Unix top command

The Unix top command is a very useful way to see what programs are currently running on the system and how heavily they are using system resources. (The command is named “top” because it shows the top users of the system.) It is a good idea to run top to check for other users running large programs before running one yourself on a shared computer (such as the Applied Math servers), and you can also use it to monitor your own programs.

Running top

To run top, simply type:

$ top

at the command line. top will fill your terminal window with a real-time display of system status, with a summary area displaying general information about memory and processor usage in the first few lines, followed by a list of processes and information about them. An example display is shown below, with the process list truncated for brevity:

top - 14:45:34 up  6:32,  2 users,  load average: 0.78, 0.61, 0.59
Tasks: 110 total,   3 running, 106 sleeping,   0 stopped,   1 zombie
Cpu(s): 75.0%us,  0.6%sy,  0.0%ni, 24.4%id,  0.0%wa,  0.0%hi,  0.0%si,
0.0%st
Mem:    507680k total,   491268k used,    16412k free,    24560k buffers
Swap:        0k total,        0k used,        0k free,   316368k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 2342 uwhpsc    20   0  8380 1212  796 R  300  0.2   0:28.55 jacobi2d.exe
  842 root      20   0  100m  24m 9780 R    2  5.0 178:27.50 Xorg
 1051 uwhpsc    20   0 40236  11m 8904 S    1  2.3   0:01.53 xfce4-terminal
    1 root      20   0  3528 1864 1304 S    0  0.4   0:01.12 init
    2 root      20   0     0    0    0 S    0  0.0   0:00.00 kthreadd

Summary area

The summary area shows a great deal of information about the general state of the system. The main highlights are the third through fifth lines, which show CPU and memory usage.

CPU is given as percentages spent doing various tasks. The abbreviations have the following meanings:

• us – time spent running non-niced user processes (most of your programs will fall into this category)
• sy – time spent running the Linux kernel and its processes
• ni – time spent running niced use processes
• id – time spent idle
• wa – time spent waiting for I/O

The fourth and fifth lines show the usage of physical (i.e. actual RAM chips) and virtual memory. The first three fields are mostly self-explanatory, though on Linux the used memory includes disk cache. (Linux uses RAM that’s not allocated by programs to cache data from the disk, which can improve the computer’s performance because RAM is much faster than disk.) buffers gives the amount of memory used for I/O buffering, and cached is the amount used by the disk cache. Programs’ memory allocation takes priority over buffering and caching, so the total amount of memory available is the sum free + buffers + cached.

Task area

The task area gives a sorted list of the processes currently running on your system. By default, the list is sorted in descending order of CPU usage; the example display above is sorted by memory usage instead. Many different fields can be displayed in the task area; the default fields are:

• PID: Process ID number. Useful for killing processes.
• USER: Name of the user running the process.
• PR: Relative priority of the process.
• NI: Nice value of the process. Negative nice values give a process higher priority, while positive ones give it lower priority.
• VIRT: Total amount of memory used by the process. This includes all code, data, and shared libraries being used. The field name comes from the fact that this includes memory that has been swapped to disk, so it measures the total virtual memory used, not necessarily how much of it is currently present in RAM.
• RES: Total amount of physical (“resident”) memory used by the process.
• S: Process status. The most common values are S for “Sleeping” or R for “Running”.
• %CPU: Percent of CPU time being used by the process. This is relative to a single CPU; in a multiprocessor system, it may be higher than 100%.
• %MEM: Percent of available RAM being used by the process. This does not include data that has been swapped to disk.
• TIME+: Total CPU time used by the process since it started. This only counts the time the process has spent using the CPU, not time spent sleeping.
• COMMAND: The name of the program.

Interactive commands

There are many useful commands you can issue to top; only a few of them are listed here. For more information, see the top manual page.

Command	Meaning
q	Quit
?	Help
u	Select processes belonging to a particular user. Useful on shared systems.
k	Kill a process
F	Select which field to sort by

Further reading

For more information, see the quick reference guide at http://www.oreillynet.com/linux/cmd/cmd.csp?path=t/top, or type man top in a Unix terminal window to read the manual page.