RSS
also: Resident Set Size, resident memory, physical memory usage
Resident Set Size (RSS) is the portion of a process's memory that is currently held in physical RAM, as opposed to swapped to disk or stored in the page cache.
Resident Set Size (RSS) measures how much physical memory (RAM) a running process is actively using at any given moment. It excludes memory that has been swapped to disk and may include shared libraries used by multiple processes.
You can view RSS for running processes using tools like ps, top, or htop. For example, ps aux displays RSS in the RSS column, showing memory in kilobytes.
RSS is useful for understanding real memory consumption, though it can be misleading when processes share libraries—the same shared memory may be counted in the RSS of multiple processes. For more accurate memory profiling, tools like pmap or smaps provide detailed breakdowns of memory usage.