path: root/Documentation/admin-guide/cgroup-v1/memory.rst

==========================
Memory Resource Controller
==========================

NOTE:
      This document is hopelessly outdated and it asks for a complete
      rewrite. It still contains a useful information so we are keeping it
      here but make sure to check the current code if you need a deeper
      understanding.

NOTE:
      The Memory Resource Controller has generically been referred to as the
      memory controller in this document. Do not confuse memory controller
      used here with the memory controller that is used in hardware.

(For editors) In this document:
      When we mention a cgroup (cgroupfs's directory) with memory controller,
      we call it "memory cgroup". When you see git-log and source code, you'll
      see patch's title and function names tend to use "memcg".
      In this document, we avoid using it.

Benefits and Purpose of the memory controller
=============================================

The memory controller isolates the memory behaviour of a group of tasks
from the rest of the system. The article on LWN [12] mentions some probable
uses of the memory controller. The memory controller can be used to

a. Isolate an application or a group of applications
   Memory-hungry applications can be isolated and limited to a smaller
   amount of memory.
b. Create a cgroup with a limited amount of memory; this can be used
   as a good alternative to booting with mem=XXXX.
c. Virtualization solutions can control the amount of memory they want
   to assign to a virtual machine instance.
d. A CD/DVD burner could control the amount of memory used by the
   rest of the system to ensure that burning does not fail due to lack
   of available memory.
e. There are several other use cases; find one or use the controller just
   for fun (to learn and hack on the VM subsystem).

Current Status: linux-2.6.34-mmotm(development version of 2010/April)

Features:

 - accounting anonymous pages, file caches, swap caches usage and limiting them.
 - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
 - optionally, memory+swap usage can be accounted and limited.
 - hierarchical accounting
 - soft limit
 - moving (recharging) account at moving a task is selectable.
 - usage threshold notifier
 - memory pressure notifier
 - oom-killer disable knob and oom-notifier
 - Root cgroup has no limit controls.

 Kernel memory support is a work in progress, and the current version provides
 basically functionality. (See Section 2.7)

Brief summary of control files.

==================================== ==========================================
 tasks				     attach a task(thread) and show list of
				     threads
 cgroup.procs			     show list of processes
 cgroup.event_control		     an interface for event_fd()
 memory.usage_in_bytes		     show current usage for memory
				     (See 5.5 for details)
 memory.memsw.usage_in_bytes	     show current usage for memory+Swap
				     (See 5.5 for details)
 memory.limit_in_bytes		     set/show limit of memory usage
 memory.memsw.limit_in_bytes	     set/show limit of memory+Swap usage
 memory.failcnt			     show the number of memory usage hits limits
 memory.memsw.failcnt		     show the number of memory+Swap hits limits
 memory.max_usage_in_bytes	     show max memory usage recorded
 memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
 memory.soft_limit_in_bytes	     set/show soft limit of memory usage
 memory.stat			     show various statistics
 memory.use_hierarchy		     set/show hierarchical account enabled
 memory.force_empty		     trigger forced page reclaim
 memory.pressure_level		     set memory pressure notifications
 memory.swappiness		     set/show swappiness parameter of vmscan
				     (See sysctl's vm.swappiness)
 memory.move_charge_at_immigrate     set/show controls of moving charges
 memory.oom_control		     set/show oom controls.
 memory.numa_stat		     show the number of memory usage per numa
				     node
 memory.kmem.limit_in_bytes          set/show hard limit for kernel memory
                                     This knob is deprecated and shouldn't be
                                     used. It is planned that this be removed in
                                     the foreseeable future.
 memory.kmem.usage_in_bytes          show current kernel memory allocation
 memory.kmem.failcnt                 show the number of kernel memory usage
				     hits limits
 memory.kmem.max_usage_in_bytes      show max kernel memory usage recorded

 memory.kmem.tcp.limit_in_bytes      set/show hard limit for tcp buf memory
 memory.kmem.tcp.usage_in_bytes      show current tcp buf memory allocation
 memory.kmem.tcp.failcnt             show the number of tcp buf memory usage
				     hits limits
 memory.kmem.tcp.max_usage_in_bytes  show max tcp buf memory usage recorded
==================================== ==========================================

