1==========================
2Memory Resource Controller
3==========================
4
5.. caution::
6      This document is hopelessly outdated and it asks for a complete
7      rewrite. It still contains a useful information so we are keeping it
8      here but make sure to check the current code if you need a deeper
9      understanding.
10
11.. note::
12      The Memory Resource Controller has generically been referred to as the
13      memory controller in this document. Do not confuse memory controller
14      used here with the memory controller that is used in hardware.
15
16.. hint::
17      When we mention a cgroup (cgroupfs's directory) with memory controller,
18      we call it "memory cgroup". When you see git-log and source code, you'll
19      see patch's title and function names tend to use "memcg".
20      In this document, we avoid using it.
21
22Benefits and Purpose of the memory controller
23=============================================
24
25The memory controller isolates the memory behaviour of a group of tasks
26from the rest of the system. The article on LWN [12]_ mentions some probable
27uses of the memory controller. The memory controller can be used to
28
29a. Isolate an application or a group of applications
30   Memory-hungry applications can be isolated and limited to a smaller
31   amount of memory.
32b. Create a cgroup with a limited amount of memory; this can be used
33   as a good alternative to booting with mem=XXXX.
34c. Virtualization solutions can control the amount of memory they want
35   to assign to a virtual machine instance.
36d. A CD/DVD burner could control the amount of memory used by the
37   rest of the system to ensure that burning does not fail due to lack
38   of available memory.
39e. There are several other use cases; find one or use the controller just
40   for fun (to learn and hack on the VM subsystem).
41
42Current Status: linux-2.6.34-mmotm(development version of 2010/April)
43
44Features:
45
46 - accounting anonymous pages, file caches, swap caches usage and limiting them.
47 - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
48 - optionally, memory+swap usage can be accounted and limited.
49 - hierarchical accounting
50 - soft limit
51 - moving (recharging) account at moving a task is selectable.
52 - usage threshold notifier
53 - memory pressure notifier
54 - oom-killer disable knob and oom-notifier
55 - Root cgroup has no limit controls.
56
57 Kernel memory support is a work in progress, and the current version provides
58 basically functionality. (See :ref:`section 2.7
59 <cgroup-v1-memory-kernel-extension>`)
60
61Brief summary of control files.
62
63==================================== ==========================================
64 tasks				     attach a task(thread) and show list of
65				     threads
66 cgroup.procs			     show list of processes
67 cgroup.event_control		     an interface for event_fd()
68				     This knob is not available on CONFIG_PREEMPT_RT systems.
69 memory.usage_in_bytes		     show current usage for memory
70				     (See 5.5 for details)
71 memory.memsw.usage_in_bytes	     show current usage for memory+Swap
72				     (See 5.5 for details)
73 memory.limit_in_bytes		     set/show limit of memory usage
74 memory.memsw.limit_in_bytes	     set/show limit of memory+Swap usage
75 memory.failcnt			     show the number of memory usage hits limits
76 memory.memsw.failcnt		     show the number of memory+Swap hits limits
77 memory.max_usage_in_bytes	     show max memory usage recorded
78 memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
79 memory.soft_limit_in_bytes	     set/show soft limit of memory usage
80				     This knob is not available on CONFIG_PREEMPT_RT systems.
81                                     This knob is deprecated and shouldn't be
82                                     used.
83 memory.stat			     show various statistics
84 memory.use_hierarchy		     set/show hierarchical account enabled
85                                     This knob is deprecated and shouldn't be
86                                     used.
87 memory.force_empty		     trigger forced page reclaim
88 memory.pressure_level		     set memory pressure notifications
89                                     This knob is deprecated and shouldn't be
90                                     used.
91 memory.swappiness		     set/show swappiness parameter of vmscan
92				     (See sysctl's vm.swappiness)
93				     Per memcg knob does not exist in cgroup v2.
94 memory.move_charge_at_immigrate     This knob is deprecated.
95 memory.oom_control		     set/show oom controls.
96                                     This knob is deprecated and shouldn't be
97                                     used.
98 memory.numa_stat		     show the number of memory usage per numa
99				     node
100 memory.kmem.limit_in_bytes          Deprecated knob to set and read the kernel
101                                     memory hard limit. Kernel hard limit is not
102                                     supported since 5.16. Writing any value to
103                                     do file will not have any effect same as if
104                                     nokmem kernel parameter was specified.
105                                     Kernel memory is still charged and reported
106                                     by memory.kmem.usage_in_bytes.
