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Based on kernel version 4.0. Page generated on 2015-04-14 21:20 EST.

1	Memory Resource Controller
2	
3	NOTE: This document is hopelessly outdated and it asks for a complete
4	      rewrite. It still contains a useful information so we are keeping it
5	      here but make sure to check the current code if you need a deeper
6	      understanding.
7	
8	NOTE: The Memory Resource Controller has generically been referred to as the
9	      memory controller in this document. Do not confuse memory controller
10	      used here with the memory controller that is used in hardware.
11	
12	(For editors)
13	In this document:
14	      When we mention a cgroup (cgroupfs's directory) with memory controller,
15	      we call it "memory cgroup". When you see git-log and source code, you'll
16	      see patch's title and function names tend to use "memcg".
17	      In this document, we avoid using it.
18	
19	Benefits and Purpose of the memory controller
20	
21	The memory controller isolates the memory behaviour of a group of tasks
22	from the rest of the system. The article on LWN [12] mentions some probable
23	uses of the memory controller. The memory controller can be used to
24	
25	a. Isolate an application or a group of applications
26	   Memory-hungry applications can be isolated and limited to a smaller
27	   amount of memory.
28	b. Create a cgroup with a limited amount of memory; this can be used
29	   as a good alternative to booting with mem=XXXX.
30	c. Virtualization solutions can control the amount of memory they want
31	   to assign to a virtual machine instance.
32	d. A CD/DVD burner could control the amount of memory used by the
33	   rest of the system to ensure that burning does not fail due to lack
34	   of available memory.
35	e. There are several other use cases; find one or use the controller just
36	   for fun (to learn and hack on the VM subsystem).
37	
38	Current Status: linux-2.6.34-mmotm(development version of 2010/April)
39	
40	Features:
41	 - accounting anonymous pages, file caches, swap caches usage and limiting them.
42	 - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
43	 - optionally, memory+swap usage can be accounted and limited.
44	 - hierarchical accounting
45	 - soft limit
46	 - moving (recharging) account at moving a task is selectable.
47	 - usage threshold notifier
48	 - memory pressure notifier
49	 - oom-killer disable knob and oom-notifier
50	 - Root cgroup has no limit controls.
51	
52	 Kernel memory support is a work in progress, and the current version provides
53	 basically functionality. (See Section 2.7)
54	
55	Brief summary of control files.
56	
57	 tasks				 # attach a task(thread) and show list of threads
58	 cgroup.procs			 # show list of processes
59	 cgroup.event_control		 # an interface for event_fd()
60	 memory.usage_in_bytes		 # show current usage for memory
61					 (See 5.5 for details)
62	 memory.memsw.usage_in_bytes	 # show current usage for memory+Swap
63					 (See 5.5 for details)
64	 memory.limit_in_bytes		 # set/show limit of memory usage
65	 memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage
66	 memory.failcnt			 # show the number of memory usage hits limits
67	 memory.memsw.failcnt		 # show the number of memory+Swap hits limits
68	 memory.max_usage_in_bytes	 # show max memory usage recorded
69	 memory.memsw.max_usage_in_bytes # show max memory+Swap usage recorded
70	 memory.soft_limit_in_bytes	 # set/show soft limit of memory usage
71	 memory.stat			 # show various statistics
72	 memory.use_hierarchy		 # set/show hierarchical account enabled
73	 memory.force_empty		 # trigger forced move charge to parent
74	 memory.pressure_level		 # set memory pressure notifications
75	 memory.swappiness		 # set/show swappiness parameter of vmscan
76					 (See sysctl's vm.swappiness)
77	 memory.move_charge_at_immigrate # set/show controls of moving charges
78	 memory.oom_control		 # set/show oom controls.
