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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1	Memory Resource Controller
2	
3	NOTE: The Memory Resource Controller has generically been referred to as the
4	      memory controller in this document. Do not confuse memory controller
5	      used here with the memory controller that is used in hardware.
6	
7	(For editors)
8	In this document:
9	      When we mention a cgroup (cgroupfs's directory) with memory controller,
10	      we call it "memory cgroup". When you see git-log and source code, you'll
11	      see patch's title and function names tend to use "memcg".
12	      In this document, we avoid using it.
13	
14	Benefits and Purpose of the memory controller
15	
16	The memory controller isolates the memory behaviour of a group of tasks
17	from the rest of the system. The article on LWN [12] mentions some probable
18	uses of the memory controller. The memory controller can be used to
19	
20	a. Isolate an application or a group of applications
21	   Memory-hungry applications can be isolated and limited to a smaller
22	   amount of memory.
23	b. Create a cgroup with a limited amount of memory; this can be used
24	   as a good alternative to booting with mem=XXXX.
25	c. Virtualization solutions can control the amount of memory they want
26	   to assign to a virtual machine instance.
27	d. A CD/DVD burner could control the amount of memory used by the
28	   rest of the system to ensure that burning does not fail due to lack
29	   of available memory.
30	e. There are several other use cases; find one or use the controller just
31	   for fun (to learn and hack on the VM subsystem).
32	
33	Current Status: linux-2.6.34-mmotm(development version of 2010/April)
34	
35	Features:
36	 - accounting anonymous pages, file caches, swap caches usage and limiting them.
37	 - pages are linked to per-memcg LRU exclusively, and there is no global LRU.
38	 - optionally, memory+swap usage can be accounted and limited.
39	 - hierarchical accounting
40	 - soft limit
41	 - moving (recharging) account at moving a task is selectable.
42	 - usage threshold notifier
43	 - memory pressure notifier
44	 - oom-killer disable knob and oom-notifier
45	 - Root cgroup has no limit controls.
46	
47	 Kernel memory support is a work in progress, and the current version provides
48	 basically functionality. (See Section 2.7)
49	
50	Brief summary of control files.
51	
52	 tasks				 # attach a task(thread) and show list of threads
53	 cgroup.procs			 # show list of processes
54	 cgroup.event_control		 # an interface for event_fd()
55	 memory.usage_in_bytes		 # show current res_counter usage for memory
56					 (See 5.5 for details)
57	 memory.memsw.usage_in_bytes	 # show current res_counter usage for memory+Swap
58					 (See 5.5 for details)
59	 memory.limit_in_bytes		 # set/show limit of memory usage
60	 memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage
61	 memory.failcnt			 # show the number of memory usage hits limits
62	 memory.memsw.failcnt		 # show the number of memory+Swap hits limits
63	 memory.max_usage_in_bytes	 # show max memory usage recorded
64	 memory.memsw.max_usage_in_bytes # show max memory+Swap usage recorded
65	 memory.soft_limit_in_bytes	 # set/show soft limit of memory usage
66	 memory.stat			 # show various statistics
67	 memory.use_hierarchy		 # set/show hierarchical account enabled
68	 memory.force_empty		 # trigger forced move charge to parent
69	 memory.pressure_level		 # set memory pressure notifications
70	 memory.swappiness		 # set/show swappiness parameter of vmscan
71					 (See sysctl's vm.swappiness)
72	 memory.move_charge_at_immigrate # set/show controls of moving charges
73	 memory.oom_control		 # set/show oom controls.
74	 memory.numa_stat		 # show the number of memory usage per numa node
75	
76	 memory.kmem.limit_in_bytes      # set/show hard limit for kernel memory
77	 memory.kmem.usage_in_bytes      # show current kernel memory allocation
78	 memory.kmem.failcnt             # show the number of kernel memory usage hits limits
79	 memory.kmem.max_usage_in_bytes  # show max kernel memory usage recorded
80	
81	 memory.kmem.tcp.limit_in_bytes  # set/show hard limit for tcp buf memory
82	 memory.kmem.tcp.usage_in_bytes  # show current tcp buf memory allocation
83	 memory.kmem.tcp.failcnt            # show the number of tcp buf memory usage hits limits
84	 memory.kmem.tcp.max_usage_in_bytes # show max tcp buf memory usage recorded
85	
86	1. History
87	
88	The memory controller has a long history. A request for comments for the memory
89	controller was posted by Balbir Singh [1]. At the time the RFC was posted
90	there were several implementations for memory control. The goal of the
91	RFC was to build consensus and agreement for the minimal features required
92	for memory control. The first RSS controller was posted by Balbir Singh[2]
