技术咨询：atop中MEM百分比反映PSIZE还是仅RSIZE及相关逻辑

Great question—this is definitely one of those details where the atop man page can feel frustratingly vague! Let's break this down clearly:

1. The MEM percentage always reflects RSIZE (RSS), not PSIZE #

Atop's MEM column calculates the percentage of system physical memory that a process is actively using, which maps directly to the process's RSIZE (resident set size). This is the portion of the process's memory that's actually loaded into physical RAM, not swapped out or sitting as unused virtual memory.

PSIZE (or VSIZE in some atop versions, referring to the process's total virtual memory size) includes a lot of memory that doesn't consume physical system resources—things like unallocated virtual address space, swapped-out pages, or shared library code that hasn't been loaded into RAM yet. Using PSIZE to calculate a "memory usage percentage" relative to system physical memory would be misleading, since it doesn't reflect actual memory pressure on the system.

2. Atop does not use PSIZE to adjust the MEM metric, even when available #

Even when PSIZE data is accessible to atop, the tool keeps MEM focused on physical memory utilization via RSIZE. This is intentional: the MEM column is designed to help you quickly assess which processes are contributing to physical memory exhaustion, which is the key metric for diagnosing system memory pressure.

If you want to track a process's total virtual memory footprint, atop provides separate columns (like VSIZE or PSIZE, depending on your version) to show that data—no overlap with the MEM percentage calculation.

Why the man page feels ambiguous #

The man page's definition ("percentage of the system's available capacity for this resource that the process is using") uses vague language, but "resource" here explicitly refers to physical memory. This makes sense because system-level memory constraints almost always relate to physical RAM, not virtual memory (which is largely limited by disk swap space, not the system's core memory hardware).

内容的提问来源于stack exchange，提问作者sourcejedi