How to use the command `systemd-cgtop` (with examples)
- Linux
- December 25, 2023
The systemd-cgtop command is used to display the top control groups of the local Linux control group hierarchy, ordered by their CPU, memory, or disk I/O load. It provides an interactive view similar to the top command, but specific to control groups. By default, it shows the CPU usage percentage, memory usage in bytes, and I/O operations per second for each control group.
Use case 1: Start an interactive view
Code:
systemd-cgtop
Motivation:
Starting an interactive view is the simplest and most common use case of the systemd-cgtop command. It allows you to monitor the CPU, memory, and disk I/O load of control groups in real-time, helping you identify any resource-intensive processes or groups of processes.
Explanation:
Running the systemd-cgtop command without any additional arguments starts the interactive view of the control groups. It continuously updates the display based on the current resource usage of the control groups.
Example output:
Path Tasks Δ CPU Memory Input/s Output/s
/ 463 +2 0.8% 4.9G 0B 0B
/system 225 +1 0.6% 4.1G 0B 0B
/systemd 226 0.5% 116.0M 0B 0B
/docker 210 0.3% 3.3G 0B 0B
The output displays the path of each control group, number of tasks running within it, the change in the number of tasks since the last update, CPU usage percentage, memory usage in bytes, input and output operations per second.
Use case 2: Change the sort order
Code:
systemd-cgtop --order=cpu
Motivation: Changing the sort order allows you to prioritize control groups based on their CPU usage. This can be useful when you want to identify the most resource-intensive control groups and how they are affecting the system.
Explanation:
The --order option is used to specify the sort order of the control groups. In this example, we have set it to cpu to order the control groups based on their CPU usage. Other available options for --order are memory, path, tasks, and io.
Example output:
Path Tasks Δ CPU Memory Input/s Output/s
/system 225 +1 0.6% 4.1G 0B 0B
/docker 210 0.3% 3.3G 0B 0B
/systemd 226 0.5% 116.0M 0B 0B
/ 463 +2 0.8% 4.9G 0B 0B
The output now shows the control groups ordered by their CPU usage in descending order.
Use case 3: Show the CPU usage by time instead of percentage
Code:
systemd-cgtop --cpu=percentage
Motivation: Displaying the CPU usage by time instead of percentage allows you to see the actual CPU usage duration for each control group, giving you more insight into their resource consumption.
Explanation:
The --cpu option is used to specify the mode of displaying CPU usage. By default, systemd-cgtop displays the CPU usage as a percentage. In this example, we have set it to percentage to show the CPU usage by time in milliseconds.
Example output:
Path Tasks Δ CPU (ms) Memory Input/s Output/s
/ 463 +2 190.2ms 4.9G 0B 0B
/system 225 +1 168.7ms 4.1G 0B 0B
/systemd 226 145.4ms 116.0M 0B 0B
/docker 210 102.3ms 3.3G 0B 0B
The output now includes the CPU usage in milliseconds for each control group.
Use case 4: Change the update interval
Code:
systemd-cgtop --delay=2s
Motivation:
Changing the update interval allows you to control the frequency at which systemd-cgtop updates the display. Increasing the interval can reduce the system load caused by the command.
Explanation:
The --delay option is used to specify the update interval of systemd-cgtop. In this example, we have set it to 2 seconds using the s suffix. You can also use other time units such as ms (milliseconds), us (microseconds), and min (minutes).
Example output (after waiting for 2 seconds):
Path Tasks Δ CPU Memory Input/s Output/s
/ 620 +3 1.2% 4.9G 0B 0B
/system 292 0.8% 4.1G 0B 0B
/systemd 292 0.6% 116.0M 0B 0B
/docker 260 0.4% 3.3G 0B 0B
The output now reflects the updated resource usage after the specified delay of 2 seconds.
Use case 5: Only count userspace processes
Code:
systemd-cgtop -P
Motivation:
By default, systemd-cgtop counts both userspace processes and kernel threads in the control groups. In some cases, you may only want to monitor userspace processes, ignoring the kernel threads that don’t directly impact the resource usage.
Explanation:
The -P option is a shorthand for --peers. It tells systemd-cgtop to only count userspace processes (tasks) and exclude kernel threads from the displayed statistics.
Example output:
Path Tasks Δ CPU Memory Input/s Output/s
/ 449 +2 0.5% 5.0G 0B 0B
/system 218 +1 0.4% 3.5G 0B 0B
/systemd 221 0.3% 87.5M 0B 0B
/docker 207 0.2% 4.6G 0B 0B
The output now excludes kernel threads, showing only userspace processes for each control group.
Conclusion:
The systemd-cgtop command is a powerful tool for monitoring and analyzing the resource usage of control groups in real-time. By using different options and arguments, you can customize the display to fit your specific needs, such as changing the sort order, displaying CPU usage by time, adjusting update intervals, and excluding kernel threads. These use cases can help you identify and address performance bottlenecks in your system.