79	 memory.numa_stat		 # show the number of memory usage per numa node
80	
81	 memory.kmem.limit_in_bytes      # set/show hard limit for kernel memory
82	 memory.kmem.usage_in_bytes      # show current kernel memory allocation
83	 memory.kmem.failcnt             # show the number of kernel memory usage hits limits
84	 memory.kmem.max_usage_in_bytes  # show max kernel memory usage recorded
85	
86	 memory.kmem.tcp.limit_in_bytes  # set/show hard limit for tcp buf memory
87	 memory.kmem.tcp.usage_in_bytes  # show current tcp buf memory allocation
88	 memory.kmem.tcp.failcnt            # show the number of tcp buf memory usage hits limits
89	 memory.kmem.tcp.max_usage_in_bytes # show max tcp buf memory usage recorded
90	
91	1. History
92	
93	The memory controller has a long history. A request for comments for the memory
94	controller was posted by Balbir Singh [1]. At the time the RFC was posted
95	there were several implementations for memory control. The goal of the
96	RFC was to build consensus and agreement for the minimal features required
97	for memory control. The first RSS controller was posted by Balbir Singh[2]
98	in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
99	RSS controller. At OLS, at the resource management BoF, everyone suggested
100	that we handle both page cache and RSS together. Another request was raised
101	to allow user space handling of OOM. The current memory controller is
102	at version 6; it combines both mapped (RSS) and unmapped Page
103	Cache Control [11].
104	
105	2. Memory Control
106	
107	Memory is a unique resource in the sense that it is present in a limited
108	amount. If a task requires a lot of CPU processing, the task can spread
109	its processing over a period of hours, days, months or years, but with
110	memory, the same physical memory needs to be reused to accomplish the task.
111	
112	The memory controller implementation has been divided into phases. These
113	are:
114	
115	1. Memory controller
116	2. mlock(2) controller
117	3. Kernel user memory accounting and slab control
118	4. user mappings length controller
119	
120	The memory controller is the first controller developed.
121	
122	2.1. Design
123	
124	The core of the design is a counter called the page_counter. The
125	page_counter tracks the current memory usage and limit of the group of
126	processes associated with the controller. Each cgroup has a memory controller
127	specific data structure (mem_cgroup) associated with it.
128	
129	2.2. Accounting
130	
131			+--------------------+
132			|  mem_cgroup        |
133			|  (page_counter)    |
134			+--------------------+
135			 /            ^      \
136			/             |       \
137	           +---------------+  |        +---------------+
138	           | mm_struct     |  |....    | mm_struct     |
139	           |               |  |        |               |
140	           +---------------+  |        +---------------+
141	                              |
142	                              + --------------+
143	                                              |
144	           +---------------+           +------+--------+
145	           | page          +---------->  page_cgroup|
146	           |               |           |               |
147	           +---------------+           +---------------+
148	
149	             (Figure 1: Hierarchy of Accounting)
150	
151	
152	Figure 1 shows the important aspects of the controller
153	
154	1. Accounting happens per cgroup
155	2. Each mm_struct knows about which cgroup it belongs to
156	3. Each page has a pointer to the page_cgroup, which in turn knows the
157	   cgroup it belongs to
158	
159	The accounting is done as follows: mem_cgroup_charge_common() is invoked to
160	set up the necessary data structures and check if the cgroup that is being
161	charged is over its limit. If it is, then reclaim is invoked on the cgroup.
162	More details can be found in the reclaim section of this document.
163	If everything goes well, a page meta-data-structure called page_cgroup is
164	updated. page_cgroup has its own LRU on cgroup.
165	(*) page_cgroup structure is allocated at boot/memory-hotplug time.
166	
167	2.2.1 Accounting details
168	
169	All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
170	Some pages which are never reclaimable and will not be on the LRU
171	are not accounted. We just account pages under usual VM management.
172	
173	RSS pages are accounted at page_fault unless they've already been accounted
174	for earlier. A file page will be accounted for as Page Cache when it's
175	inserted into inode (radix-tree). While it's mapped into the page tables of
176	processes, duplicate accounting is carefully avoided.
177	
178	An RSS page is unaccounted when it's fully unmapped. A PageCache page is
179	unaccounted when it's removed from radix-tree. Even if RSS pages are fully
180	unmapped (by kswapd), they may exist as SwapCache in the system until they
181	are really freed. Such SwapCaches are also accounted.
182	A swapped-in page is not accounted until it's mapped.
183	
184	Note: The kernel does swapin-readahead and reads multiple swaps at once.
185	This means swapped-in pages may contain pages for other tasks than a task
186	causing page fault. So, we avoid accounting at swap-in I/O.
187	
188	At page migration, accounting information is kept.
189	
190	Note: we just account pages-on-LRU because our purpose is to control amount
191	of used pages; not-on-LRU pages tend to be out-of-control from VM view.
