[codedokode]

You are right. RSS values cannot be added. Imagine if there are 3 processes, each use 100 Mb of shared memory and 100 Mb of private. Summing the RSS we will get 200 + 200 + 200 = 600 Mb, however the true memory consumption is 100 (shared) + 100 + 100 + 100 = 400 Mb.

To count the memory properly, you should divide size of shared segments by number of processes using it. This is how PSS in Linux is implemented. You can safely add PSS values of different processes. If you have swap, you should also sum swap usage by processes.

As I understand, you can reliably count memory usage only in Linux because other OSes like Windows or Mac do not provide PSS in their Task Managers. Windows Task Manager at least has a documentation that describes what "memory" column means, and I failed to find such documentation for Mac, so we can safely assume that it displays random numbers remotely related to memory usage.