[mgedmin]

You can't really do sums of top columns, because some memory is shared and you'll end up double-counting it.

And you can't just subtract the shared memory numbers, because different sets of pages are shared between different sets of processes, and top doesn't give enough information to figure out what's actually happening where.

Running the pmaps tool on all pids and summing the Pss number is perhaps the closest you can get to the actual memory use.