93	in Feb 2007. Pavel Emelianov [3][4][5] has since posted three versions of the
94	RSS controller. At OLS, at the resource management BoF, everyone suggested
95	that we handle both page cache and RSS together. Another request was raised
96	to allow user space handling of OOM. The current memory controller is
97	at version 6; it combines both mapped (RSS) and unmapped Page
98	Cache Control [11].
99	
100	2. Memory Control
101	
102	Memory is a unique resource in the sense that it is present in a limited
103	amount. If a task requires a lot of CPU processing, the task can spread
104	its processing over a period of hours, days, months or years, but with
105	memory, the same physical memory needs to be reused to accomplish the task.
106	
107	The memory controller implementation has been divided into phases. These
108	are:
109	
110	1. Memory controller
111	2. mlock(2) controller
112	3. Kernel user memory accounting and slab control
113	4. user mappings length controller
114	
115	The memory controller is the first controller developed.
116	
117	2.1. Design
118	
119	The core of the design is a counter called the res_counter. The res_counter
120	tracks the current memory usage and limit of the group of processes associated
121	with the controller. Each cgroup has a memory controller specific data
122	structure (mem_cgroup) associated with it.
123	
124	2.2. Accounting
125	
126			+--------------------+
127			|  mem_cgroup     |
128			|  (res_counter)     |
129			+--------------------+
130			 /            ^      \
131			/             |       \
132	           +---------------+  |        +---------------+
133	           | mm_struct     |  |....    | mm_struct     |
134	           |               |  |        |               |
135	           +---------------+  |        +---------------+
136	                              |
137	                              + --------------+
138	                                              |
139	           +---------------+           +------+--------+
140	           | page          +---------->  page_cgroup|
141	           |               |           |               |
142	           +---------------+           +---------------+
143	
144	             (Figure 1: Hierarchy of Accounting)
145	
146	
147	Figure 1 shows the important aspects of the controller
148	
149	1. Accounting happens per cgroup
150	2. Each mm_struct knows about which cgroup it belongs to
151	3. Each page has a pointer to the page_cgroup, which in turn knows the
152	   cgroup it belongs to
153	
154	The accounting is done as follows: mem_cgroup_charge_common() is invoked to
155	set up the necessary data structures and check if the cgroup that is being
156	charged is over its limit. If it is, then reclaim is invoked on the cgroup.
157	More details can be found in the reclaim section of this document.
158	If everything goes well, a page meta-data-structure called page_cgroup is
159	updated. page_cgroup has its own LRU on cgroup.
160	(*) page_cgroup structure is allocated at boot/memory-hotplug time.
161	
162	2.2.1 Accounting details
163	
164	All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
165	Some pages which are never reclaimable and will not be on the LRU
166	are not accounted. We just account pages under usual VM management.
167	
168	RSS pages are accounted at page_fault unless they've already been accounted
169	for earlier. A file page will be accounted for as Page Cache when it's
170	inserted into inode (radix-tree). While it's mapped into the page tables of
171	processes, duplicate accounting is carefully avoided.
172	
173	An RSS page is unaccounted when it's fully unmapped. A PageCache page is
174	unaccounted when it's removed from radix-tree. Even if RSS pages are fully
175	unmapped (by kswapd), they may exist as SwapCache in the system until they
176	are really freed. Such SwapCaches are also accounted.
177	A swapped-in page is not accounted until it's mapped.
178	
179	Note: The kernel does swapin-readahead and reads multiple swaps at once.
180	This means swapped-in pages may contain pages for other tasks than a task
181	causing page fault. So, we avoid accounting at swap-in I/O.
182	
183	At page migration, accounting information is kept.
184	
185	Note: we just account pages-on-LRU because our purpose is to control amount
186	of used pages; not-on-LRU pages tend to be out-of-control from VM view.
187	
188	2.3 Shared Page Accounting
189	
190	Shared pages are accounted on the basis of the first touch approach. The
191	cgroup that first touches a page is accounted for the page. The principle
192	behind this approach is that a cgroup that aggressively uses a shared
193	page will eventually get charged for it (once it is uncharged from
194	the cgroup that brought it in -- this will happen on memory pressure).
