Source: https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt

CGROUPS

Control Groups provide a mechanism for aggregation/participating sets of tasks, and all their future children, into hierarchical groups with specialized behaviour.

Definitions

cgroup

A cgroup associates a set of tasks with a set of parameters for one or more subsystems.

Term | Example ------------------|--------- Set of Tasks | process ids Set of parameters | 1 or 2 CPUs Subsystem | CPUSET, MEMORY % or nodes.

subsystem

A subsystem is a module that makes use of the task grouping facilities provided by cgroup to treat group of tasks in particular ways.

• c**group** provides the grouping of tasks with a set of parameters.

A subsystem is typically a "resource controller" that schedules a resource or applies per-cgroup limits. But it may be anything that wants to act on a group of processes e.g. a virtualization subsystem.

Hierarchy

A hierarchy is a set of cgroups arranged in a tree, such that every task in the system is in exactly one of the cgroups in the hierarchy, and a set of subsystems; each sub-system has system specific state attached to each cgroup in the hierarchy. Each hierarchy has an instance of the cgroup virtual filesystem associated with it.

Rule 1. A single hierarchy can have one or more subsystems attached to it.

Example: You create a single hierarchy for memory and cpu. It's good if you have one to one mapping. You can have:

1. /cg1: 40% memory, 60% CPU
2. /cg2: 60% memory, 40% CPU

Rule 2: Any single subsystem (such as cpu) subsystem cannot be attached to more than one hierarchy if one of those hierarchies has a different subsystem attached to it already.

Rule 3: A task can belong to only one cgroup in a hierarchy. All tasks on a system by default are member of default cgroup of that hierarchy, which is known as root cgroup.

Rule 4: Any process that forks itself creates a child task. By default child task inherits the cgroup membership of its parent but can be moved to different cgroups as needed.

source: https://access.redhat.com/documentation/en-US/RedHatEnterpriseLinux/6/html/ResourceManagementGuide/sec-RelationshipsBetweenSubsystemsHierarchiesControlGroupsandTasks.html