107 memory.kmem.usage_in_bytes          show current kernel memory allocation
108 memory.kmem.failcnt                 show the number of kernel memory usage
109				     hits limits
110 memory.kmem.max_usage_in_bytes      show max kernel memory usage recorded
111
112 memory.kmem.tcp.limit_in_bytes      set/show hard limit for tcp buf memory
113                                     This knob is deprecated and shouldn't be
114                                     used.
115 memory.kmem.tcp.usage_in_bytes      show current tcp buf memory allocation
116                                     This knob is deprecated and shouldn't be
117                                     used.
118 memory.kmem.tcp.failcnt             show the number of tcp buf memory usage
119				     hits limits
120                                     This knob is deprecated and shouldn't be
121                                     used.
122 memory.kmem.tcp.max_usage_in_bytes  show max tcp buf memory usage recorded
123                                     This knob is deprecated and shouldn't be
124                                     used.
125==================================== ==========================================
126
1271. History
128==========
129
130The memory controller has a long history. A request for comments for the memory
131controller was posted by Balbir Singh [1]_. At the time the RFC was posted
132there were several implementations for memory control. The goal of the
133RFC was to build consensus and agreement for the minimal features required
134for memory control. The first RSS controller was posted by Balbir Singh [2]_
135in Feb 2007. Pavel Emelianov [3]_ [4]_ [5]_ has since posted three versions
136of the RSS controller. At OLS, at the resource management BoF, everyone
137suggested that we handle both page cache and RSS together. Another request was
138raised to allow user space handling of OOM. The current memory controller is
139at version 6; it combines both mapped (RSS) and unmapped Page
140Cache Control [11]_.
141
1422. Memory Control
143=================
144
145Memory is a unique resource in the sense that it is present in a limited
146amount. If a task requires a lot of CPU processing, the task can spread
147its processing over a period of hours, days, months or years, but with
148memory, the same physical memory needs to be reused to accomplish the task.
149
150The memory controller implementation has been divided into phases. These
151are:
152
1531. Memory controller
1542. mlock(2) controller
1553. Kernel user memory accounting and slab control
1564. user mappings length controller
157
158The memory controller is the first controller developed.
159
1602.1. Design
161-----------
162
163The core of the design is a counter called the page_counter. The
164page_counter tracks the current memory usage and limit of the group of
165processes associated with the controller. Each cgroup has a memory controller
166specific data structure (mem_cgroup) associated with it.
167
1682.2. Accounting
169---------------
170
171.. code-block::
172   :caption: Figure 1: Hierarchy of Accounting
173
174		+--------------------+
175		|  mem_cgroup        |
176		|  (page_counter)    |
177		+--------------------+
178		 /            ^      \
179		/             |       \
180           +---------------+  |        +---------------+
181           | mm_struct     |  |....    | mm_struct     |
182           |               |  |        |               |
183           +---------------+  |        +---------------+
184                              |
185                              + --------------+
186                                              |
187           +---------------+           +------+--------+
188           | page          +---------->  page_cgroup|
189           |               |           |               |
190           +---------------+           +---------------+
191
192
193
194Figure 1 shows the important aspects of the controller
195
1961. Accounting happens per cgroup
1972. Each mm_struct knows about which cgroup it belongs to
1983. Each page has a pointer to the page_cgroup, which in turn knows the
199   cgroup it belongs to
200
201The accounting is done as follows: mem_cgroup_charge_common() is invoked to
202set up the necessary data structures and check if the cgroup that is being
203charged is over its limit. If it is, then reclaim is invoked on the cgroup.
204More details can be found in the reclaim section of this document.
205If everything goes well, a page meta-data-structure called page_cgroup is
206updated. page_cgroup has its own LRU on cgroup.
207(*) page_cgroup structure is allocated at boot/memory-hotplug time.
208
2092.2.1 Accounting details
210------------------------
211
212All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
213Some pages which are never reclaimable and will not be on the LRU
214are not accounted. We just account pages under usual VM management.
215
216RSS pages are accounted at page_fault unless they've already been accounted
217for earlier. A file page will be accounted for as Page Cache when it's
218inserted into inode (xarray). While it's mapped into the page tables of
219processes, duplicate accounting is carefully avoided.
220
221An RSS page is unaccounted when it's fully unmapped. A PageCache page is
222unaccounted when it's removed from xarray. Even if RSS pages are fully
223unmapped (by kswapd), they may exist as SwapCache in the system until they
224are really freed. Such SwapCaches are also accounted.
