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