How to Use the Command 'cgclassify' (with examples)

The command cgclassify is a tool used in Linux to move running processes to specific control groups (cgroups). Cgroups are a feature provided by the Linux kernel that helps manage, allocate, prioritize, deny, or control resource usage (CPU, memory, disk I/O, etc.) for a collection of processes. This command acts as a utility for assigning PIDs (Process IDs) to the appropriate cgroups, allowing system administrators to effectively manage system resource utilization based on their needs.

Use Case 1: Move a Process to a Specific Cgroup in the CPU Hierarchy

Code:

cgclassify -g cpu:student 1234

Motivation:

Imagine a scenario where you are running a multi-tenant application platform, and each tenant’s processes need to be isolated within their own cgroup to prevent resource hogging. By moving a process with a specific PID to the designated cgroup, you can ensure that the process’s resource usage is managed within its allocated limits, thereby maintaining system performance and fairness among all tenants.

Explanation:

• cgclassify: The command being executed to classify and move processes into specified cgroups.
• -g cpu:student: The -g flag specifies the target cgroup path and hierarchy. Here, it indicates that the process should be moved into the CPU resource sub-system under the cgroup named “student.”
• 1234: Represents the PID of the process that is to be moved into the specified cgroup. Replace 1234 with the actual PID of the process of interest.

Example Output:

Assuming successful execution and the cgroup “student” exists within the CPU hierarchy, the output might not show anything explicitly unless there is an error, as the command typically runs silently when successful. However, seeing no output usually confirms that the task was moved successfully.

Use Case 2: Move a Process Based on Configuration File

Code:

cgclassify 1234

Motivation:

When managing a complex system with multiple processes, it can be cumbersome to manually assign each process to its designated cgroup. Utilizing a set configuration file like /etc/cgrules.conf, which outlines rules for cgroup classification based on process properties, can automate this process. This is particularly useful when rules apply consistently across system startups and reconfigurations.

Explanation:

• cgclassify: The command being run to classify and arrange processes into appropriate cgroups.
• 1234: The PID of the process to be moved. In this scenario, cgclassify uses the defined rules in the configuration file /etc/cgrules.conf to determine the appropriate cgroup for the process.

Example Output:

The command usually provides no output unless there’s a problem. If there’s an issue with the configuration file or the command’s execution, you might see an error message. In normal circumstances, the process gets relocated silently as defined in /etc/cgrules.conf.

Use Case 3: Move a Process to a Specific Cgroup with the Sticky Option

Code:

cgclassify --sticky -g cpu:/student 1234

Motivation:

There are situations where you not only need to move a process into a cgroup but also ensure that any automatic cgroup management daemons, like cgred, do not alter the assignment contrary to wishes. This could be instrumental when a specific process and its children must maintain access to predetermined resources regardless of system-wide rules applied by cgred.

Explanation:

• cgclassify: The command being executed to classify and move processes into cgroups.
• --sticky: This flag instructs the system to maintain the process in the specified cgroup, effectively preventing the cgred daemon from moving it based on /etc/cgrules.conf.
• -g cpu:/student: Sets the target cgroup to the “student” group within the CPU hierarchy.
• 1234: PID of the process that needs classification into the specified cgroup.

Example Output:

Like most system commands, cgclassify --sticky will not emit a typical success message but will instead allow systems administrators to see the effects via monitoring tools or by checking the cgroup file system for the assigned processes.

Conclusion:

Using cgclassify, system administrators can maintain organized control over application processes, ensuring that resource allocation rules are adhered to in Linux environments. By leveraging this tool along with cgroups, it’s possible to optimize and stabilize system performance where multiple processes, services, or tenants require distinct resource management profiles.