What is the best way to calculate the real and combined memory usage of forked processes? - linux

Question

I am working on a per process memory monitoring (Bash) script but it turns out to be more of a headache than I thought. Especially on forked processes such as PostgreSQL. There are a couple of reasons:
RSS is a potential value to be used as memory usage, however this also contains shared libraries etc which are used in other processes
PSS is another potential value which (should) show only the private memory of a process. Problem here is that PSS can only be retrieved from /proc//smaps which requires elevated capability privileges (or root)
USS (calculated as Private_Dirty + Private_Clean, source How does smem calculate RSS, USS and PSS?) could also be a potential candidate but here again we need access to /proc//smaps
For now I am trying to solve the forked process problem by looping through each PID's smaps (as suggested in https://www.depesz.com/2012/06/09/how-much-ram-is-postgresql-using/), for example:
for pid in $(pgrep -a -f "postgres" | awk '{print $1}' | tr "\n" " " ); do grep "^Pss:" /proc/$pid/smaps; done
Maybe some of the postgres processes should be excluded, I am not sure.
Using this method to calculate and sum the PSS and USS values, resulting in:
PSS: 4817 MB - USS: 4547 MB - RES: 6176 MB - VIRT: 26851 MB used
Obviously this only works with elevated privileges, which I would prefer to avoid. If these values actually represent the truth is not known because other tools/commands show yet again different values.
Unfortunately top and htop are unable to combine the postgres processes. atop is able to do this and seems to be (from a feeling) the most accurate with the following values:
NPROCS SYSCPU USRCPU VSIZE RSIZE PSIZE SWAPSZ RDDSK WRDSK RNET SNET MEM CMD 1/1
27 56m50s 16m40s 5.4G 1.1G 0K 2308K 0K 0K 0 0 11% postgres
Now to the question: What is the suggested and best way to retrieve the most accurate memory usage of an application with forked processes, such as PostgreSQL?
And in case atop already does an accurate calculation, how does atop get the to RSIZE value? Note that this value shown as root and non-root user, which would probably mean that /proc/<pid>/smaps is not used for the calculation.
Please comment if more information is needed.
EDIT: I actually found a bug in my pgrep pattern in my final script and it falsely parsed a lot more than just the postgres processes.
The new output now shows the same RES value as seen in atop RSIZE:
Script output:
PSS: 205 MB - USS: 60 MB - RES: 1162 MB - VIRT: 5506 MB
atop summarized postgres output:
NPROCS SYSCPU USRCPU VSIZE RSIZE PSIZE SWAPSZ RDDSK WRDSK RNET SNET MEM CMD
27 0.04s 0.10s 5.4G 1.1G 0K 2308K 0K 32K 0 0 11% postgres
But the question remains of course. Unless I am now using the most accurate way with the summarized RSS (RES) memory value. Let me know your thoughts, thanks :)