225A swapped-in page is accounted after adding into swapcache.
226
227Note: The kernel does swapin-readahead and reads multiple swaps at once.
228Since page's memcg recorded into swap whatever memsw enabled, the page will
229be accounted after swapin.
230
231At page migration, accounting information is kept.
232
233Note: we just account pages-on-LRU because our purpose is to control amount
234of used pages; not-on-LRU pages tend to be out-of-control from VM view.
235
2362.3 Shared Page Accounting
237--------------------------
238
239Shared pages are accounted on the basis of the first touch approach. The
240cgroup that first touches a page is accounted for the page. The principle
241behind this approach is that a cgroup that aggressively uses a shared
242page will eventually get charged for it (once it is uncharged from
243the cgroup that brought it in -- this will happen on memory pressure).
244
2452.4 Swap Extension
246--------------------------------------
247
248Swap usage is always recorded for each of cgroup. Swap Extension allows you to
249read and limit it.
250
251When CONFIG_SWAP is enabled, following files are added.
252
253 - memory.memsw.usage_in_bytes.
254 - memory.memsw.limit_in_bytes.
255
256memsw means memory+swap. Usage of memory+swap is limited by
257memsw.limit_in_bytes.
258
259Example: Assume a system with 4G of swap. A task which allocates 6G of memory
260(by mistake) under 2G memory limitation will use all swap.
261In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
262By using the memsw limit, you can avoid system OOM which can be caused by swap
263shortage.
264
2652.4.1 why 'memory+swap' rather than swap
266~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
267
268The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
269to move account from memory to swap...there is no change in usage of
270memory+swap. In other words, when we want to limit the usage of swap without
271affecting global LRU, memory+swap limit is better than just limiting swap from
272an OS point of view.
273
2742.4.2. What happens when a cgroup hits memory.memsw.limit_in_bytes
275~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
276
277When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
278in this cgroup. Then, swap-out will not be done by cgroup routine and file
279caches are dropped. But as mentioned above, global LRU can do swapout memory
280from it for sanity of the system's memory management state. You can't forbid
281it by cgroup.
282
2832.5 Reclaim
284-----------
285
286Each cgroup maintains a per cgroup LRU which has the same structure as
287global VM. When a cgroup goes over its limit, we first try
288to reclaim memory from the cgroup so as to make space for the new
289pages that the cgroup has touched. If the reclaim is unsuccessful,
290an OOM routine is invoked to select and kill the bulkiest task in the
291cgroup. (See :ref:`10. OOM Control <cgroup-v1-memory-oom-control>` below.)
292
293The reclaim algorithm has not been modified for cgroups, except that
294pages that are selected for reclaiming come from the per-cgroup LRU
295list.
296
297.. note::
298   Reclaim does not work for the root cgroup, since we cannot set any
299   limits on the root cgroup.
300
301.. note::
302   When panic_on_oom is set to "2", the whole system will panic.
303
304When oom event notifier is registered, event will be delivered.
305(See :ref:`oom_control <cgroup-v1-memory-oom-control>` section)
306
3072.6 Locking
308-----------
309
310Lock order is as follows::
311
312  folio_lock
313    mm->page_table_lock or split pte_lock
314      folio_memcg_lock (memcg->move_lock)
315        mapping->i_pages lock
316          lruvec->lru_lock.
317
318Per-node-per-memcgroup LRU (cgroup's private LRU) is guarded by
319lruvec->lru_lock; the folio LRU flag is cleared before
320isolating a page from its LRU under lruvec->lru_lock.
321
322.. _cgroup-v1-memory-kernel-extension:
323
3242.7 Kernel Memory Extension
325-----------------------------------------------
326
327With the Kernel memory extension, the Memory Controller is able to limit
328the amount of kernel memory used by the system. Kernel memory is fundamentally
329different than user memory, since it can't be swapped out, which makes it
330possible to DoS the system by consuming too much of this precious resource.
331
332Kernel memory accounting is enabled for all memory cgroups by default. But
333it can be disabled system-wide by passing cgroup.memory=nokmem to the kernel
334at boot time. In this case, kernel memory will not be accounted at all.
335
336Kernel memory limits are not imposed for the root cgroup. Usage for the root
337cgroup may or may not be accounted. The memory used is accumulated into
338memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
339(currently only for tcp).
340
341The main "kmem" counter is fed into the main counter, so kmem charges will
342also be visible from the user counter.
343
344Currently no soft limit is implemented for kernel memory. It is future work
345to trigger slab reclaim when those limits are reached.
