Tips for using top command in Linux

First, let me introduce the meaning of some fields in top:

VIRT：virtual memory usage

1. The virtual memory size "required" by the process, including libraries, code, data, etc. used by the process
2. If the process requests 100m of memory, but actually only uses 10m, then it will increase by 100m, not the actual usage.

RES：resident memory usage

1. The memory size currently used by the process, excluding swap out
2. Contains sharing of other processes
3. If you apply for 100m of memory and actually use 10m, it will only grow by 10m, which is the opposite of VIRT
4. Regarding the memory occupied by the library, it only counts the memory size occupied by the loaded library file

SHR: shared memory

1. In addition to the shared memory of its own process, it also includes the shared memory of other processes
2. Although the process only uses a few shared library functions, it includes the size of the entire shared library
3. Formula for calculating the physical memory size occupied by a process: RES – SHR
4. After swap out, it will drop

DATA

1. Memory occupied by data. If top is not displayed, press the f key to display it.
2. The data space actually required by the program is the one that is actually used during operation.

While top is running, you can use top's internal commands to control how the process is displayed. The internal commands are as follows:

s – Change the screen update frequency
l – turns on or off the display of the first line of the first part of the top information
t – turns off or on the display of the Tasks information in the second line of the first part and the CPU information in the third line
m – turns off or on the display of the Mem information in the fourth line of the first part and the Swap information in the fifth line
N – Lists processes in order of PID size
P – Sort the process list by CPU usage
M – Sort the process list by memory usage
h – Display help
n – Sets the number of processes to display in the process list
q – quit top
s – Change the screen update period

Column name meaning

a PID process id
b PPID parent process id
c RUSER Real user name
d UID User ID of the process owner
e USER User name of the process owner
f GROUP Group name of the process owner
g TTY The name of the terminal that started the process. Processes not started from the terminal are displayed as ?
h PR Priority
i NI nice value. Negative values ​​indicate high priority, positive values ​​indicate low priority
j P The last used CPU, only meaningful in a multi-CPU environment
k %CPU The percentage of CPU time occupied since the last update
l TIME The total CPU time used by the process, in seconds
m TIME+ Total CPU time used by the process, in units of 1/100 seconds
n %MEM The percentage of physical memory used by the process
o VIRT The total amount of virtual memory used by the process, in kb. VIRT=SWAP+RES
p SWAP The size of the virtual memory used by the process that is swapped out, in kb.
q RES The size of the physical memory used by the process that has not been swapped out, in kb. RES=CODE+DATA
r CODE The physical memory size occupied by the executable code, in kb
s DATA The physical memory size occupied by the part other than the executable code (data segment + stack), in kb
t SHR shared memory size, unit KB
u nFLT Number of page faults
v nDRT The number of pages modified since the last write.
w S Process status. (D = uninterruptible sleep state, R = run, S = sleep, T = trace/stop, Z = zombie process)
x COMMAND command name/command line
y WCHAN If the process is sleeping, the name of the sleeping system function is displayed
z Flags Task flags, refer to sched.h

By default, only the more important PID, USER, PR, NI, VIRT, RES, SHR, S, %CPU, %MEM, TIME+, and COMMAND columns are displayed. You can use the following shortcut keys to change the displayed content.

The f keys can be used to select the displayed content. After pressing the f key, a list of columns will be displayed. Press az to display or hide the corresponding columns, and finally press the Enter key to confirm.
Press the o key to change the order in which the columns are displayed. Pressing lowercase az moves the corresponding column to the right, while uppercase AZ moves the corresponding column to the left. Finally, press Enter to confirm.
Press the uppercase F or O key, then press az to sort the processes by the corresponding column. The capital R key can reverse the current sorting.

How to use top:

Use format:

top [-] [d] [p] [q] [c] [C] [S] [s] [n]

Parameter Description:

d: Specifies the time interval between two screen information refreshes. Of course, users can use the s interactive command to change it.

p: Monitor the status of only a certain process by specifying the monitoring process ID.

q: This option will make top refresh without any delay. If the calling program has superuser privileges, top will run at the highest possible priority.

S: Specifies the accumulation mode.

s: Make the top command run in safe mode. This will remove the potential dangers of interactive commands.

i: Make top not display any idle or zombie processes.

c: Display the entire command line instead of just the command name.