195	
196	But see section 8.2: when moving a task to another cgroup, its pages may
197	be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
198	
199	Exception: If CONFIG_MEMCG_SWAP is not used.
200	When you do swapoff and make swapped-out pages of shmem(tmpfs) to
201	be backed into memory in force, charges for pages are accounted against the
202	caller of swapoff rather than the users of shmem.
203	
204	2.4 Swap Extension (CONFIG_MEMCG_SWAP)
205	
206	Swap Extension allows you to record charge for swap. A swapped-in page is
207	charged back to original page allocator if possible.
208	
209	When swap is accounted, following files are added.
210	 - memory.memsw.usage_in_bytes.
211	 - memory.memsw.limit_in_bytes.
212	
213	memsw means memory+swap. Usage of memory+swap is limited by
214	memsw.limit_in_bytes.
215	
216	Example: Assume a system with 4G of swap. A task which allocates 6G of memory
217	(by mistake) under 2G memory limitation will use all swap.
218	In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
219	By using the memsw limit, you can avoid system OOM which can be caused by swap
220	shortage.
221	
222	* why 'memory+swap' rather than swap.
223	The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
224	to move account from memory to swap...there is no change in usage of
225	memory+swap. In other words, when we want to limit the usage of swap without
226	affecting global LRU, memory+swap limit is better than just limiting swap from
227	an OS point of view.
228	
229	* What happens when a cgroup hits memory.memsw.limit_in_bytes
230	When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
231	in this cgroup. Then, swap-out will not be done by cgroup routine and file
232	caches are dropped. But as mentioned above, global LRU can do swapout memory
233	from it for sanity of the system's memory management state. You can't forbid
234	it by cgroup.
235	
236	2.5 Reclaim
237	
238	Each cgroup maintains a per cgroup LRU which has the same structure as
239	global VM. When a cgroup goes over its limit, we first try
240	to reclaim memory from the cgroup so as to make space for the new
241	pages that the cgroup has touched. If the reclaim is unsuccessful,
242	an OOM routine is invoked to select and kill the bulkiest task in the
243	cgroup. (See 10. OOM Control below.)
244	
245	The reclaim algorithm has not been modified for cgroups, except that
246	pages that are selected for reclaiming come from the per-cgroup LRU
247	list.
248	
249	NOTE: Reclaim does not work for the root cgroup, since we cannot set any
250	limits on the root cgroup.
251	
252	Note2: When panic_on_oom is set to "2", the whole system will panic.
253	
254	When oom event notifier is registered, event will be delivered.
255	(See oom_control section)
256	
257	2.6 Locking
258	
259	   lock_page_cgroup()/unlock_page_cgroup() should not be called under
260	   mapping->tree_lock.
261	
262	   Other lock order is following:
263	   PG_locked.
264	   mm->page_table_lock
265	       zone->lru_lock
266		  lock_page_cgroup.
267	  In many cases, just lock_page_cgroup() is called.
268	  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
269	  zone->lru_lock, it has no lock of its own.
270	
271	2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM)
272	
273	WARNING: Current implementation lacks reclaim support. That means allocation
274		 attempts will fail when close to the limit even if there are plenty of
275		 kmem available for reclaim. That makes this option unusable in real
276		 life so DO NOT SELECT IT unless for development purposes.
277	
278	With the Kernel memory extension, the Memory Controller is able to limit
279	the amount of kernel memory used by the system. Kernel memory is fundamentally
280	different than user memory, since it can't be swapped out, which makes it
281	possible to DoS the system by consuming too much of this precious resource.
282	
283	Kernel memory won't be accounted at all until limit on a group is set. This
284	allows for existing setups to continue working without disruption.  The limit
285	cannot be set if the cgroup have children, or if there are already tasks in the
286	cgroup. Attempting to set the limit under those conditions will return -EBUSY.
287	When use_hierarchy == 1 and a group is accounted, its children will
288	automatically be accounted regardless of their limit value.
289	
290	After a group is first limited, it will be kept being accounted until it
291	is removed. The memory limitation itself, can of course be removed by writing
292	-1 to memory.kmem.limit_in_bytes. In this case, kmem will be accounted, but not
293	limited.
294	
295	Kernel memory limits are not imposed for the root cgroup. Usage for the root
296	cgroup may or may not be accounted. The memory used is accumulated into
297	memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
298	(currently only for tcp).