346
3472.7.1 Current Kernel Memory resources accounted
348-----------------------------------------------
349
350stack pages:
351  every process consumes some stack pages. By accounting into
352  kernel memory, we prevent new processes from being created when the kernel
353  memory usage is too high.
354
355slab pages:
356  pages allocated by the SLAB or SLUB allocator are tracked. A copy
357  of each kmem_cache is created every time the cache is touched by the first time
358  from inside the memcg. The creation is done lazily, so some objects can still be
359  skipped while the cache is being created. All objects in a slab page should
360  belong to the same memcg. This only fails to hold when a task is migrated to a
361  different memcg during the page allocation by the cache.
362
363sockets memory pressure:
364  some sockets protocols have memory pressure
365  thresholds. The Memory Controller allows them to be controlled individually
366  per cgroup, instead of globally.
367
368tcp memory pressure:
369  sockets memory pressure for the tcp protocol.
370
3712.7.2 Common use cases
372----------------------
373
374Because the "kmem" counter is fed to the main user counter, kernel memory can
375never be limited completely independently of user memory. Say "U" is the user
376limit, and "K" the kernel limit. There are three possible ways limits can be
377set:
378
379U != 0, K = unlimited:
380    This is the standard memcg limitation mechanism already present before kmem
381    accounting. Kernel memory is completely ignored.
382
383U != 0, K < U:
384    Kernel memory is a subset of the user memory. This setup is useful in
385    deployments where the total amount of memory per-cgroup is overcommitted.
386    Overcommitting kernel memory limits is definitely not recommended, since the
387    box can still run out of non-reclaimable memory.
388    In this case, the admin could set up K so that the sum of all groups is
389    never greater than the total memory, and freely set U at the cost of his
390    QoS.
391
392    .. warning::
393       In the current implementation, memory reclaim will NOT be triggered for
394       a cgroup when it hits K while staying below U, which makes this setup
395       impractical.
396
397U != 0, K >= U:
398    Since kmem charges will also be fed to the user counter and reclaim will be
399    triggered for the cgroup for both kinds of memory. This setup gives the
400    admin a unified view of memory, and it is also useful for people who just
401    want to track kernel memory usage.
402
4033. User Interface
404=================
405
406To use the user interface:
407
4081. Enable CONFIG_CGROUPS and CONFIG_MEMCG options
4092. Prepare the cgroups (see :ref:`Why are cgroups needed?
410   <cgroups-why-needed>` for the background information)::
411
412	# mount -t tmpfs none /sys/fs/cgroup
413	# mkdir /sys/fs/cgroup/memory
414	# mount -t cgroup none /sys/fs/cgroup/memory -o memory
415
4163. Make the new group and move bash into it::
417
418	# mkdir /sys/fs/cgroup/memory/0
419	# echo $$ > /sys/fs/cgroup/memory/0/tasks
420
4214. Since now we're in the 0 cgroup, we can alter the memory limit::
422
423	# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
424
425   The limit can now be queried::
426
427	# cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
428	4194304
429
430.. note::
431   We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
432   mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes,
433   Gibibytes.)
434
435.. note::
436   We can write "-1" to reset the ``*.limit_in_bytes(unlimited)``.
437
438.. note::
439   We cannot set limits on the root cgroup any more.
440
441
442We can check the usage::
443
444  # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
445  1216512
446
447A successful write to this file does not guarantee a successful setting of
448this limit to the value written into the file. This can be due to a
449number of factors, such as rounding up to page boundaries or the total
450availability of memory on the system. The user is required to re-read
451this file after a write to guarantee the value committed by the kernel::
452
453  # echo 1 > memory.limit_in_bytes
454  # cat memory.limit_in_bytes
455  4096
456
457The memory.failcnt field gives the number of times that the cgroup limit was
458exceeded.
459
460The memory.stat file gives accounting information. Now, the number of
461caches, RSS and Active pages/Inactive pages are shown.
462
4634. Testing
464==========
465
466For testing features and implementation, see memcg_test.txt.
467
468Performance test is also important. To see pure memory controller's overhead,
469testing on tmpfs will give you good numbers of small overheads.
470Example: do kernel make on tmpfs.
471
472Page-fault scalability is also important. At measuring parallel
473page fault test, multi-process test may be better than multi-thread
474test because it has noise of shared objects/status.
475
476But the above two are testing extreme situations.
477Trying usual test under memory controller is always helpful.