192	
193	2.3 Shared Page Accounting
194	
195	Shared pages are accounted on the basis of the first touch approach. The
196	cgroup that first touches a page is accounted for the page. The principle
197	behind this approach is that a cgroup that aggressively uses a shared
198	page will eventually get charged for it (once it is uncharged from
199	the cgroup that brought it in -- this will happen on memory pressure).
200	
201	But see section 8.2: when moving a task to another cgroup, its pages may
202	be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
203	
204	Exception: If CONFIG_MEMCG_SWAP is not used.
205	When you do swapoff and make swapped-out pages of shmem(tmpfs) to
206	be backed into memory in force, charges for pages are accounted against the
207	caller of swapoff rather than the users of shmem.
208	
209	2.4 Swap Extension (CONFIG_MEMCG_SWAP)
210	
211	Swap Extension allows you to record charge for swap. A swapped-in page is
212	charged back to original page allocator if possible.
213	
214	When swap is accounted, following files are added.
215	 - memory.memsw.usage_in_bytes.
216	 - memory.memsw.limit_in_bytes.
217	
218	memsw means memory+swap. Usage of memory+swap is limited by
219	memsw.limit_in_bytes.
220	
221	Example: Assume a system with 4G of swap. A task which allocates 6G of memory
222	(by mistake) under 2G memory limitation will use all swap.
223	In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
224	By using the memsw limit, you can avoid system OOM which can be caused by swap
225	shortage.
226	
227	* why 'memory+swap' rather than swap.
228	The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
229	to move account from memory to swap...there is no change in usage of
230	memory+swap. In other words, when we want to limit the usage of swap without
231	affecting global LRU, memory+swap limit is better than just limiting swap from
232	an OS point of view.
233	
234	* What happens when a cgroup hits memory.memsw.limit_in_bytes
235	When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
236	in this cgroup. Then, swap-out will not be done by cgroup routine and file
237	caches are dropped. But as mentioned above, global LRU can do swapout memory
238	from it for sanity of the system's memory management state. You can't forbid
239	it by cgroup.
240	
241	2.5 Reclaim
242	
243	Each cgroup maintains a per cgroup LRU which has the same structure as
244	global VM. When a cgroup goes over its limit, we first try
245	to reclaim memory from the cgroup so as to make space for the new
246	pages that the cgroup has touched. If the reclaim is unsuccessful,
247	an OOM routine is invoked to select and kill the bulkiest task in the
248	cgroup. (See 10. OOM Control below.)
249	
250	The reclaim algorithm has not been modified for cgroups, except that
251	pages that are selected for reclaiming come from the per-cgroup LRU
252	list.
253	
254	NOTE: Reclaim does not work for the root cgroup, since we cannot set any
255	limits on the root cgroup.
256	
257	Note2: When panic_on_oom is set to "2", the whole system will panic.
258	
259	When oom event notifier is registered, event will be delivered.
260	(See oom_control section)
261	
262	2.6 Locking
263	
264	   lock_page_cgroup()/unlock_page_cgroup() should not be called under
265	   mapping->tree_lock.
266	
267	   Other lock order is following:
268	   PG_locked.
269	   mm->page_table_lock
270	       zone->lru_lock
271		  lock_page_cgroup.
272	  In many cases, just lock_page_cgroup() is called.
273	  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
274	  zone->lru_lock, it has no lock of its own.
275	
276	2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM)
277	
278	WARNING: Current implementation lacks reclaim support. That means allocation
279		 attempts will fail when close to the limit even if there are plenty of
280		 kmem available for reclaim. That makes this option unusable in real
281		 life so DO NOT SELECT IT unless for development purposes.
282	
283	With the Kernel memory extension, the Memory Controller is able to limit
284	the amount of kernel memory used by the system. Kernel memory is fundamentally
285	different than user memory, since it can't be swapped out, which makes it
286	possible to DoS the system by consuming too much of this precious resource.
287	
288	Kernel memory won't be accounted at all until limit on a group is set. This
289	allows for existing setups to continue working without disruption.  The limit
290	cannot be set if the cgroup have children, or if there are already tasks in the
291	cgroup. Attempting to set the limit under those conditions will return -EBUSY.
292	When use_hierarchy == 1 and a group is accounted, its children will
293	automatically be accounted regardless of their limit value.
294	
295	After a group is first limited, it will be kept being accounted until it
296	is removed. The memory limitation itself, can of course be removed by writing
297	-1 to memory.kmem.limit_in_bytes. In this case, kmem will be accounted, but not
298	limited.
