid)+"/statm").readlines()[0].split()[1:3])
print "only %.1f%% reported as shared" % ((sizel[1]/sizel[0])*100)
os.wait()

-- 
Pádraig Brady - http://www.pixelbeat.org
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