478
479.. _cgroup-v1-memory-test-troubleshoot:
480
4814.1 Troubleshooting
482-------------------
483
484Sometimes a user might find that the application under a cgroup is
485terminated by the OOM killer. There are several causes for this:
486
4871. The cgroup limit is too low (just too low to do anything useful)
4882. The user is using anonymous memory and swap is turned off or too low
489
490A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
491some of the pages cached in the cgroup (page cache pages).
492
493To know what happens, disabling OOM_Kill as per :ref:`"10. OOM Control"
494<cgroup-v1-memory-oom-control>` (below) and seeing what happens will be
495helpful.
496
497.. _cgroup-v1-memory-test-task-migration:
498
4994.2 Task migration
500------------------
501
502When a task migrates from one cgroup to another, its charge is not
503carried forward by default. The pages allocated from the original cgroup still
504remain charged to it, the charge is dropped when the page is freed or
505reclaimed.
506
507You can move charges of a task along with task migration.
508See :ref:`8. "Move charges at task migration" <cgroup-v1-memory-move-charges>`
509
5104.3 Removing a cgroup
511---------------------
512
513A cgroup can be removed by rmdir, but as discussed in :ref:`sections 4.1
514<cgroup-v1-memory-test-troubleshoot>` and :ref:`4.2
515<cgroup-v1-memory-test-task-migration>`, a cgroup might have some charge
516associated with it, even though all tasks have migrated away from it. (because
517we charge against pages, not against tasks.)
518
519We move the stats to parent, and no change on the charge except uncharging
520from the child.
521
522Charges recorded in swap information is not updated at removal of cgroup.
523Recorded information is discarded and a cgroup which uses swap (swapcache)
524will be charged as a new owner of it.
525
5265. Misc. interfaces
527===================
528
5295.1 force_empty
530---------------
531  memory.force_empty interface is provided to make cgroup's memory usage empty.
532  When writing anything to this::
533
534    # echo 0 > memory.force_empty
535
536  the cgroup will be reclaimed and as many pages reclaimed as possible.
537
538  The typical use case for this interface is before calling rmdir().
539  Though rmdir() offlines memcg, but the memcg may still stay there due to
540  charged file caches. Some out-of-use page caches may keep charged until
541  memory pressure happens. If you want to avoid that, force_empty will be useful.
542
5435.2 stat file
544-------------
545
546memory.stat file includes following statistics:
547
548  * per-memory cgroup local status
549
550    =============== ===============================================================
551    cache           # of bytes of page cache memory.
552    rss             # of bytes of anonymous and swap cache memory (includes
553                    transparent hugepages).
554    rss_huge        # of bytes of anonymous transparent hugepages.
555    mapped_file     # of bytes of mapped file (includes tmpfs/shmem)
556    pgpgin          # of charging events to the memory cgroup. The charging
557                    event happens each time a page is accounted as either mapped
558                    anon page(RSS) or cache page(Page Cache) to the cgroup.
559    pgpgout         # of uncharging events to the memory cgroup. The uncharging
560                    event happens each time a page is unaccounted from the
561                    cgroup.
562    swap            # of bytes of swap usage
563    swapcached      # of bytes of swap cached in memory
564    dirty           # of bytes that are waiting to get written back to the disk.
565    writeback       # of bytes of file/anon cache that are queued for syncing to
566                    disk.
567    inactive_anon   # of bytes of anonymous and swap cache memory on inactive
568                    LRU list.
569    active_anon     # of bytes of anonymous and swap cache memory on active
570                    LRU list.
571    inactive_file   # of bytes of file-backed memory and MADV_FREE anonymous
572                    memory (LazyFree pages) on inactive LRU list.
573    active_file     # of bytes of file-backed memory on active LRU list.
574    unevictable     # of bytes of memory that cannot be reclaimed (mlocked etc).
575    =============== ===============================================================
576
577  * status considering hierarchy (see memory.use_hierarchy settings):
578
579    ========================= ===================================================
580    hierarchical_memory_limit # of bytes of memory limit with regard to
581                              hierarchy
582                              under which the memory cgroup is
583    hierarchical_memsw_limit  # of bytes of memory+swap limit with regard to
584                              hierarchy under which memory cgroup is.