1. History
==========

The memory controller has a long history. A request for comments for the memory
controller was posted by Balbir Singh [1]. At the time the RFC was posted
there were several implementations for memory control. The goal of the
RFC was to build consensus and agreement for the minimal features required
for memory control. The first RSS controller was posted by Balbir Singh[2]
in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
RSS controller. At OLS, at the resource management BoF, everyone suggested
that we handle both page cache and RSS together. Another request was raised
to allow user space handling of OOM. The current memory controller is
at version 6; it combines both mapped (RSS) and unmapped Page
Cache Control [11].

2. Memory Control
=================

Memory is a unique resource in the sense that it is present in a limited
amount. If a task requires a lot of CPU processing, the task can spread
its processing over a period of hours, days, months or years, but with
memory, the same physical memory needs to be reused to accomplish the task.

The memory controller implementation has been divided into phases. These
are:

1. Memory controller
2. mlock(2) controller
3. Kernel user memory accounting and slab control
4. user mappings length controller

The memory controller is the first controller developed.

2.1. Design
-----------

The core of the design is a counter called the page_counter. The
page_counter tracks the current memory usage and limit of the group of
processes associated with the controller. Each cgroup has a memory controller
specific data structure (mem_cgroup) associated with it.

2.2. Accounting
---------------

::

		+--------------------+
		|  mem_cgroup        |
		|  (page_counter)    |
		+--------------------+
		 /            ^      \
		/             |       \
           +---------------+  |        +---------------+
           | mm_struct     |  |....    | mm_struct     |
           |               |  |        |               |
           +---------------+  |        +---------------+
                              |
                              + --------------+
                                              |
           +---------------+           +------+--------+
           | page          +---------->  page_cgroup|
           |               |           |               |
           +---------------+           +---------------+

             (Figure 1: Hierarchy of Accounting)


Figure 1 shows the important aspects of the controller

1. Accounting happens per cgroup
2. Each mm_struct knows about which cgroup it belongs to
3. Each page has a pointer to the page_cgroup, which in turn knows the
   cgroup it belongs to

The accounting is done as follows: mem_cgroup_charge_common() is invoked to
set up the necessary data structures and check if the cgroup that is being
charged is over its limit. If it is, then reclaim is invoked on the cgroup.
More details can be found in the reclaim section of this document.
If everything goes well, a page meta-data-structure called page_cgroup is
updated. page_cgroup has its own LRU on cgroup.
(*) page_cgroup structure is allocated at boot/memory-hotplug time.

2.2.1 Accounting details
------------------------

All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
Some pages which are never reclaimable and will not be on the LRU
are not accounted. We just account pages under usual VM management.

RSS pages are accounted at page_fault unless they've already been accounted
for earlier. A file page will be accounted for as Page Cache when it's
inserted into inode (radix-tree). While it's mapped into the page tables of
processes, duplicate accounting is carefully avoided.

An RSS page is unaccounted when it's fully unmapped. A PageCache page is
unaccounted when it's removed from radix-tree. Even if RSS pages are fully
unmapped (by kswapd), they may exist as SwapCache in the system until they
are really freed. Such SwapCaches are also accounted.
A swapped-in page is accounted after adding into swapcache.

Note: The kernel does swapin-readahead and reads multiple swaps at once.
Since page's memcg recorded into swap whatever memsw enabled, the page will
be accounted after swapin.

At page migration, accounting information is kept.

Note: we just account pages-on-LRU because our purpose is to control amount
of used pages; not-on-LRU pages tend to be out-of-control from VM view.

2.3 Shared Page Accounting
--------------------------

Shared pages are accounted on the basis of the first touch approach. The
cgroup that first touches a page is accounted for the page. The principle
behind this approach is that a cgroup that aggressively uses a shared
page will eventually get charged for it (once it is uncharged from
the cgroup that brought it in -- this will happen on memory pressure).

But see section 8.2: when moving a task to another cgroup, its pages may
be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.

2.4 Swap Extension
--------------------------------------

Swap usage is always recorded for each of cgroup. Swap Extension allows you to
read and limit it.

