Linux Administration Cookbook

cgroups (Linux control groups)

Here's an excerpt from the Linux Programmer's Manual:

"Control groups, usually referred to as cgroups, are a Linux kernel feature that allows processes to be organized into hierarchical groups whose usage of various types of resources can then be limited and monitored. The kernel's cgroup interface is provided through a pseudo-filesystem called cgroupfs. Grouping is implemented in the core cgroup kernel code, while resource tracking and limits are implemented in a set of per-resource-type subsystems (memory, CPU, and so on)."

In practice, this means that the kernel has the ability to group processes together into a stack and that it can then control and monitor the resource usage of it.