585
586    total_<counter>           # hierarchical version of <counter>, which in
587                              addition to the cgroup's own value includes the
588                              sum of all hierarchical children's values of
589                              <counter>, i.e. total_cache
590    ========================= ===================================================
591
592  * additional vm parameters (depends on CONFIG_DEBUG_VM):
593
594    ========================= ========================================
595    recent_rotated_anon       VM internal parameter. (see mm/vmscan.c)
596    recent_rotated_file       VM internal parameter. (see mm/vmscan.c)
597    recent_scanned_anon       VM internal parameter. (see mm/vmscan.c)
598    recent_scanned_file       VM internal parameter. (see mm/vmscan.c)
599    ========================= ========================================
600
601.. hint::
602	recent_rotated means recent frequency of LRU rotation.
603	recent_scanned means recent # of scans to LRU.
604	showing for better debug please see the code for meanings.
605
606.. note::
607	Only anonymous and swap cache memory is listed as part of 'rss' stat.
608	This should not be confused with the true 'resident set size' or the
609	amount of physical memory used by the cgroup.
610
611	'rss + mapped_file" will give you resident set size of cgroup.
612
613	Note that some kernel configurations might account complete larger
614	allocations (e.g., THP) towards 'rss' and 'mapped_file', even if
615	only some, but not all that memory is mapped.
616
617	(Note: file and shmem may be shared among other cgroups. In that case,
618	mapped_file is accounted only when the memory cgroup is owner of page
619	cache.)
620
6215.3 swappiness
622--------------
623
624Overrides /proc/sys/vm/swappiness for the particular group. The tunable
625in the root cgroup corresponds to the global swappiness setting.
626
627Please note that unlike during the global reclaim, limit reclaim
628enforces that 0 swappiness really prevents from any swapping even if
629there is a swap storage available. This might lead to memcg OOM killer
630if there are no file pages to reclaim.
631
6325.4 failcnt
633-----------
634
635A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
636This failcnt(== failure count) shows the number of times that a usage counter
637hit its limit. When a memory cgroup hits a limit, failcnt increases and
638memory under it will be reclaimed.
639
640You can reset failcnt by writing 0 to failcnt file::
641
642	# echo 0 > .../memory.failcnt
643
6445.5 usage_in_bytes
645------------------
646
647For efficiency, as other kernel components, memory cgroup uses some optimization
648to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
649method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
650value for efficient access. (Of course, when necessary, it's synchronized.)
651If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
652value in memory.stat(see 5.2).
653
6545.6 numa_stat
655-------------
656
657This is similar to numa_maps but operates on a per-memcg basis.  This is
658useful for providing visibility into the numa locality information within
659an memcg since the pages are allowed to be allocated from any physical
660node.  One of the use cases is evaluating application performance by
661combining this information with the application's CPU allocation.
662
663Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
664per-node page counts including "hierarchical_<counter>" which sums up all
665hierarchical children's values in addition to the memcg's own value.
666
667The output format of memory.numa_stat is::
668
669  total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
670  file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
671  anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
672  unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
673  hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
674
675The "total" count is sum of file + anon + unevictable.
676
6776. Hierarchy support
678====================
679
680The memory controller supports a deep hierarchy and hierarchical accounting.
681The hierarchy is created by creating the appropriate cgroups in the
682cgroup filesystem. Consider for example, the following cgroup filesystem
683hierarchy::
684
685	       root
686	     /  |   \
687            /	|    \
688	   a	b     c
689		      | \
690		      |  \
691		      d   e
692
693In the diagram above, with hierarchical accounting enabled, all memory
694usage of e, is accounted to its ancestors up until the root (i.e, c and root).
695If one of the ancestors goes over its limit, the reclaim algorithm reclaims
696from the tasks in the ancestor and the children of the ancestor.
697
6986.1 Hierarchical accounting and reclaim
699---------------------------------------
700
701Hierarchical accounting is enabled by default. Disabling the hierarchical
702accounting is deprecated. An attempt to do it will result in a failure
703and a warning printed to dmesg.
704
705For compatibility reasons writing 1 to memory.use_hierarchy will always pass::
706
707	# echo 1 > memory.use_hierarchy
708
7097. Soft limits (DEPRECATED)
710===========================
711
712THIS IS DEPRECATED!
713
714Soft limits allow for greater sharing of memory. The idea behind soft limits
715is to allow control groups to use as much of the memory as needed, provided
716
717a. There is no memory contention
718b. They do not exceed their hard limit
719
720When the system detects memory contention or low memory, control groups
721are pushed back to their soft limits. If the soft limit of each control
722group is very high, they are pushed back as much as possible to make
723sure that one control group does not starve the others of memory.
