Vsz

Vsz, typically seen as VSZ in documentation, is a common abbreviation in computing for the virtual memory size of a process on Unix-like systems. It denotes the total size of the process's virtual address space, including all code, data, heap, stack, and memory-mapped regions, regardless of whether those pages are currently loaded in physical RAM or backed by swap.

VSZ is not a direct measure of actual memory usage. The portion of a process that is resident in RAM is described by RSS (resident set size). Some tools also use PSS (proportional set size), which apportions shared pages among processes according to usage. In practice, VSZ can be larger than RSS or PSS, because virtual memory includes space for, among other things, loaded libraries, code segments that may be swapped out, and mapped files.

How VSZ is reported varies by tool. In ps and top, VSZ is usually shown in kilobytes and represents the virtual address space size. In /proc/[pid]/stat, the vsize field provides the same information in bytes. Interpreting VSZ involves considering the system’s memory management behavior: a large VSZ does not necessarily indicate high memory pressure; it may reflect large address spaces created by memory-mapped files, shared libraries, or overcommit settings.

Outside computing, Vsz can be used as an acronym in other domains, but in technical contexts VSZ as virtual memory size is the most widely recognized meaning. For a fuller understanding, compare VSZ with RSS and PSS when diagnosing memory usage.