Master Linux Memory: Understand VIRT, RES, SWAP and Diagnose Low Memory

Memory Basics

First run the top command to view memory-related fields.

The fields VIRT , RES , and SWAP represent virtual memory, resident (physical) memory, and swapped memory respectively.

Physical memory ( RES ) is the actual RAM in use. When RAM is insufficient, the OS moves rarely used pages to the swap area on disk; swapped pages are brought back when needed. Swap acts as a temporary storage, like a small warehouse for infrequently used data.

Virtual memory ( VIRT ) equals physical memory plus swap space. Programs operate on virtual addresses, which the OS maps to physical memory or swap.

Quickly Check Memory Status

Run free to see key metrics:

total – total physical memory size

used – amount of physical memory currently used

free – amount of idle physical memory

shared – memory shared among processes

buffers/cached – memory used for buffers and cache

swap – usage status of swap space

Common Memory Questions

1. Is low free space a sign of insufficient memory? In Linux, free physical memory is often low because the system uses idle memory for caching to improve performance. When memory runs low, Linux releases cache to satisfy program needs.

2. What is the difference between buffer and cache? Buffers store metadata of disk files (e.g., attributes, directory structures), while cache stores the actual file contents.

3. How to determine if memory is truly insufficient? Two main signs: continuous page swapping and a high number of major page faults. Major faults occur when a page is not in memory and must be fetched from disk. Use sar -B 1 3 to view metrics such as pgpgin/pgpgout (page in/out), fault (minor faults), and majflt (major faults). The vmstat 1 5 command also shows memory information, where the memory column displays memory stats and si/so indicate swap in/out.

Additional tip: after running top, the SWAP column may be hidden. Press f to enter column edit mode, then press p to select the SWAP column, and hit Enter to return to the top view with SWAP displayed.