When CONFIG_SWAP is enabled, following files are added.

 - memory.memsw.usage_in_bytes.
 - memory.memsw.limit_in_bytes.

memsw means memory+swap. Usage of memory+swap is limited by
memsw.limit_in_bytes.

Example: Assume a system with 4G of swap. A task which allocates 6G of memory
(by mistake) under 2G memory limitation will use all swap.
In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
By using the memsw limit, you can avoid system OOM which can be caused by swap
shortage.

**why 'memory+swap' rather than swap**

The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
to move account from memory to swap...there is no change in usage of
memory+swap. In other words, when we want to limit the usage of swap without
affecting global LRU, memory+swap limit is better than just limiting swap from
an OS point of view.

**What happens when a cgroup hits memory.memsw.limit_in_bytes**

When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
in this cgroup. Then, swap-out will not be done by cgroup routine and file
caches are dropped. But as mentioned above, global LRU can do swapout memory
from it for sanity of the system's memory management state. You can't forbid
it by cgroup.

2.5 Reclaim
-----------

Each cgroup maintains a per cgroup LRU which has the same structure as
global VM. When a cgroup goes over its limit, we first try
to reclaim memory from the cgroup so as to make space for the new
pages that the cgroup has touched. If the reclaim is unsuccessful,
an OOM routine is invoked to select and kill the bulkiest task in the
cgroup. (See 10. OOM Control below.)

The reclaim algorithm has not been modified for cgroups, except that
pages that are selected for reclaiming come from the per-cgroup LRU
list.

NOTE:
  Reclaim does not work for the root cgroup, since we cannot set any
  limits on the root cgroup.

Note2:
  When panic_on_oom is set to "2", the whole system will panic.

When oom event notifier is registered, event will be delivered.
(See oom_control section)

2.6 Locking
-----------

   lock_page_cgroup()/unlock_page_cgroup() should not be called under
   the i_pages lock.

   Other lock order is following:

   PG_locked.
     mm->page_table_lock
         pgdat->lru_lock
	   lock_page_cgroup.

  In many cases, just lock_page_cgroup() is called.

  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
  pgdat->lru_lock, it has no lock of its own.

2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM)
-----------------------------------------------

With the Kernel memory extension, the Memory Controller is able to limit
the amount of kernel memory used by the system. Kernel memory is fundamentally
different than user memory, since it can't be swapped out, which makes it
possible to DoS the system by consuming too much of this precious resource.

Kernel memory accounting is enabled for all memory cgroups by default. But
it can be disabled system-wide by passing cgroup.memory=nokmem to the kernel
at boot time. In this case, kernel memory will not be accounted at all.

Kernel memory limits are not imposed for the root cgroup. Usage for the root
cgroup may or may not be accounted. The memory used is accumulated into
memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
(currently only for tcp).

The main "kmem" counter is fed into the main counter, so kmem charges will
also be visible from the user counter.

Currently no soft limit is implemented for kernel memory. It is future work
to trigger slab reclaim when those limits are reached.

2.7.1 Current Kernel Memory resources accounted
-----------------------------------------------

stack pages:
  every process consumes some stack pages. By accounting into
  kernel memory, we prevent new processes from being created when the kernel
  memory usage is too high.

slab pages:
  pages allocated by the SLAB or SLUB allocator are tracked. A copy
  of each kmem_cache is created every time the cache is touched by the first time
  from inside the memcg. The creation is done lazily, so some objects can still be
  skipped while the cache is being created. All objects in a slab page should
  belong to the same memcg. This only fails to hold when a task is migrated to a
  different memcg during the page allocation by the cache.

sockets memory pressure:
  some sockets protocols have memory pressure
  thresholds. The Memory Controller allows them to be controlled individually
  per cgroup, instead of globally.

tcp memory pressure:
  sockets memory pressure for the tcp protocol.

2.7.2 Common use cases
----------------------

Because the "kmem" counter is fed to the main user counter, kernel memory can
never be limited completely independently of user memory. Say "U" is the user
limit, and "K" the kernel limit. There are three possible ways limits can be
set:

U != 0, K = unlimited:
    This is the standard memcg limitation mechanism already present before kmem
    accounting. Kernel memory is completely ignored.

