I used the following one liner (obtained from an old thread on this site) to look for RSS size of a process and convert it to Mb (I am using process nfsmapid as an example):
ps -eo rss,args | nawk 'END { print s/1024, "Mb" } /nfsmapid/ { s += $1 }'
I found that the RSS Mb displayed was always higher than if I took the RSS value and converted it manually to Mb.
What I found using this one liner was that the displayed value of $1 also included the RSS for the nawk process.
Using the one liner
ps -eo rss,args | nawk 'END { print s/1024, "Mb" } /nfsmapid/ { s += $1 }'
875.742 MB
Just displaying the RSS process
ps -eo rss,args | nawk '/nfsmapid/ {print $1/1024," " $2}'
874.461 /usr/lib/nfs/nfsmapid
1.28125 nawk
If you add 1.28125 to 874.461, the result is 875.742(25) MB
Using "+= $1" is great for collating the RSS of multiple processes with the same name
No doubt there is some way to stop this from occurring (I don't have time to investigate at the moment)