299	
300	Kernel memory limits are not imposed for the root cgroup. Usage for the root
301	cgroup may or may not be accounted. The memory used is accumulated into
302	memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
303	(currently only for tcp).
304	The main "kmem" counter is fed into the main counter, so kmem charges will
305	also be visible from the user counter.
306	
307	Currently no soft limit is implemented for kernel memory. It is future work
308	to trigger slab reclaim when those limits are reached.
309	
310	2.7.1 Current Kernel Memory resources accounted
311	
312	* stack pages: every process consumes some stack pages. By accounting into
313	kernel memory, we prevent new processes from being created when the kernel
314	memory usage is too high.
315	
316	* slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy
317	of each kmem_cache is created every time the cache is touched by the first time
318	from inside the memcg. The creation is done lazily, so some objects can still be
319	skipped while the cache is being created. All objects in a slab page should
320	belong to the same memcg. This only fails to hold when a task is migrated to a
321	different memcg during the page allocation by the cache.
322	
323	* sockets memory pressure: some sockets protocols have memory pressure
324	thresholds. The Memory Controller allows them to be controlled individually
325	per cgroup, instead of globally.
326	
327	* tcp memory pressure: sockets memory pressure for the tcp protocol.
328	
329	2.7.2 Common use cases
330	
331	Because the "kmem" counter is fed to the main user counter, kernel memory can
332	never be limited completely independently of user memory. Say "U" is the user
333	limit, and "K" the kernel limit. There are three possible ways limits can be
334	set:
335	
336	    U != 0, K = unlimited:
337	    This is the standard memcg limitation mechanism already present before kmem
338	    accounting. Kernel memory is completely ignored.
339	
340	    U != 0, K < U:
341	    Kernel memory is a subset of the user memory. This setup is useful in
342	    deployments where the total amount of memory per-cgroup is overcommited.
343	    Overcommiting kernel memory limits is definitely not recommended, since the
344	    box can still run out of non-reclaimable memory.
345	    In this case, the admin could set up K so that the sum of all groups is
346	    never greater than the total memory, and freely set U at the cost of his
347	    QoS.
348	
349	    U != 0, K >= U:
350	    Since kmem charges will also be fed to the user counter and reclaim will be
351	    triggered for the cgroup for both kinds of memory. This setup gives the
352	    admin a unified view of memory, and it is also useful for people who just
353	    want to track kernel memory usage.
354	
355	3. User Interface
356	
357	3.0. Configuration
358	
359	a. Enable CONFIG_CGROUPS
360	b. Enable CONFIG_MEMCG
361	c. Enable CONFIG_MEMCG_SWAP (to use swap extension)
362	d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)
363	
364	3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
365	# mount -t tmpfs none /sys/fs/cgroup
366	# mkdir /sys/fs/cgroup/memory
367	# mount -t cgroup none /sys/fs/cgroup/memory -o memory
368	
369	3.2. Make the new group and move bash into it
370	# mkdir /sys/fs/cgroup/memory/0
371	# echo $$ > /sys/fs/cgroup/memory/0/tasks
372	
373	Since now we're in the 0 cgroup, we can alter the memory limit:
374	# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
375	
376	NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
377	mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
378	
379	NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
380	NOTE: We cannot set limits on the root cgroup any more.
381	
382	# cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
383	4194304
384	
385	We can check the usage:
386	# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
387	1216512
388	
389	A successful write to this file does not guarantee a successful setting of
390	this limit to the value written into the file. This can be due to a
391	number of factors, such as rounding up to page boundaries or the total
392	availability of memory on the system. The user is required to re-read
393	this file after a write to guarantee the value committed by the kernel.
394	
395	# echo 1 > memory.limit_in_bytes
396	# cat memory.limit_in_bytes
397	4096
398	
399	The memory.failcnt field gives the number of times that the cgroup limit was
400	exceeded.
401	
402	The memory.stat file gives accounting information. Now, the number of
403	caches, RSS and Active pages/Inactive pages are shown.
404	
405	4. Testing
406	
407	For testing features and implementation, see memcg_test.txt.
408	
409	Performance test is also important. To see pure memory controller's overhead,
410	testing on tmpfs will give you good numbers of small overheads.
411	Example: do kernel make on tmpfs.
412	
413	Page-fault scalability is also important. At measuring parallel
414	page fault test, multi-process test may be better than multi-thread
415	test because it has noise of shared objects/status.
