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lastmod 2025-01-02created 2017-06-0214 edits

top - display Linux processes.

VSS vs RSS

In theory, it is said that the exact amount of memory a process occupies cannot be known. However, there are various ways to interpret the number of pages mapped to a process: VSS, RSS, USS, PSS, and so on.

VSS (Virtual Set Size)
The size of the virtual memory associated with the process.
RSS (Resident Set Size)
The number of physical pages associated with the process.
Use this one to calculate memory usage.

USS and PSS

Somewhat more meaningful figures than VSS or RSS are USS and PSS, which can be obtained with the procrank command.

  • USS (Unique Set Size): The number of pages unique to the process. It is the size of memory private to the process that is not shared.
  • PSS (Proportional Set Size): USS + (shared pages / number of sharing processes). That is, the process's own memory usage + the share of shared memory taken up by one process. If process A uses 6MB of memory and 2MB of it is the process's own region, the remaining 4MB is shared memory. If the 4MB of shared memory is shared by 4 processes, the PSS is 2MB + (4MB/4) = 3MB.

PSS divides shared pages by the number of sharing processes, giving a more accurate picture of memory usage. It can be seen as the value closest to the actual amount of memory a process uses.

Fields Management

Press f to select the displayed fields. 

Fields Management for window 1:Def, whose current sort field is %CPU
   Navigate with Up/Dn, Right selects for move then <Enter> or Left commits,
   'd' or <Space> toggles display, 's' sets sort.  Use 'q' or <Esc> to end!
  • PID - Process Id
  • USER - Effective User Name
  • PR - Priority
  • NI - Nice Value
  • VIRT - Virtual Image (KiB)
  • RES - Resident Size (KiB)
  • SHR - Shared Memory (KiB)
  • S - Process Status
  • %CPU - CPU Usage
  • %MEM - Memory Usage (RES)
  • TIME+ - CPU Time, hundredths
  • COMMAND - Command Name/Line
  • PPID - Parent Process pid
  • UID - Effective User Id
  • RUID - Real User Id
  • RUSER - Real User Name
  • SUID - Saved User Id
  • SUSER - Saved User Name
  • GID - Group Id
  • GROUP - Group Name
  • PGRP - Process Group Id
  • TTY - Controlling Tty
  • TPGID - Tty Process Grp Id
  • SID - Session Id
  • nTH - Number of Threads
  • P - Last Used Cpu (SMP)
  • TIME - CPU Time
  • SWAP - Swapped Size (KiB)
  • CODE - Code Size (KiB)
  • DATA - Data+Stack (KiB)
  • nMaj - Major Page Faults
  • nMin - Minor Page Faults
  • nDRT - Dirty Pages Count
  • WCHAN - Sleeping in Function
  • Flags - Task Flags
  • CGROUPS - Control Groups
  • SUPGIDS - Supp Groups IDs
  • SUPGRPS - Supp Groups Names
  • TGID - Thread Group Id
  • OOMa - OOMEM Adjustment
  • OOMs - OOMEM Score current
  • ENVIRON - Environment vars
  • vMj - Major Faults delta
  • vMn - Minor Faults delta
  • USED - Res+Swap Size (KiB)
  • nsIPC - IPC namespace Inode
  • nsMNT - MNT namespace Inode
  • nsNET - NET namespace Inode
  • nsPID - PID namespace Inode
  • nsUSER - USER namespace Inode
  • nsUTS - UTS namespace Inode
  • LXC - LXC container name
  • RSan - RES Anonymous (KiB)
  • RSfd - RES File-based (KiB)
  • RSlk - RES Locked (KiB)
  • RSsh - RES Shared (KiB)
  • CGNAME - Control Group name
  • NU - Last Used NUMA node

See also

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