> Resident size is the most reliable metric

Actually, Proportional Set Size is more accurate than RSS. See: https://en.wikipedia.org/wiki/Proportional_set_size

> This is what Windows task manager reports

Just to clarify, Windows Task Manager uses Private Working Set by default for process memory usage which does NOT include shared pages with other processes such as libraries or memory mapped files (hence the name “private”). It only shows the memory that maps to privately allocated physical memory per process. It’s probably closer to Resident Set on Unix.

You probably meant the memory usage in performance tab but I wanted to clarify in case people mistake it for all memory usage fields.

If you use memory-mapped files, cached pages count towards the resident set size of your process. If you use ordinary file I/O, they don't. That behavior has amusing consequences in HPC clusters that monitor the memory usage of each job and kill them if they use more memory than they requested.