724
725Please note that soft limits is a best-effort feature; it comes with
726no guarantees, but it does its best to make sure that when memory is
727heavily contended for, memory is allocated based on the soft limit
728hints/setup. Currently soft limit based reclaim is set up such that
729it gets invoked from balance_pgdat (kswapd).
730
7317.1 Interface
732-------------
733
734Soft limits can be setup by using the following commands (in this example we
735assume a soft limit of 256 MiB)::
736
737	# echo 256M > memory.soft_limit_in_bytes
738
739If we want to change this to 1G, we can at any time use::
740
741	# echo 1G > memory.soft_limit_in_bytes
742
743.. note::
744       Soft limits take effect over a long period of time, since they involve
745       reclaiming memory for balancing between memory cgroups
746
747.. note::
748       It is recommended to set the soft limit always below the hard limit,
749       otherwise the hard limit will take precedence.
750
751.. _cgroup-v1-memory-move-charges:
752
7538. Move charges at task migration (DEPRECATED!)
754===============================================
755
756THIS IS DEPRECATED!
757
758Reading memory.move_charge_at_immigrate will always return 0 and writing
759to it will always return -EINVAL.
760
7619. Memory thresholds
762====================
763
764Memory cgroup implements memory thresholds using the cgroups notification
765API (see cgroups.txt). It allows to register multiple memory and memsw
766thresholds and gets notifications when it crosses.
767
768To register a threshold, an application must:
769
770- create an eventfd using eventfd(2);
771- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
772- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
773  cgroup.event_control.
774
775Application will be notified through eventfd when memory usage crosses
776threshold in any direction.
777
778It's applicable for root and non-root cgroup.
779
780.. _cgroup-v1-memory-oom-control:
781
78210. OOM Control (DEPRECATED)
783============================
784
785THIS IS DEPRECATED!
786
787memory.oom_control file is for OOM notification and other controls.
788
789Memory cgroup implements OOM notifier using the cgroup notification
790API (See cgroups.txt). It allows to register multiple OOM notification
791delivery and gets notification when OOM happens.
792
793To register a notifier, an application must:
794
795 - create an eventfd using eventfd(2)
796 - open memory.oom_control file
797 - write string like "<event_fd> <fd of memory.oom_control>" to
798   cgroup.event_control
799
800The application will be notified through eventfd when OOM happens.
801OOM notification doesn't work for the root cgroup.
802
803You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
804
805	#echo 1 > memory.oom_control
806
807If OOM-killer is disabled, tasks under cgroup will hang/sleep
808in memory cgroup's OOM-waitqueue when they request accountable memory.
809
810For running them, you have to relax the memory cgroup's OOM status by
811
812	* enlarge limit or reduce usage.
813
814To reduce usage,
815
816	* kill some tasks.
817	* move some tasks to other group with account migration.
818	* remove some files (on tmpfs?)
819
820Then, stopped tasks will work again.
821
822At reading, current status of OOM is shown.
823
824	- oom_kill_disable 0 or 1
825	  (if 1, oom-killer is disabled)
826	- under_oom	   0 or 1
827	  (if 1, the memory cgroup is under OOM, tasks may be stopped.)
828        - oom_kill         integer counter
829          The number of processes belonging to this cgroup killed by any
830          kind of OOM killer.
831
83211. Memory Pressure (DEPRECATED)
833================================
834
835THIS IS DEPRECATED!
836
837The pressure level notifications can be used to monitor the memory
838allocation cost; based on the pressure, applications can implement
839different strategies of managing their memory resources. The pressure
840levels are defined as following:
841
842The "low" level means that the system is reclaiming memory for new
843allocations. Monitoring this reclaiming activity might be useful for
844maintaining cache level. Upon notification, the program (typically
845"Activity Manager") might analyze vmstat and act in advance (i.e.
846prematurely shutdown unimportant services).
847
848The "medium" level means that the system is experiencing medium memory
849pressure, the system might be making swap, paging out active file caches,
850etc. Upon this event applications may decide to further analyze
851vmstat/zoneinfo/memcg or internal memory usage statistics and free any
852resources that can be easily reconstructed or re-read from a disk.
853
854The "critical" level means that the system is actively thrashing, it is
855about to out of memory (OOM) or even the in-kernel OOM killer is on its
856way to trigger. Applications should do whatever they can to help the
857system. It might be too late to consult with vmstat or any other
858statistics, so it's advisable to take an immediate action.
