memcg: memory cgroup hierarchy documentation

  Because rmdir() moves all pages to parent, some out-of-use page caches can be

  moved to the parent. If you want to avoid that, force_empty will be useful.

6. Hierarchy support

6. TODO

The memory controller supports a deep hierarchy and hierarchical accounting.

The hierarchy is created by creating the appropriate cgroups in the

cgroup filesystem. Consider for example, the following cgroup filesystem

hierarchy

		root

	     /  |   \

           /	|    \

	  a	b	c

			| \

			|  \

			d   e

In the diagram above, with hierarchical accounting enabled, all memory

usage of e, is accounted to its ancestors up until the root (i.e, c and root),

that has memory.use_hierarchy enabled.  If one of the ancestors goes over its

limit, the reclaim algorithm reclaims from the tasks in the ancestor and the

children of the ancestor.

6.1 Enabling hierarchical accounting and reclaim

The memory controller by default disables the hierarchy feature. Support

can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup

# echo 1 > memory.use_hierarchy

The feature can be disabled by

# echo 0 > memory.use_hierarchy

NOTE1: Enabling/disabling will fail if the cgroup already has other

cgroups created below it.

NOTE2: This feature can be enabled/disabled per subtree.

7. TODO

1. Add support for accounting huge pages (as a separate controller)

2. Make per-cgroup scanner reclaim not-shared pages first