299	The main "kmem" counter is fed into the main counter, so kmem charges will
300	also be visible from the user counter.
301	
302	Currently no soft limit is implemented for kernel memory. It is future work
303	to trigger slab reclaim when those limits are reached.
304	
305	2.7.1 Current Kernel Memory resources accounted
306	
307	* stack pages: every process consumes some stack pages. By accounting into
308	kernel memory, we prevent new processes from being created when the kernel
309	memory usage is too high.
310	
311	* slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy
312	of each kmem_cache is created every time the cache is touched by the first time
313	from inside the memcg. The creation is done lazily, so some objects can still be
314	skipped while the cache is being created. All objects in a slab page should
315	belong to the same memcg. This only fails to hold when a task is migrated to a
316	different memcg during the page allocation by the cache.
317	
318	* sockets memory pressure: some sockets protocols have memory pressure
319	thresholds. The Memory Controller allows them to be controlled individually
320	per cgroup, instead of globally.
321	
322	* tcp memory pressure: sockets memory pressure for the tcp protocol.
323	
324	2.7.3 Common use cases
325	
326	Because the "kmem" counter is fed to the main user counter, kernel memory can
327	never be limited completely independently of user memory. Say "U" is the user
328	limit, and "K" the kernel limit. There are three possible ways limits can be
329	set:
330	
331	    U != 0, K = unlimited:
332	    This is the standard memcg limitation mechanism already present before kmem
333	    accounting. Kernel memory is completely ignored.
334	
335	    U != 0, K < U:
336	    Kernel memory is a subset of the user memory. This setup is useful in
337	    deployments where the total amount of memory per-cgroup is overcommited.
338	    Overcommiting kernel memory limits is definitely not recommended, since the
339	    box can still run out of non-reclaimable memory.
340	    In this case, the admin could set up K so that the sum of all groups is
341	    never greater than the total memory, and freely set U at the cost of his
342	    QoS.
343	
344	    U != 0, K >= U:
345	    Since kmem charges will also be fed to the user counter and reclaim will be
346	    triggered for the cgroup for both kinds of memory. This setup gives the
347	    admin a unified view of memory, and it is also useful for people who just
348	    want to track kernel memory usage.
349	
350	3. User Interface
351	
352	0. Configuration
353	
354	a. Enable CONFIG_CGROUPS
355	b. Enable CONFIG_RESOURCE_COUNTERS
356	c. Enable CONFIG_MEMCG
357	d. Enable CONFIG_MEMCG_SWAP (to use swap extension)
358	d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)
359	
360	1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
361	# mount -t tmpfs none /sys/fs/cgroup
362	# mkdir /sys/fs/cgroup/memory
363	# mount -t cgroup none /sys/fs/cgroup/memory -o memory
364	
365	2. Make the new group and move bash into it
366	# mkdir /sys/fs/cgroup/memory/0
367	# echo $$ > /sys/fs/cgroup/memory/0/tasks
368	
369	Since now we're in the 0 cgroup, we can alter the memory limit:
370	# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
371	
372	NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
373	mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
374	
375	NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
376	NOTE: We cannot set limits on the root cgroup any more.
377	
378	# cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
379	4194304
380	
381	We can check the usage:
382	# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
383	1216512
384	
385	A successful write to this file does not guarantee a successful setting of
386	this limit to the value written into the file. This can be due to a
387	number of factors, such as rounding up to page boundaries or the total
388	availability of memory on the system. The user is required to re-read
389	this file after a write to guarantee the value committed by the kernel.
390	
391	# echo 1 > memory.limit_in_bytes
392	# cat memory.limit_in_bytes
393	4096
394	
395	The memory.failcnt field gives the number of times that the cgroup limit was
396	exceeded.
397	
398	The memory.stat file gives accounting information. Now, the number of
399	caches, RSS and Active pages/Inactive pages are shown.
400	
401	4. Testing
402	
403	For testing features and implementation, see memcg_test.txt.
404	
405	Performance test is also important. To see pure memory controller's overhead,
406	testing on tmpfs will give you good numbers of small overheads.
407	Example: do kernel make on tmpfs.
408	
409	Page-fault scalability is also important. At measuring parallel
410	page fault test, multi-process test may be better than multi-thread
411	test because it has noise of shared objects/status.
