Estimating memory usage on Android and Linux

Earlier kernels export Rss and Vss of each single process to show how many page frames it occupies and how the address space it spans on a particular time. But this information is process-wide, and it doesn't help when you estimate system-wide memory usage. The problem is the lacking of information about how the processes share pages with each other. Simply summing up the Rss of all processes would over calculate the number of pages shared by multiple processes.