416	
417	But the above two are testing extreme situations.
418	Trying usual test under memory controller is always helpful.
419	
420	4.1 Troubleshooting
421	
422	Sometimes a user might find that the application under a cgroup is
423	terminated by the OOM killer. There are several causes for this:
424	
425	1. The cgroup limit is too low (just too low to do anything useful)
426	2. The user is using anonymous memory and swap is turned off or too low
427	
428	A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
429	some of the pages cached in the cgroup (page cache pages).
430	
431	To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
432	seeing what happens will be helpful.
433	
434	4.2 Task migration
435	
436	When a task migrates from one cgroup to another, its charge is not
437	carried forward by default. The pages allocated from the original cgroup still
438	remain charged to it, the charge is dropped when the page is freed or
439	reclaimed.
440	
441	You can move charges of a task along with task migration.
442	See 8. "Move charges at task migration"
443	
444	4.3 Removing a cgroup
445	
446	A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
447	cgroup might have some charge associated with it, even though all
448	tasks have migrated away from it. (because we charge against pages, not
449	against tasks.)
450	
451	We move the stats to root (if use_hierarchy==0) or parent (if
452	use_hierarchy==1), and no change on the charge except uncharging
453	from the child.
454	
455	Charges recorded in swap information is not updated at removal of cgroup.
456	Recorded information is discarded and a cgroup which uses swap (swapcache)
457	will be charged as a new owner of it.
458	
459	About use_hierarchy, see Section 6.
460	
461	5. Misc. interfaces.
462	
463	5.1 force_empty
464	  memory.force_empty interface is provided to make cgroup's memory usage empty.
465	  When writing anything to this
466	
467	  # echo 0 > memory.force_empty
468	
469	  the cgroup will be reclaimed and as many pages reclaimed as possible.
470	
471	  The typical use case for this interface is before calling rmdir().
472	  Because rmdir() moves all pages to parent, some out-of-use page caches can be
473	  moved to the parent. If you want to avoid that, force_empty will be useful.
474	
475	  Also, note that when memory.kmem.limit_in_bytes is set the charges due to
476	  kernel pages will still be seen. This is not considered a failure and the
477	  write will still return success. In this case, it is expected that
478	  memory.kmem.usage_in_bytes == memory.usage_in_bytes.
479	
480	  About use_hierarchy, see Section 6.
481	
482	5.2 stat file
483	
484	memory.stat file includes following statistics
485	
486	# per-memory cgroup local status
487	cache		- # of bytes of page cache memory.
488	rss		- # of bytes of anonymous and swap cache memory (includes
489			transparent hugepages).
490	rss_huge	- # of bytes of anonymous transparent hugepages.
491	mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
492	pgpgin		- # of charging events to the memory cgroup. The charging
493			event happens each time a page is accounted as either mapped
494			anon page(RSS) or cache page(Page Cache) to the cgroup.
495	pgpgout		- # of uncharging events to the memory cgroup. The uncharging
496			event happens each time a page is unaccounted from the cgroup.
497	swap		- # of bytes of swap usage
498	writeback	- # of bytes of file/anon cache that are queued for syncing to
499			disk.
500	inactive_anon	- # of bytes of anonymous and swap cache memory on inactive
501			LRU list.
502	active_anon	- # of bytes of anonymous and swap cache memory on active
503			LRU list.
504	inactive_file	- # of bytes of file-backed memory on inactive LRU list.
505	active_file	- # of bytes of file-backed memory on active LRU list.
506	unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).
507	
508	# status considering hierarchy (see memory.use_hierarchy settings)
509	
510	hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
511				under which the memory cgroup is
512	hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
513				hierarchy under which memory cgroup is.
514	
515	total_<counter>		- # hierarchical version of <counter>, which in
516				addition to the cgroup's own value includes the
517				sum of all hierarchical children's values of
518				<counter>, i.e. total_cache
519	
520	# The following additional stats are dependent on CONFIG_DEBUG_VM.
521	
522	recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
523	recent_rotated_file	- VM internal parameter. (see mm/vmscan.c)
524	recent_scanned_anon	- VM internal parameter. (see mm/vmscan.c)
525	recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)
526	
527	Memo:
528		recent_rotated means recent frequency of LRU rotation.
529		recent_scanned means recent # of scans to LRU.
530		showing for better debug please see the code for meanings.