412	
413	But the above two are testing extreme situations.
414	Trying usual test under memory controller is always helpful.
415	
416	4.1 Troubleshooting
417	
418	Sometimes a user might find that the application under a cgroup is
419	terminated by the OOM killer. There are several causes for this:
420	
421	1. The cgroup limit is too low (just too low to do anything useful)
422	2. The user is using anonymous memory and swap is turned off or too low
423	
424	A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
425	some of the pages cached in the cgroup (page cache pages).
426	
427	To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
428	seeing what happens will be helpful.
429	
430	4.2 Task migration
431	
432	When a task migrates from one cgroup to another, its charge is not
433	carried forward by default. The pages allocated from the original cgroup still
434	remain charged to it, the charge is dropped when the page is freed or
435	reclaimed.
436	
437	You can move charges of a task along with task migration.
438	See 8. "Move charges at task migration"
439	
440	4.3 Removing a cgroup
441	
442	A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
443	cgroup might have some charge associated with it, even though all
444	tasks have migrated away from it. (because we charge against pages, not
445	against tasks.)
446	
447	We move the stats to root (if use_hierarchy==0) or parent (if
448	use_hierarchy==1), and no change on the charge except uncharging
449	from the child.
450	
451	Charges recorded in swap information is not updated at removal of cgroup.
452	Recorded information is discarded and a cgroup which uses swap (swapcache)
453	will be charged as a new owner of it.
454	
455	About use_hierarchy, see Section 6.
456	
457	5. Misc. interfaces.
458	
459	5.1 force_empty
460	  memory.force_empty interface is provided to make cgroup's memory usage empty.
461	  When writing anything to this
462	
463	  # echo 0 > memory.force_empty
464	
465	  the cgroup will be reclaimed and as many pages reclaimed as possible.
466	
467	  The typical use case for this interface is before calling rmdir().
468	  Because rmdir() moves all pages to parent, some out-of-use page caches can be
469	  moved to the parent. If you want to avoid that, force_empty will be useful.
470	
471	  Also, note that when memory.kmem.limit_in_bytes is set the charges due to
472	  kernel pages will still be seen. This is not considered a failure and the
473	  write will still return success. In this case, it is expected that
474	  memory.kmem.usage_in_bytes == memory.usage_in_bytes.
475	
476	  About use_hierarchy, see Section 6.
477	
478	5.2 stat file
479	
480	memory.stat file includes following statistics
481	
482	# per-memory cgroup local status
483	cache		- # of bytes of page cache memory.
484	rss		- # of bytes of anonymous and swap cache memory (includes
485			transparent hugepages).
486	rss_huge	- # of bytes of anonymous transparent hugepages.
487	mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
488	pgpgin		- # of charging events to the memory cgroup. The charging
489			event happens each time a page is accounted as either mapped
490			anon page(RSS) or cache page(Page Cache) to the cgroup.
491	pgpgout		- # of uncharging events to the memory cgroup. The uncharging
492			event happens each time a page is unaccounted from the cgroup.
493	swap		- # of bytes of swap usage
494	writeback	- # of bytes of file/anon cache that are queued for syncing to
495			disk.
496	inactive_anon	- # of bytes of anonymous and swap cache memory on inactive
497			LRU list.
498	active_anon	- # of bytes of anonymous and swap cache memory on active
499			LRU list.
500	inactive_file	- # of bytes of file-backed memory on inactive LRU list.
501	active_file	- # of bytes of file-backed memory on active LRU list.
502	unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).
503	
504	# status considering hierarchy (see memory.use_hierarchy settings)
505	
506	hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
507				under which the memory cgroup is
508	hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
509				hierarchy under which memory cgroup is.
510	
511	total_<counter>		- # hierarchical version of <counter>, which in
512				addition to the cgroup's own value includes the
513				sum of all hierarchical children's values of
514				<counter>, i.e. total_cache
515	
516	# The following additional stats are dependent on CONFIG_DEBUG_VM.
517	
518	recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
519	recent_rotated_file	- VM internal parameter. (see mm/vmscan.c)
520	recent_scanned_anon	- VM internal parameter. (see mm/vmscan.c)
521	recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)
522	
523	Memo:
524		recent_rotated means recent frequency of LRU rotation.
525		recent_scanned means recent # of scans to LRU.
526		showing for better debug please see the code for meanings.