Description of common commands:

Ctrl+L: Erase and rewrite the screen

K: Terminate a process. The system will prompt the user to enter the PID of the process that needs to be terminated and what signal needs to be sent to the process. Generally, you can use signal 15 to terminate the process; if it cannot be terminated normally, use signal 9 to force the process to end. The default value is signal 15. This command is disabled in safe mode.

i: Ignore idle and zombie processes. This is a switch command.

q: exit the program

r: Rearrange the priority of a process. The system prompts the user to enter the process PID to be changed and the process priority value to be set. Entering a positive value will lower the priority, while a negative value will give the process a higher priority. The default value is 10.

S: Switch to cumulative mode.

s: Change the delay time between two refreshes. The system will prompt the user to enter a new time in seconds. If there is a decimal point, convert it to ms. If you enter a value of 0, the system will refresh continuously. The default value is 5 s. It should be noted that if the time is set too small, it is likely to cause continuous refresh, making it impossible to see the displayed situation clearly, and the system load will also increase significantly.

f or F: Add or remove items from the current display.

o or O: Change the order of displayed items

l: Toggle display of average load and startup time information.

m: Switch to display memory information.

t: Switch to display process and CPU status information.

c: Switch to display command name and full command line.

M: Sort by resident memory size.

P: Sort by CPU usage percentage.

T: Sort by time/cumulative time.

W: Write the current settings to the ~/.toprc file.

Improve your knowledge of top command

Trying to figure out what's running on your machine and which process is eating up the memory and making the system really, really slow - this is a job that the top command is perfect for.

top is a very useful program that acts similarly to the Windows Task Manager or MacOS's Activity Monitor. Running top on a *nix machine will give you a real-time view of the processes running on the system.

$ top

Depending on the version of top you run, you will see something like this:

top - 08:31:32 up 1 day, 4:09, 0 users, load average: 0.20, 0.12, 0.10
Tasks: 3 total, 1 running, 2 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.5 us, 0.3 sy, 0.0 ni, 99.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 4042284 total, 2523744 used, 1518540 free, 263776 buffers
KiB Swap: 1048572 total, 0 used, 1048572 free. 1804264 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
  1 root 20 0 21964 3632 3124 S 0.0 0.1 0:00.23 bash
 193 root 20 0 123520 29636 8640 S 0.0 0.7 0:00.58 flask
 195 root 20 0 23608 2724 2400 R 0.0 0.1 0:00.21 top

Your version of top may look different than this, particularly in the columns displayed.

How to read the output

You can tell what you're running by the output, but trying to interpret the results can be a bit confusing.

The first few lines contain a bunch of statistics (Details), followed by a table with the columns of results (Columns). Let’s start with the latter.

List

These are the processes that are running on the system. The default sorting is by CPU usage in descending order. This means that the programs at the top of the list are using more CPU resources and placing a heavier load on your system. In terms of resource usage, these programs are literally the top processes consuming the most resources. I have to say, the name top is very clever.

The rightmost COMMAND column reports the process names (the commands that started them). In this example, the process names are bash (a command interpreter in which we are running top), flask (a web framework written in Python), and top itself.

Other columns provide useful information about the process:

• PID: Process ID, a unique identifier used to locate a process
• USER: The user who runs the process
• PR: Priority of the task
• NI: Nice value, a better representation of priority
• VIRT: The size of virtual memory, in KiB (kibibytes)
• RES: resident memory size in KiB (part of physical memory and virtual memory)
• SHR: Shared memory size in KiB (part of shared memory and virtual memory)
• S: process status, usually I for idle, R for running, S for sleeping, Z for zombie process, T or t for stopped (there are other less common options)
• %CPU: CPU usage since the last screen update
• %MEM: RES resident memory usage since the last screen update
• TIME+: Total CPU usage time since the program started
• COMMAND: Start command, as described previously
  

Knowing exactly what the VIRT, RES, and SHR values ​​represent is not important in daily operations. It is important to know that the process with the highest VIRT value is the process using the most memory. When you use top to find out why your computer is running so slowly, the process with the highest VIRT value is the culprit. If you want to know the exact meaning of shared memory and physical memory, please refer to the Linux Memory Types section of the top manual page.

Yes, I said kibibytes, not kilobytes. The value 1024 often referred to as a kilobyte is actually a kibibyte. The Greek word kilo (χίλιοι) means one thousand (e.g. a kilometer is 1000 meters and a kilogram is 1000 grams). Kibi is a portmanteau of kilo and binary, meaning 1024 bytes (or 2^10). However, because this word is difficult to say, many people say kilobyte when they mean 1024 bytes. top tries to use proper terminology here, so take it as it is.

