The Linux kernel contains a large number of features, one of which is Control Groups (cgroups), which are kernel level constructs that allow for resource constraints. This is the mechanism upon which OpenShift is able to limit resources per application gear, and is combined with SELinux in order to offer the functionality of multitenancy at the Operating System level instead of relying on a form of virtualization or IaaS cloud.
For topics on Linux kernel documentation, there are a few places more authoritative than the official documentation that comes with the kernel source code; therefore, we shall refer to the discussion of cgroups definitions from a definite source:
A *cgroup* associates a set of tasks with a set of parameters for one or more subsystems.
A *subsystem* is a module that makes use of the task grouping facilities provided by cgroups to treat groups of tasks in particular ways. A subsystem is typically a "resource controller" that schedules a resource or...