527	
528	Note:
529		Only anonymous and swap cache memory is listed as part of 'rss' stat.
530		This should not be confused with the true 'resident set size' or the
531		amount of physical memory used by the cgroup.
532		'rss + file_mapped" will give you resident set size of cgroup.
533		(Note: file and shmem may be shared among other cgroups. In that case,
534		 file_mapped is accounted only when the memory cgroup is owner of page
535		 cache.)
536	
537	5.3 swappiness
538	
539	Overrides /proc/sys/vm/swappiness for the particular group. The tunable
540	in the root cgroup corresponds to the global swappiness setting.
541	
542	Please note that unlike during the global reclaim, limit reclaim
543	enforces that 0 swappiness really prevents from any swapping even if
544	there is a swap storage available. This might lead to memcg OOM killer
545	if there are no file pages to reclaim.
546	
547	5.4 failcnt
548	
549	A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
550	This failcnt(== failure count) shows the number of times that a usage counter
551	hit its limit. When a memory cgroup hits a limit, failcnt increases and
552	memory under it will be reclaimed.
553	
554	You can reset failcnt by writing 0 to failcnt file.
555	# echo 0 > .../memory.failcnt
556	
557	5.5 usage_in_bytes
558	
559	For efficiency, as other kernel components, memory cgroup uses some optimization
560	to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
561	method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
562	value for efficient access. (Of course, when necessary, it's synchronized.)
563	If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
564	value in memory.stat(see 5.2).
565	
566	5.6 numa_stat
567	
568	This is similar to numa_maps but operates on a per-memcg basis.  This is
569	useful for providing visibility into the numa locality information within
570	an memcg since the pages are allowed to be allocated from any physical
571	node.  One of the use cases is evaluating application performance by
572	combining this information with the application's CPU allocation.
573	
574	Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
575	per-node page counts including "hierarchical_<counter>" which sums up all
576	hierarchical children's values in addition to the memcg's own value.
577	
578	The output format of memory.numa_stat is:
579	
580	total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
581	file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
582	anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
583	unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
584	hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
585	
586	The "total" count is sum of file + anon + unevictable.
587	
588	6. Hierarchy support
589	
590	The memory controller supports a deep hierarchy and hierarchical accounting.
591	The hierarchy is created by creating the appropriate cgroups in the
592	cgroup filesystem. Consider for example, the following cgroup filesystem
593	hierarchy
594	
595		       root
596		     /  |   \
597	            /	|    \
598		   a	b     c
599			      | \
600			      |  \
601			      d   e
602	
603	In the diagram above, with hierarchical accounting enabled, all memory
604	usage of e, is accounted to its ancestors up until the root (i.e, c and root),
605	that has memory.use_hierarchy enabled. If one of the ancestors goes over its
606	limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
607	children of the ancestor.
608	
609	6.1 Enabling hierarchical accounting and reclaim
610	
611	A memory cgroup by default disables the hierarchy feature. Support
612	can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
613	
614	# echo 1 > memory.use_hierarchy
615	
616	The feature can be disabled by
617	
618	# echo 0 > memory.use_hierarchy
619	
620	NOTE1: Enabling/disabling will fail if either the cgroup already has other
621	       cgroups created below it, or if the parent cgroup has use_hierarchy
622	       enabled.
623	
624	NOTE2: When panic_on_oom is set to "2", the whole system will panic in
625	       case of an OOM event in any cgroup.
626	
627	7. Soft limits
628	
629	Soft limits allow for greater sharing of memory. The idea behind soft limits
630	is to allow control groups to use as much of the memory as needed, provided
631	
632	a. There is no memory contention
633	b. They do not exceed their hard limit
634	
635	When the system detects memory contention or low memory, control groups
636	are pushed back to their soft limits. If the soft limit of each control
637	group is very high, they are pushed back as much as possible to make
638	sure that one control group does not starve the others of memory.
639	
640	Please note that soft limits is a best-effort feature; it comes with
641	no guarantees, but it does its best to make sure that when memory is
642	heavily contended for, memory is allocated based on the soft limit
643	hints/setup. Currently soft limit based reclaim is set up such that
644	it gets invoked from balance_pgdat (kswapd).