531	
532	Note:
533		Only anonymous and swap cache memory is listed as part of 'rss' stat.
534		This should not be confused with the true 'resident set size' or the
535		amount of physical memory used by the cgroup.
536		'rss + file_mapped" will give you resident set size of cgroup.
537		(Note: file and shmem may be shared among other cgroups. In that case,
538		 file_mapped is accounted only when the memory cgroup is owner of page
539		 cache.)
540	
541	5.3 swappiness
542	
543	Overrides /proc/sys/vm/swappiness for the particular group. The tunable
544	in the root cgroup corresponds to the global swappiness setting.
545	
546	Please note that unlike during the global reclaim, limit reclaim
547	enforces that 0 swappiness really prevents from any swapping even if
548	there is a swap storage available. This might lead to memcg OOM killer
549	if there are no file pages to reclaim.
550	
551	5.4 failcnt
552	
553	A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
554	This failcnt(== failure count) shows the number of times that a usage counter
555	hit its limit. When a memory cgroup hits a limit, failcnt increases and
556	memory under it will be reclaimed.
557	
558	You can reset failcnt by writing 0 to failcnt file.
559	# echo 0 > .../memory.failcnt
560	
561	5.5 usage_in_bytes
562	
563	For efficiency, as other kernel components, memory cgroup uses some optimization
564	to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
565	method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
566	value for efficient access. (Of course, when necessary, it's synchronized.)
567	If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
568	value in memory.stat(see 5.2).
569	
570	5.6 numa_stat
571	
572	This is similar to numa_maps but operates on a per-memcg basis.  This is
573	useful for providing visibility into the numa locality information within
574	an memcg since the pages are allowed to be allocated from any physical
575	node.  One of the use cases is evaluating application performance by
576	combining this information with the application's CPU allocation.
577	
578	Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
579	per-node page counts including "hierarchical_<counter>" which sums up all
580	hierarchical children's values in addition to the memcg's own value.
581	
582	The output format of memory.numa_stat is:
583	
584	total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
585	file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
586	anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
587	unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
588	hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
589	
590	The "total" count is sum of file + anon + unevictable.
591	
592	6. Hierarchy support
593	
594	The memory controller supports a deep hierarchy and hierarchical accounting.
595	The hierarchy is created by creating the appropriate cgroups in the
596	cgroup filesystem. Consider for example, the following cgroup filesystem
597	hierarchy
598	
599		       root
600		     /  |   \
601	            /	|    \
602		   a	b     c
603			      | \
604			      |  \
605			      d   e
606	
607	In the diagram above, with hierarchical accounting enabled, all memory
608	usage of e, is accounted to its ancestors up until the root (i.e, c and root),
609	that has memory.use_hierarchy enabled. If one of the ancestors goes over its
610	limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
611	children of the ancestor.
612	
613	6.1 Enabling hierarchical accounting and reclaim
614	
615	A memory cgroup by default disables the hierarchy feature. Support
616	can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
617	
618	# echo 1 > memory.use_hierarchy
619	
620	The feature can be disabled by
621	
622	# echo 0 > memory.use_hierarchy
623	
624	NOTE1: Enabling/disabling will fail if either the cgroup already has other
625	       cgroups created below it, or if the parent cgroup has use_hierarchy
626	       enabled.
627	
628	NOTE2: When panic_on_oom is set to "2", the whole system will panic in
629	       case of an OOM event in any cgroup.
630	
631	7. Soft limits
632	
633	Soft limits allow for greater sharing of memory. The idea behind soft limits
634	is to allow control groups to use as much of the memory as needed, provided
635	
636	a. There is no memory contention
637	b. They do not exceed their hard limit
638	
639	When the system detects memory contention or low memory, control groups
640	are pushed back to their soft limits. If the soft limit of each control
641	group is very high, they are pushed back as much as possible to make
642	sure that one control group does not starve the others of memory.
643	
644	Please note that soft limits is a best-effort feature; it comes with
645	no guarantees, but it does its best to make sure that when memory is
646	heavily contended for, memory is allocated based on the soft limit
647	hints/setup. Currently soft limit based reclaim is set up such that
648	it gets invoked from balance_pgdat (kswapd).