U != 0, K < U:
    Kernel memory is a subset of the user memory. This setup is useful in
    deployments where the total amount of memory per-cgroup is overcommited.
    Overcommiting kernel memory limits is definitely not recommended, since the
    box can still run out of non-reclaimable memory.
    In this case, the admin could set up K so that the sum of all groups is
    never greater than the total memory, and freely set U at the cost of his
    QoS.

WARNING:
    In the current implementation, memory reclaim will NOT be
    triggered for a cgroup when it hits K while staying below U, which makes
    this setup impractical.

U != 0, K >= U:
    Since kmem charges will also be fed to the user counter and reclaim will be
    triggered for the cgroup for both kinds of memory. This setup gives the
    admin a unified view of memory, and it is also useful for people who just
    want to track kernel memory usage.

3. User Interface
=================

3.0. Configuration
------------------

a. Enable CONFIG_CGROUPS
b. Enable CONFIG_MEMCG
c. Enable CONFIG_MEMCG_SWAP (to use swap extension)
d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)

3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
-------------------------------------------------------------------

::

	# mount -t tmpfs none /sys/fs/cgroup
	# mkdir /sys/fs/cgroup/memory
	# mount -t cgroup none /sys/fs/cgroup/memory -o memory

3.2. Make the new group and move bash into it::

	# mkdir /sys/fs/cgroup/memory/0
	# echo $$ > /sys/fs/cgroup/memory/0/tasks

Since now we're in the 0 cgroup, we can alter the memory limit::

	# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes

NOTE:
  We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
  mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes,
  Gibibytes.)

NOTE:
  We can write "-1" to reset the ``*.limit_in_bytes(unlimited)``.

NOTE:
  We cannot set limits on the root cgroup any more.

::

  # cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
  4194304

We can check the usage::

  # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
  1216512

A successful write to this file does not guarantee a successful setting of
this limit to the value written into the file. This can be due to a
number of factors, such as rounding up to page boundaries or the total
availability of memory on the system. The user is required to re-read
this file after a write to guarantee the value committed by the kernel::

  # echo 1 > memory.limit_in_bytes
  # cat memory.limit_in_bytes
  4096

The memory.failcnt field gives the number of times that the cgroup limit was
exceeded.

The memory.stat file gives accounting information. Now, the number of
caches, RSS and Active pages/Inactive pages are shown.

4. Testing
==========

For testing features and implementation, see memcg_test.txt.

Performance test is also important. To see pure memory controller's overhead,
testing on tmpfs will give you good numbers of small overheads.
Example: do kernel make on tmpfs.

Page-fault scalability is also important. At measuring parallel
page fault test, multi-process test may be better than multi-thread
test because it has noise of shared objects/status.

But the above two are testing extreme situations.
Trying usual test under memory controller is always helpful.

4.1 Troubleshooting
-------------------

Sometimes a user might find that the application under a cgroup is
terminated by the OOM killer. There are several causes for this:

1. The cgroup limit is too low (just too low to do anything useful)
2. The user is using anonymous memory and swap is turned off or too low

A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
some of the pages cached in the cgroup (page cache pages).

To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
seeing what happens will be helpful.

4.2 Task migration
------------------

When a task migrates from one cgroup to another, its charge is not
carried forward by default. The pages allocated from the original cgroup still
remain charged to it, the charge is dropped when the page is freed or
reclaimed.

You can move charges of a task along with task migration.
See 8. "Move charges at task migration"

4.3 Removing a cgroup
---------------------

A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
cgroup might have some charge associated with it, even though all
tasks have migrated away from it. (because we charge against pages, not
against tasks.)

We move the stats to root (if use_hierarchy==0) or parent (if
use_hierarchy==1), and no change on the charge except uncharging
from the child.

Charges recorded in swap information is not updated at removal of cgroup.
Recorded information is discarded and a cgroup which uses swap (swapcache)
will be charged as a new owner of it.

About use_hierarchy, see Section 6.

5. Misc. interfaces
===================

5.1 force_empty
---------------