645	
646	7.1 Interface
647	
648	Soft limits can be setup by using the following commands (in this example we
649	assume a soft limit of 256 MiB)
650	
651	# echo 256M > memory.soft_limit_in_bytes
652	
653	If we want to change this to 1G, we can at any time use
654	
655	# echo 1G > memory.soft_limit_in_bytes
656	
657	NOTE1: Soft limits take effect over a long period of time, since they involve
658	       reclaiming memory for balancing between memory cgroups
659	NOTE2: It is recommended to set the soft limit always below the hard limit,
660	       otherwise the hard limit will take precedence.
661	
662	8. Move charges at task migration
663	
664	Users can move charges associated with a task along with task migration, that
665	is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
666	This feature is not supported in !CONFIG_MMU environments because of lack of
667	page tables.
668	
669	8.1 Interface
670	
671	This feature is disabled by default. It can be enabled (and disabled again) by
672	writing to memory.move_charge_at_immigrate of the destination cgroup.
673	
674	If you want to enable it:
675	
676	# echo (some positive value) > memory.move_charge_at_immigrate
677	
678	Note: Each bits of move_charge_at_immigrate has its own meaning about what type
679	      of charges should be moved. See 8.2 for details.
680	Note: Charges are moved only when you move mm->owner, in other words,
681	      a leader of a thread group.
682	Note: If we cannot find enough space for the task in the destination cgroup, we
683	      try to make space by reclaiming memory. Task migration may fail if we
684	      cannot make enough space.
685	Note: It can take several seconds if you move charges much.
686	
687	And if you want disable it again:
688	
689	# echo 0 > memory.move_charge_at_immigrate
690	
691	8.2 Type of charges which can be moved
692	
693	Each bit in move_charge_at_immigrate has its own meaning about what type of
694	charges should be moved. But in any case, it must be noted that an account of
695	a page or a swap can be moved only when it is charged to the task's current
696	(old) memory cgroup.
697	
698	  bit | what type of charges would be moved ?
699	 -----+------------------------------------------------------------------------
700	   0  | A charge of an anonymous page (or swap of it) used by the target task.
701	      | You must enable Swap Extension (see 2.4) to enable move of swap charges.
702	 -----+------------------------------------------------------------------------
703	   1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
704	      | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
705	      | anonymous pages, file pages (and swaps) in the range mmapped by the task
706	      | will be moved even if the task hasn't done page fault, i.e. they might
707	      | not be the task's "RSS", but other task's "RSS" that maps the same file.
708	      | And mapcount of the page is ignored (the page can be moved even if
709	      | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
710	      | enable move of swap charges.
711	
712	8.3 TODO
713	
714	- All of moving charge operations are done under cgroup_mutex. It's not good
715	  behavior to hold the mutex too long, so we may need some trick.
716	
717	9. Memory thresholds
718	
719	Memory cgroup implements memory thresholds using the cgroups notification
720	API (see cgroups.txt). It allows to register multiple memory and memsw
721	thresholds and gets notifications when it crosses.
722	
723	To register a threshold, an application must:
724	- create an eventfd using eventfd(2);
725	- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
726	- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
727	  cgroup.event_control.
728	
729	Application will be notified through eventfd when memory usage crosses
730	threshold in any direction.
731	
732	It's applicable for root and non-root cgroup.
733	
734	10. OOM Control
735	
736	memory.oom_control file is for OOM notification and other controls.
737	
738	Memory cgroup implements OOM notifier using the cgroup notification
739	API (See cgroups.txt). It allows to register multiple OOM notification
740	delivery and gets notification when OOM happens.
741	
742	To register a notifier, an application must:
743	 - create an eventfd using eventfd(2)
744	 - open memory.oom_control file
745	 - write string like "<event_fd> <fd of memory.oom_control>" to
746	   cgroup.event_control
747	
748	The application will be notified through eventfd when OOM happens.
749	OOM notification doesn't work for the root cgroup.
750	
751	You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
752	
753		#echo 1 > memory.oom_control
754	
755	If OOM-killer is disabled, tasks under cgroup will hang/sleep
756	in memory cgroup's OOM-waitqueue when they request accountable memory.
757	
758	For running them, you have to relax the memory cgroup's OOM status by
759		* enlarge limit or reduce usage.
760	To reduce usage,
761		* kill some tasks.
762		* move some tasks to other group with account migration.
763		* remove some files (on tmpfs?)
764	
765	Then, stopped tasks will work again.
766	
767	At reading, current status of OOM is shown.
768		oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
769		under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
770					 be stopped.)