649	
650	7.1 Interface
651	
652	Soft limits can be setup by using the following commands (in this example we
653	assume a soft limit of 256 MiB)
654	
655	# echo 256M > memory.soft_limit_in_bytes
656	
657	If we want to change this to 1G, we can at any time use
658	
659	# echo 1G > memory.soft_limit_in_bytes
660	
661	NOTE1: Soft limits take effect over a long period of time, since they involve
662	       reclaiming memory for balancing between memory cgroups
663	NOTE2: It is recommended to set the soft limit always below the hard limit,
664	       otherwise the hard limit will take precedence.
665	
666	8. Move charges at task migration
667	
668	Users can move charges associated with a task along with task migration, that
669	is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
670	This feature is not supported in !CONFIG_MMU environments because of lack of
671	page tables.
672	
673	8.1 Interface
674	
675	This feature is disabled by default. It can be enabled (and disabled again) by
676	writing to memory.move_charge_at_immigrate of the destination cgroup.
677	
678	If you want to enable it:
679	
680	# echo (some positive value) > memory.move_charge_at_immigrate
681	
682	Note: Each bits of move_charge_at_immigrate has its own meaning about what type
683	      of charges should be moved. See 8.2 for details.
684	Note: Charges are moved only when you move mm->owner, in other words,
685	      a leader of a thread group.
686	Note: If we cannot find enough space for the task in the destination cgroup, we
687	      try to make space by reclaiming memory. Task migration may fail if we
688	      cannot make enough space.
689	Note: It can take several seconds if you move charges much.
690	
691	And if you want disable it again:
692	
693	# echo 0 > memory.move_charge_at_immigrate
694	
695	8.2 Type of charges which can be moved
696	
697	Each bit in move_charge_at_immigrate has its own meaning about what type of
698	charges should be moved. But in any case, it must be noted that an account of
699	a page or a swap can be moved only when it is charged to the task's current
700	(old) memory cgroup.
701	
702	  bit | what type of charges would be moved ?
703	 -----+------------------------------------------------------------------------
704	   0  | A charge of an anonymous page (or swap of it) used by the target task.
705	      | You must enable Swap Extension (see 2.4) to enable move of swap charges.
706	 -----+------------------------------------------------------------------------
707	   1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
708	      | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
709	      | anonymous pages, file pages (and swaps) in the range mmapped by the task
710	      | will be moved even if the task hasn't done page fault, i.e. they might
711	      | not be the task's "RSS", but other task's "RSS" that maps the same file.
712	      | And mapcount of the page is ignored (the page can be moved even if
713	      | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
714	      | enable move of swap charges.
715	
716	8.3 TODO
717	
718	- All of moving charge operations are done under cgroup_mutex. It's not good
719	  behavior to hold the mutex too long, so we may need some trick.
720	
721	9. Memory thresholds
722	
723	Memory cgroup implements memory thresholds using the cgroups notification
724	API (see cgroups.txt). It allows to register multiple memory and memsw
725	thresholds and gets notifications when it crosses.
726	
727	To register a threshold, an application must:
728	- create an eventfd using eventfd(2);
729	- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
730	- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
731	  cgroup.event_control.
732	
733	Application will be notified through eventfd when memory usage crosses
734	threshold in any direction.
735	
736	It's applicable for root and non-root cgroup.
737	
738	10. OOM Control
739	
740	memory.oom_control file is for OOM notification and other controls.
741	
742	Memory cgroup implements OOM notifier using the cgroup notification
743	API (See cgroups.txt). It allows to register multiple OOM notification
744	delivery and gets notification when OOM happens.
745	
746	To register a notifier, an application must:
747	 - create an eventfd using eventfd(2)
748	 - open memory.oom_control file
749	 - write string like "<event_fd> <fd of memory.oom_control>" to
750	   cgroup.event_control
751	
752	The application will be notified through eventfd when OOM happens.
753	OOM notification doesn't work for the root cgroup.
754	
755	You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
756	
757		#echo 1 > memory.oom_control
758	
759	If OOM-killer is disabled, tasks under cgroup will hang/sleep
760	in memory cgroup's OOM-waitqueue when they request accountable memory.
761	
762	For running them, you have to relax the memory cgroup's OOM status by
763		* enlarge limit or reduce usage.
764	To reduce usage,
765		* kill some tasks.
766		* move some tasks to other group with account migration.
767		* remove some files (on tmpfs?)
768	
769	Then, stopped tasks will work again.
770	
771	At reading, current status of OOM is shown.
772		oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
773		under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
774					 be stopped.)