Screen Update Instructions

Real-time screen updating is one of the really cool things Linux programs can do. This means that programs can update the content they display in real time, so it appears dynamic, even though they use text. Very cool! In our case, the update interval is important because some statistics (%CPU and %MEM) are based on the values ​​from the last screen update.

Because we are running in a persistent application, we can type commands to modify the configuration in real time (rather than stopping the application and running it again with a different command line option).

Pressing h invokes the help screen, which also shows the default delay (the time interval between screen updates). This value defaults to (approximately) 3 seconds, but you can change it by typing d (roughly meaning delay) or s (probably meaning screen or seconds).

detail

There is a lot of useful information above the process list. Some details seem a bit strange and confusing. But once you take the time to go through them one by one, you'll find that they can be very useful in a pinch.

The first line contains general information about the system:

• top: We are running top! Hello! top!
• XX:YY:XX: Current time, updated every time the screen is updated
• up (followed by X day, YY:ZZ): the system's uptime, or how long it has been since the system was booted
• Load average (followed by three numbers): The system load in the past one minute, five minutes, and 15 minutes.
  

The second line (Task) shows information about the running task and is self-explanatory. It shows the total number of processes and the number of running, sleeping, stopped, and zombie processes. This is actually the sum of the S (status) column above.

The third line (%Cpu(s)) shows the CPU usage by type. The data are the values ​​between screen refreshes. The values ​​are:

• us: user process
• sy: system process
• ni: nice user process
• id: CPU idle time. A high value indicates that the system is relatively idle.
• wa: Waiting time, or the time spent waiting for I/O to complete
• hi: time consumed in hardware interrupts
• si: time consumed in software interrupts
• st: "Time stolen from this VM by the hypervisor"
  

You can expand or collapse the Task and %Cpu(s) rows by clicking the t (toggle).

The fourth line (KiB Mem) and the fifth line (KiB Swap) provide information about the memory and swap space. These values ​​are:

• Total memory capacity
• Used Memory
• Free Memory
• Memory buffer value
• Swap space cache value
  

By default they are displayed in KiB, but pressing E (extend memory scaling) you can cycle through different units: KiB, MiB, GiB, TiB, PiB, EiB (kilobytes, megabytes, gigabytes, terabytes, petabytes, and exabytes)

The top manual page has more information on options and configuration items. You can run man top to view the documentation on your system. There are also many HTML man pages, but please be aware that these may be specific to different versions of top.

Two top alternatives

You don't have to always use top to check the system status. You can use other tools to assist in troubleshooting depending on your situation, especially if you want a more graphical or professional interface.

htop

htop is like top, but it brings something very useful to the table: it can display CPU and memory usage in a graphical interface.

This is what htop looks like in the same environment where we just ran top. The display is simpler, but the functionality is rich.

Task stats, load, uptime and process list are still there, but there's a nice, colorful, dynamic per-core CPU usage, and a graphical memory usage chart.

Here's what the different colors mean (you can also get help with this information by pressing h).

CPU task priority or type:

• Blue: Low priority
• Green: Normal priority
• Red: Kernel tasks
• Blue: Virtual tasks
• The value at the end of the bar graph is the percentage of CPU used.
  

Memory:

• Green: used memory
• Blue: Buffered memory
• Yellow: Cache memory
• The values ​​at the end of the bar graph show the used and total memory.
  

If colors don't work for you, you can disable them by running htop -C; htop will then use different symbols to display CPU and memory types.

At the bottom there is a set of active shortcut keytips that can be used to manipulate the filter results or change the sort order. Try some of the shortcut keys to see what they do. Be careful when trying F9 though; it will bring up a list of signals that will kill (ie stop) a process. I recommend exploring these options outside of a production environment.

Hisham Muhammad, the author of htop (yes, htop is named after Hisham), gave a short talk at FOSDEM 2018 in February. He explained that htop not only has a simple graphical interface, but also a more modern way of displaying process information statistics, which are not available in previous tools (such as top).

You can read more about htop in the man page or on the htop website. (Hint: The website background is a dynamic htop.)

docker stats

If you are using Docker, you can run docker stats to generate a rich and contextual interface for container status.

This may be more helpful than top because it is not categorized by process but by container. This is especially useful when a container is running slowly, and it is faster to see which container is consuming the most resources than to run top and then find the container's progress.

With the help of the above explanation of top and htop terms, it should be easier for you to understand those in docker stats. However, the docker stats documentation provides a thorough description of each column.

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