Chapter 1. Introduction to Control Groups (Cgroups)
1.1. What are Control Groups
The control groups, abbreviated as cgroups in this guide, are a Linux kernel feature that allows you to allocate resources — such as CPU time, system memory, network bandwidth, or combinations of these resources — among hierarchically ordered groups of processes running on a system. By using cgroups, system administrators gain fine-grained control over allocating, prioritizing, denying, managing, and monitoring system resources. Hardware resources can be smartly divided up among applications and users, increasing overall efficiency.
Control Groups provide a way to hierarchically group and label processes, and to apply resource limits to them. Traditionally, all processes received similar amounts of system resources that the administrator could modulate with the process niceness value. With this approach, applications that involved a large number of processes received more resources than applications with few processes, regardless of the relative importance of these applications.
Red Hat Enterprise Linux 7 moves the resource management settings from the process level to the application level by binding the system of cgroup hierarchies with the systemd unit tree. Therefore, you can manage system resources with
systemctlcommands, or by modifying systemd unit files. See Chapter 2, Using Control Groups for details.
In previous versions of Red Hat Enterprise Linux, system administrators built custom cgroup hierarchies with the use of the
cgconfigcommand from the libcgroup package. This package is now deprecated, and it is not recommended to use it since it can easily create conflicts with the default cgroup hierarchy. However, libcgroup is still available to cover for certain specific cases, where systemd is not yet applicable, most notably for using the net-prio subsystem. See Chapter 3, Using libcgroup Tools.
The aforementioned tools provide a high-level interface to interact with cgroup controllers (also known as subsystems) in Linux kernel. The main cgroup controllers for resource management are cpu, memory, and blkio, see Available Controllers in Red Hat Enterprise Linux 7 for the list of controllers enabled by default. For detailed description of resource controllers and their configurable parameters, see Controller-Specific Kernel Documentation.