Detailed explanation of using top command to analyze Linux system performance

Introduction to Linux top command

The top command is a commonly used performance analysis tool under Linux. It can display the resource usage of each process in the system in real time, similar to the Windows Task Manager.

top displays the current processes and other conditions of the system. It is a dynamic display process, that is, the current status can be continuously refreshed by pressing keys by the user. If the command is executed in the foreground, it will occupy the foreground exclusively until the user terminates the program. To be more precise, the top command provides real-time status monitoring of the system processor. It will display a list of tasks that are most "sensitive" to the CPU in the system. The command can sort tasks by CPU usage, memory usage, and execution time; and many features of the command can be set through interactive commands or in personal custom files.

In the Linux system, the top command is the most convenient tool for analyzing system performance, and top is also an interactive tool; through the top command, you can clearly understand the information of the executing process, including process ID, memory usage, CPU usage, etc. In fact, it is similar to the task manager of Windows.

The results of running the top command are as follows:

The first line is the task queue information, which is similar to the execution result of the uptime command. The specific parameters are as follows:

• 19:40:19 — refers to the current system time
• up 35 days, 20:32 — means the system has been running for 35 days, 20 hours, and 32 minutes (meaning the system has not been restarted during this period and has been running continuously)
• 1 users — There is currently 1 user logged into the system
• Load average: 0.00, 0.00, 0.00 — The three numbers after load average are the load conditions for 1 minute, 5 minutes, and 15 minutes respectively.
• The load average data is calculated by checking the number of active processes every 5 seconds using a specific algorithm. If this number is divided by the number of logical CPUs, a result above 5 indicates that the system is overloaded.

The second line, Tasks — Tasks (processes), the specific information is as follows:

• The system now has a total of 81 processes, of which 1 is running, 80 are sleeping, 0 are stopped, and 0 are zombies. Pay special attention to the zombie processes.

The third line contains CPU status information. The specific attributes are as follows:

• 0.0%us — The percentage of CPU used by user space.
• 0.0%sy — The percentage of CPU used by kernel space.
• 0.0%ni — The percentage of CPU used by processes that have changed their priority
• 100.0%id — idle CPU percentage
• 0.0%wa — The percentage of CPU used by IO waiting
• 0.0%hi — The percentage of CPU occupied by hardware IRQ
• 0.0%si — The percentage of CPU occupied by software interrupts
• 0.0%st — I don’t know about this either…

The fourth line, Mem memory status, the specific information is as follows:

• 1029828k total — Total physical memory (1GB)
• 804628k used — Total memory in use (767M)
• 228200k free — Total free memory (217M)
• 167208k buffers — Amount of cached memory (159M)

The fifth line contains the swap partition information. The specific information is as follows:

• 0k total — Total swap size (0K)
• 0k used — Total swap used (0K)
• 0k free — Total free swap space (0K)
• 401672k cached — Total amount of cached swap (383M)

Line 6, blank line.

From the seventh line onwards: Status monitoring of each process (task), column information is as follows:

• PID — process id
• USER — The process owner
• PR — Process Priority
• NI — nice value. Negative values ​​indicate high priority, positive values ​​indicate low priority
• VIRT — The total amount of virtual memory used by the process, in kilobytes. VIRT=SWAP+RES
• RES — The size of the physical memory used by the process that has not been swapped out, in kb. RES=CODE+DATA
• SHR — Shared memory size in kb
• S — Process status. D = Uninterruptible sleep state R = Run S = Sleep T = Trace/Stop Z = Zombie process
• %CPU — The percentage of CPU time used since the last update
• %MEM — The percentage of physical memory used by the process
• TIME+ — The total CPU time used by the process, in units of 1/100 seconds
• COMMAND — Process name (command name/command line)

top command parameters:

• -b batch processing
• -c Display complete COMMAND
• -I Ignore invalidation process
• -s confidentiality mode
• -S Cumulative mode
• -i<time> Set the interval time
• -u<username> specifies the username
• -p<process number> specifies the process
• -n<number of times> loop display times

Interactive commands of top (enter the following commands while the top command is running):

• q: Exit the program.
• h or ?: Display the help screen and give a brief summary of the command.
• 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.
• 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. This is the recommended way to write top configuration files.
• 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.
• l: Switch to display average load and startup time information. The first hidden line is displayed.
• m: Switch to display memory information. The hidden memory line is displayed
• t: Switch to display process and CPU status information. The hidden CPU line is displayed.
• c: Switch to display the command name and full command line. Displays the complete command. This feature is very useful.
• 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 5s. 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.

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