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This document summarizes the usage of top, which displays processes sorted by CPU usage or memory usage.

1. top

1.1. top 

$ top
top - 10:27:27 up 36 min,  3 users,  load average: 0.00, 0.01, 0.05
Tasks: 238 total,   1 running, 237 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.2 us,  0.1 sy,  0.0 ni, 99.7 id,  0.1 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   8052812 total,  1053584 used,  6999228 free,    49428 buffers
KiB Swap:  8265724 total,        0 used,  8265724 free.   541164 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 1848 root      20   0 1175416 103756  61724 S   0.7  1.3   0:12.55 compiz
 1349 root      20   0  601840  69188  53208 S   0.3  0.9   0:05.81 Xorg
 2824 root      20   0   30372   3544   2976 R   0.3  0.0   0:00.06 top
    1 root      20   0   34024   4464   2616 S   0.0  0.1   0:00.80 init
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.00 kthreadd
    3 root      20   0       0      0      0 S   0.0  0.0   0:00.52 ksoftirqd/0
    5 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/0:+
    7 root      20   0       0      0      0 S   0.0  0.0   0:00.33 rcu_preempt
    8 root      20   0       0      0      0 S   0.0  0.0   0:00.00 rcu_sched
    9 root      20   0       0      0      0 S   0.0  0.0   0:00.00 rcu_bh
   10 root      rt   0       0      0      0 S   0.0  0.0   0:00.00 migration/0
   11 root      rt   0       0      0      0 S   0.0  0.0   0:00.01 watchdog/0
   12 root      rt   0       0      0      0 S   0.0  0.0   0:00.01 watchdog/1
   13 root      rt   0       0      0      0 S   0.0  0.0   0:00.00 migration/1
   14 root      20   0       0      0      0 S   0.0  0.0   0:00.44 ksoftirqd/1
   16 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/1:+
   17 root      rt   0       0      0      0 S   0.0  0.0   0:00.00 watchdog/2
[Shell 1] top

[Shell 1] shows what can be checked through top. The upper part outputs CPU and memory information, and the lower part outputs process information sorted in descending order by CPU usage.

1.1.1. CPU Information

The %Cpu(s) part in the upper part of [Shell 1] represents the average CPU usage of all CPU cores. Each column has the following meaning:

  • us (user) : CPU usage rate for running user code of processes without nice value applied (un-niced, nice = 0). Represents usage rate of most user processes
  • sy (system) : CPU usage rate for running kernel code, excluding usage/idle rates of id, wa, hi, si
  • ni (nice) : CPU usage rate for running user code of processes with nice value applied (niced)
  • id (idle) : CPU idle rate excluding I/O Wait
  • wa (wait) : CPU idle rate due to I/O Wait
  • hi (hardware interrupt) : CPU usage rate used for pure hardware interrupt processing. Represents CPU usage rate for processing the top halves part that only sets interrupt flags in the kernel
  • si (sotware interrupt) : CPU usage rate of bottom halves that actually process interrupts according to interrupt flags set by top halves
  • st (steal) : CPU usage rate stolen by the hypervisor or other virtual machines when the kernel runs inside a virtual machine controlled by a hypervisor

1.1.2. Memory Information

Between the CPU information and process information in [Shell 1], memory usage is displayed. Each item has the following meaning:

  • Mem total : Total memory capacity
  • Mem used : Memory capacity in use
  • Mem free : Memory capacity not in use
  • Swap total : Total swap capacity
  • Swap used : Swap capacity in use
  • Swap free : Swap capacity not in use
  • buffers : Memory capacity used as kernel buffer
  • cached Mem : Memory capacity used as kernel cache

1.1.3. Process Information

The lower part of [Shell 1] outputs process information. Each column has the following meaning:

  • PID : Process ID
  • USER : Process owner
  • PR : Scheduling priority actually used during kernel scheduling. Can have values “0 ~ 39, rt”, and in the case of numbers, lower values have higher priority. rt means Real Time Scheduling Priority and has higher priority than priority 0
  • NI : nice value. Can have values “-20 ~ 19”, and lower numbers have higher priority. “20 + NI” becomes PR
  • VIRT : Virtual memory capacity. Means the sum of all memory capacity and swap capacity allocated for the process, even if not currently in use
  • RES : Actual memory capacity currently in use. Part of VIRT
  • SHR : Shared memory capacity. Part of RES
  • S : Process state
  • %CPU : CPU usage rate
  • %MEM : Memory usage rate
  • TIME+ : Process runtime
  • COMMAND : Process command

1.1.4. Shortcuts

  • 1 : Outputs usage per CPU core
  • SHIFT + M : Sorts by memory usage
  • SHIFT + P : Sorts by CPU usage
  • SHIFT + T : Sorts by runtime

2. References

tmuxvmstat