771	
772	11. Memory Pressure
773	
774	The pressure level notifications can be used to monitor the memory
775	allocation cost; based on the pressure, applications can implement
776	different strategies of managing their memory resources. The pressure
777	levels are defined as following:
778	
779	The "low" level means that the system is reclaiming memory for new
780	allocations. Monitoring this reclaiming activity might be useful for
781	maintaining cache level. Upon notification, the program (typically
782	"Activity Manager") might analyze vmstat and act in advance (i.e.
783	prematurely shutdown unimportant services).
784	
785	The "medium" level means that the system is experiencing medium memory
786	pressure, the system might be making swap, paging out active file caches,
787	etc. Upon this event applications may decide to further analyze
788	vmstat/zoneinfo/memcg or internal memory usage statistics and free any
789	resources that can be easily reconstructed or re-read from a disk.
790	
791	The "critical" level means that the system is actively thrashing, it is
792	about to out of memory (OOM) or even the in-kernel OOM killer is on its
793	way to trigger. Applications should do whatever they can to help the
794	system. It might be too late to consult with vmstat or any other
795	statistics, so it's advisable to take an immediate action.
796	
797	The events are propagated upward until the event is handled, i.e. the
798	events are not pass-through. Here is what this means: for example you have
799	three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
800	and C, and suppose group C experiences some pressure. In this situation,
801	only group C will receive the notification, i.e. groups A and B will not
802	receive it. This is done to avoid excessive "broadcasting" of messages,
803	which disturbs the system and which is especially bad if we are low on
804	memory or thrashing. So, organize the cgroups wisely, or propagate the
805	events manually (or, ask us to implement the pass-through events,
806	explaining why would you need them.)
807	
808	The file memory.pressure_level is only used to setup an eventfd. To
809	register a notification, an application must:
810	
811	- create an eventfd using eventfd(2);
812	- open memory.pressure_level;
813	- write string like "<event_fd> <fd of memory.pressure_level> <level>"
814	  to cgroup.event_control.
815	
816	Application will be notified through eventfd when memory pressure is at
817	the specific level (or higher). Read/write operations to
818	memory.pressure_level are no implemented.
819	
820	Test:
821	
822	   Here is a small script example that makes a new cgroup, sets up a
823	   memory limit, sets up a notification in the cgroup and then makes child
824	   cgroup experience a critical pressure:
825	
826	   # cd /sys/fs/cgroup/memory/
827	   # mkdir foo
828	   # cd foo
829	   # cgroup_event_listener memory.pressure_level low &
830	   # echo 8000000 > memory.limit_in_bytes
831	   # echo 8000000 > memory.memsw.limit_in_bytes
832	   # echo $$ > tasks
833	   # dd if=/dev/zero | read x
834	
835	   (Expect a bunch of notifications, and eventually, the oom-killer will
836	   trigger.)
837	
838	12. TODO
839	
840	1. Make per-cgroup scanner reclaim not-shared pages first
841	2. Teach controller to account for shared-pages
842	3. Start reclamation in the background when the limit is
843	   not yet hit but the usage is getting closer
844	
845	Summary
846	
847	Overall, the memory controller has been a stable controller and has been
848	commented and discussed quite extensively in the community.
849	
850	References
851	
852	1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
853	2. Singh, Balbir. Memory Controller (RSS Control),
854	   http://lwn.net/Articles/222762/
855	3. Emelianov, Pavel. Resource controllers based on process cgroups
856	   http://lkml.org/lkml/2007/3/6/198
857	4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
858	   http://lkml.org/lkml/2007/4/9/78
859	5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
860	   http://lkml.org/lkml/2007/5/30/244
861	6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
862	7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
863	   subsystem (v3), http://lwn.net/Articles/235534/
864	8. Singh, Balbir. RSS controller v2 test results (lmbench),
865	   http://lkml.org/lkml/2007/5/17/232
866	9. Singh, Balbir. RSS controller v2 AIM9 results
867	   http://lkml.org/lkml/2007/5/18/1
868	10. Singh, Balbir. Memory controller v6 test results,
869	    http://lkml.org/lkml/2007/8/19/36
870	11. Singh, Balbir. Memory controller introduction (v6),
871	    http://lkml.org/lkml/2007/8/17/69
872	12. Corbet, Jonathan, Controlling memory use in cgroups,
873	    http://lwn.net/Articles/243795/
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