859
860By default, events are propagated upward until the event is handled, i.e. the
861events are not pass-through. For example, you have three cgroups: A->B->C. Now
862you set up an event listener on cgroups A, B and C, and suppose group C
863experiences some pressure. In this situation, only group C will receive the
864notification, i.e. groups A and B will not receive it. This is done to avoid
865excessive "broadcasting" of messages, which disturbs the system and which is
866especially bad if we are low on memory or thrashing. Group B, will receive
867notification only if there are no event listeners for group C.
868
869There are three optional modes that specify different propagation behavior:
870
871 - "default": this is the default behavior specified above. This mode is the
872   same as omitting the optional mode parameter, preserved by backwards
873   compatibility.
874
875 - "hierarchy": events always propagate up to the root, similar to the default
876   behavior, except that propagation continues regardless of whether there are
877   event listeners at each level, with the "hierarchy" mode. In the above
878   example, groups A, B, and C will receive notification of memory pressure.
879
880 - "local": events are pass-through, i.e. they only receive notifications when
881   memory pressure is experienced in the memcg for which the notification is
882   registered. In the above example, group C will receive notification if
883   registered for "local" notification and the group experiences memory
884   pressure. However, group B will never receive notification, regardless if
885   there is an event listener for group C or not, if group B is registered for
886   local notification.
887
888The level and event notification mode ("hierarchy" or "local", if necessary) are
889specified by a comma-delimited string, i.e. "low,hierarchy" specifies
890hierarchical, pass-through, notification for all ancestor memcgs. Notification
891that is the default, non pass-through behavior, does not specify a mode.
892"medium,local" specifies pass-through notification for the medium level.
893
894The file memory.pressure_level is only used to setup an eventfd. To
895register a notification, an application must:
896
897- create an eventfd using eventfd(2);
898- open memory.pressure_level;
899- write string as "<event_fd> <fd of memory.pressure_level> <level[,mode]>"
900  to cgroup.event_control.
901
902Application will be notified through eventfd when memory pressure is at
903the specific level (or higher). Read/write operations to
904memory.pressure_level are no implemented.
905
906Test:
907
908   Here is a small script example that makes a new cgroup, sets up a
909   memory limit, sets up a notification in the cgroup and then makes child
910   cgroup experience a critical pressure::
911
912	# cd /sys/fs/cgroup/memory/
913	# mkdir foo
914	# cd foo
915	# cgroup_event_listener memory.pressure_level low,hierarchy &
916	# echo 8000000 > memory.limit_in_bytes
917	# echo 8000000 > memory.memsw.limit_in_bytes
918	# echo $$ > tasks
919	# dd if=/dev/zero | read x
920
921   (Expect a bunch of notifications, and eventually, the oom-killer will
922   trigger.)
923
92412. TODO
925========
926
9271. Make per-cgroup scanner reclaim not-shared pages first
9282. Teach controller to account for shared-pages
9293. Start reclamation in the background when the limit is
930   not yet hit but the usage is getting closer
931
932Summary
933=======
934
935Overall, the memory controller has been a stable controller and has been
936commented and discussed quite extensively in the community.
937
938References
939==========
940
941.. [1] Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
942.. [2] Singh, Balbir. Memory Controller (RSS Control),
943   http://lwn.net/Articles/222762/
944.. [3] Emelianov, Pavel. Resource controllers based on process cgroups
945   https://lore.kernel.org/r/45ED7DEC.7010403@sw.ru
946.. [4] Emelianov, Pavel. RSS controller based on process cgroups (v2)
947   https://lore.kernel.org/r/461A3010.90403@sw.ru
948.. [5] Emelianov, Pavel. RSS controller based on process cgroups (v3)
949   https://lore.kernel.org/r/465D9739.8070209@openvz.org
950
9516. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
9527. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
953   subsystem (v3), http://lwn.net/Articles/235534/
9548. Singh, Balbir. RSS controller v2 test results (lmbench),
955   https://lore.kernel.org/r/464C95D4.7070806@linux.vnet.ibm.com
9569. Singh, Balbir. RSS controller v2 AIM9 results
957   https://lore.kernel.org/r/464D267A.50107@linux.vnet.ibm.com
95810. Singh, Balbir. Memory controller v6 test results,
959    https://lore.kernel.org/r/20070819094658.654.84837.sendpatchset@balbir-laptop
960
961.. [11] Singh, Balbir. Memory controller introduction (v6),
962   https://lore.kernel.org/r/20070817084228.26003.12568.sendpatchset@balbir-laptop
963.. [12] Corbet, Jonathan, Controlling memory use in cgroups,
964   http://lwn.net/Articles/243795/
965