The top Utility

The top utility is shipped on most platforms and with most releases of Linux. top reports on processor activity in real time and is most useful for obtaining an overall picture of the processes that are using the system and the resources they are using. top lets you specify the delay between measurements and the number of measurements to take. top also allows a batch mode of operation that writes the data in a normal text format. Another option to top lets you specify that the only processes that are displayed are those that are actually running.

top reports the approximate amount of memory a process is using. You can configure top to show a process hierarchy.

Use top as a first step in your analysis to see which processes are running in the system and how much of the system time they are consuming. By running top over a period of time and directing the output to a file, you can see which processes are running when. You can also approximate when a process stops, when large amounts of memory are consumed, and when one or two processes start to consume large amounts of processor time. For example, if top output indicates that a particular process is consuming most of the processing time, you can use other tools to identify the location within the application where the time is being consumed. The same holds true for memory usage. If a large amount of memory is suddenly being consumed or a gradual buildup in memory consumption has occurred, use a lower-level trace tool to identify where the increase in memory usage is occurring.

The following is a sample of the top report. The display is updated by default every 5 seconds.

 # top   8:24 am  up 4 days, 19:31,  1 user,  load average: 0.16, 0.06, 0.09   57 processes: 55 sleeping, 2 running, 0 zombie, 0 stopped   CPU states:  2.1% user,  0.6% system,  0.0% nice,  4.3% idle    Mem:   252976K av,  211516K used,   41460K free,       0K shrd,   29820K buff     Swap:  522072K av,   27060K used,  495012K free                   76836K cached PID    USER    PRI  NI  SIZE  RSS SHARE STAT %CPU  %MEM   TIME  COMMAND 20269  root    16   0   224   224  188    R   98.4  0.0   0:09  pi_watch 20270  root    10   0  1040  1040  828    R    1.9  0.4   0:00  top 1      root     9   0   464   428  412    S    0.0  0.1   0:08  init 2      root     9   0     0     0    0    SW   0.0  0.0   0:00  keventd 3      root    19  19     0     0    0    SWN  0.0  0.0   0:00  ksoftirqd_CPU0 4      root     9   0     0     0    0    SW   0.0  0.0   0:09  kswapd 5      root     9   0     0     0    0    SW   0.0  0.0   0:00  bdflush 6      root     9   0     0     0    0    SW   0.0  0.0   0:02  kupdated 8      root     9   0     0     0    0    SW   0.0  0.0   0:00  khubd 10     root    -1 -20     0     0    0    SW   0.0  0.0   0:00  mdrecoveryd 14     root     9   0     0     0    0    SW   0.0  0.0   0:14  kjournald 127    root     9   0     0     0    0    SW   0.0  0.0   0:00  kjournald 128    root     9   0     0     0    0    SW   0.0  0.0   0:00  kjournald 129    root     9   0     0     0    0    SW   0.0  0.0   0:00  kjournald 428    root     9   0   536   528  456    S    0.0  0.2   0:00  syslogd 432    root     9   0   428   412  376    S    0.0  0.1   0:00  klogd 449    rpc      9   0   560   556  484    S    0.0  0.2   0:00  portmap 468    rpcuser  9   0   660   576  576    S    0.0  0.2   0:00  rpc.statd 581    root     9   0   972   732  732    S    0.0  0.2   0:00  sshd 594    root     9   0   716   588  588    S    0.0  0.2   0:00  xinetd 615    root     9   0  1644  1260 1068    S    0.0  0.4   0:14  sendmail 624    smmsp    8   0  1176   772  688    S    0.0  0.3   0:00  sendmail 634    root     9   0   400   368  352    S    0.0  0.1   0:14  gpm 643    root     9   0   604   564  528    S    0.0  0.2   0:00  crond 674    xfs      9   0 6136 4  552 1032    S    0.0  1.7   0:06  xfs 692    daemon   9   0   512   492  456    S    0.0  0.1   0:00  atd 701    root     9   0   816   612  612    S    0.0  0.2   0:00  login 702    root     9   0   388   344  344    S    0.0  0.1   0:00  mingetty 703    root     9   0   388   344  344    S    0.0  0.1   0:00  mingetty 704    root     9   0   388   344  344    S    0.0  0.1   0:00  mingetty 705    root     9   0   388   344  344    S    0.0  0.1   0:00  mingetty 706    root     9   0   388   344  344    S    0.0  0.1   0:00  mingetty 709    root     9   0  1384  1028 1028    S    0.0  0.4   0:00  bash 826    root     9   0   484   460  420    S    0.0  0.1   0:09  pam_timestamp_c 16054  root     9   0  8624  8192 5476    S    0.0  3.2   4:02  gnome-terminal 16055  root     9   0  1528  1528 1164    S    0.0  0.6   0:01  bash 19021  root     9   0  1856  1752  900    S    0.0  0.6   0:02  oprofiled 20175  root     9   0  1512  1512 1152    S    0.0  0.5   0:00  bash

When active processes are displayed, only the actual process that is running is shownno information about memory or CPU is displayed.

While top is running, you can add or modify some options to change what is being reported. The program accepts options from the keyboard. You can also create a configuration file to define what to measure and how to measure it. Use the info top command to obtain online documentation on these capabilities.

Some of the more useful top parameters are as follows:

`d`	Delay between updates to the data.
`p`	Displays only the processes specified. Up to 20 processes can be specified.
`S`	Displays a summary of time being spent by the process and its children. Also displays dead time.
`I`	Does not report idle processes.
`H`	Shows all threads by process.
`N`	Number of times to produce the reports.

If top does not work on a system, ensure that the /proc file support is active on the system. Active /proc file support is required for top to run.