775	
776	11. Memory Pressure
777	
778	The pressure level notifications can be used to monitor the memory
779	allocation cost; based on the pressure, applications can implement
780	different strategies of managing their memory resources. The pressure
781	levels are defined as following:
782	
783	The "low" level means that the system is reclaiming memory for new
784	allocations. Monitoring this reclaiming activity might be useful for
785	maintaining cache level. Upon notification, the program (typically
786	"Activity Manager") might analyze vmstat and act in advance (i.e.
787	prematurely shutdown unimportant services).
788	
789	The "medium" level means that the system is experiencing medium memory
790	pressure, the system might be making swap, paging out active file caches,
791	etc. Upon this event applications may decide to further analyze
792	vmstat/zoneinfo/memcg or internal memory usage statistics and free any
793	resources that can be easily reconstructed or re-read from a disk.
794	
795	The "critical" level means that the system is actively thrashing, it is
796	about to out of memory (OOM) or even the in-kernel OOM killer is on its
797	way to trigger. Applications should do whatever they can to help the
798	system. It might be too late to consult with vmstat or any other
799	statistics, so it's advisable to take an immediate action.
800	
801	The events are propagated upward until the event is handled, i.e. the
802	events are not pass-through. Here is what this means: for example you have
803	three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
804	and C, and suppose group C experiences some pressure. In this situation,
805	only group C will receive the notification, i.e. groups A and B will not
806	receive it. This is done to avoid excessive "broadcasting" of messages,
807	which disturbs the system and which is especially bad if we are low on
808	memory or thrashing. So, organize the cgroups wisely, or propagate the
809	events manually (or, ask us to implement the pass-through events,
810	explaining why would you need them.)
811	
812	The file memory.pressure_level is only used to setup an eventfd. To
813	register a notification, an application must:
814	
815	- create an eventfd using eventfd(2);
816	- open memory.pressure_level;
817	- write string like "<event_fd> <fd of memory.pressure_level> <level>"
818	  to cgroup.event_control.
819	
820	Application will be notified through eventfd when memory pressure is at
821	the specific level (or higher). Read/write operations to
822	memory.pressure_level are no implemented.
823	
824	Test:
825	
826	   Here is a small script example that makes a new cgroup, sets up a
827	   memory limit, sets up a notification in the cgroup and then makes child
828	   cgroup experience a critical pressure:
829	
830	   # cd /sys/fs/cgroup/memory/
831	   # mkdir foo
832	   # cd foo
833	   # cgroup_event_listener memory.pressure_level low &
834	   # echo 8000000 > memory.limit_in_bytes
835	   # echo 8000000 > memory.memsw.limit_in_bytes
836	   # echo $$ > tasks
837	   # dd if=/dev/zero | read x
838	
839	   (Expect a bunch of notifications, and eventually, the oom-killer will
840	   trigger.)
841	
842	12. TODO
843	
844	1. Make per-cgroup scanner reclaim not-shared pages first
845	2. Teach controller to account for shared-pages
846	3. Start reclamation in the background when the limit is
847	   not yet hit but the usage is getting closer
848	
849	Summary
850	
851	Overall, the memory controller has been a stable controller and has been
852	commented and discussed quite extensively in the community.
853	
854	References
855	
856	1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
857	2. Singh, Balbir. Memory Controller (RSS Control),
858	   http://lwn.net/Articles/222762/
859	3. Emelianov, Pavel. Resource controllers based on process cgroups
860	   http://lkml.org/lkml/2007/3/6/198
861	4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
862	   http://lkml.org/lkml/2007/4/9/78
863	5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
864	   http://lkml.org/lkml/2007/5/30/244
865	6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
866	7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
867	   subsystem (v3), http://lwn.net/Articles/235534/
868	8. Singh, Balbir. RSS controller v2 test results (lmbench),
869	   http://lkml.org/lkml/2007/5/17/232
870	9. Singh, Balbir. RSS controller v2 AIM9 results
871	   http://lkml.org/lkml/2007/5/18/1
872	10. Singh, Balbir. Memory controller v6 test results,
873	    http://lkml.org/lkml/2007/8/19/36
874	11. Singh, Balbir. Memory controller introduction (v6),
875	    http://lkml.org/lkml/2007/8/17/69
876	12. Corbet, Jonathan, Controlling memory use in cgroups,
877	    http://lwn.net/Articles/243795/
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