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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 EST.

1	Memory Resource Controller
3	NOTE: This document is hopelessly outdated and it asks for a complete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.
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.
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.
19	Benefits and Purpose of the memory controller
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
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).
38	Current Status: linux-2.6.34-mmotm(development version of 2010/April)
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.
52	 Kernel memory support is a work in progress, and the current version provides
53	 basically functionality. (See Section 2.7)
55	Brief summary of control files.
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
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
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
91	1. History
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].
105	2. Memory Control
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.
112	The memory controller implementation has been divided into phases. These
113	are:
115	1. Memory controller
116	2. mlock(2) controller
117	3. Kernel user memory accounting and slab control
118	4. user mappings length controller
120	The memory controller is the first controller developed.
122	2.1. Design
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.
129	2.2. Accounting
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	           +---------------+           +---------------+
149	             (Figure 1: Hierarchy of Accounting)
152	Figure 1 shows the important aspects of the controller
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
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.
167	2.2.1 Accounting details
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.
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.
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.
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.
188	At page migration, accounting information is kept.
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.
193	2.3 Shared Page Accounting
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).
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.
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.
209	2.4 Swap Extension (CONFIG_MEMCG_SWAP)
211	Swap Extension allows you to record charge for swap. A swapped-in page is
212	charged back to original page allocator if possible.
214	When swap is accounted, following files are added.
215	 - memory.memsw.usage_in_bytes.
216	 - memory.memsw.limit_in_bytes.
218	memsw means memory+swap. Usage of memory+swap is limited by
219	memsw.limit_in_bytes.
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.
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.
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.
241	2.5 Reclaim
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.)
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.
254	NOTE: Reclaim does not work for the root cgroup, since we cannot set any
255	limits on the root cgroup.
257	Note2: When panic_on_oom is set to "2", the whole system will panic.
259	When oom event notifier is registered, event will be delivered.
260	(See oom_control section)
262	2.6 Locking
264	   lock_page_cgroup()/unlock_page_cgroup() should not be called under
265	   mapping->tree_lock.
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.
276	2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM)
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.
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.
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.
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.
302	Currently no soft limit is implemented for kernel memory. It is future work
303	to trigger slab reclaim when those limits are reached.
305	2.7.1 Current Kernel Memory resources accounted
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.
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.
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.
322	* tcp memory pressure: sockets memory pressure for the tcp protocol.
324	2.7.2 Common use cases
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:
331	    U != 0, K = unlimited:
332	    This is the standard memcg limitation mechanism already present before kmem
333	    accounting. Kernel memory is completely ignored.
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	    WARNING: In the current implementation, memory reclaim will NOT be
344	    triggered for a cgroup when it hits K while staying below U, which makes
345	    this setup impractical.
347	    U != 0, K >= U:
348	    Since kmem charges will also be fed to the user counter and reclaim will be
349	    triggered for the cgroup for both kinds of memory. This setup gives the
350	    admin a unified view of memory, and it is also useful for people who just
351	    want to track kernel memory usage.
353	3. User Interface
355	3.0. Configuration
357	a. Enable CONFIG_CGROUPS
358	b. Enable CONFIG_MEMCG
359	c. Enable CONFIG_MEMCG_SWAP (to use swap extension)
360	d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)
362	3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
363	# mount -t tmpfs none /sys/fs/cgroup
364	# mkdir /sys/fs/cgroup/memory
365	# mount -t cgroup none /sys/fs/cgroup/memory -o memory
367	3.2. Make the new group and move bash into it
368	# mkdir /sys/fs/cgroup/memory/0
369	# echo $$ > /sys/fs/cgroup/memory/0/tasks
371	Since now we're in the 0 cgroup, we can alter the memory limit:
372	# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
374	NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
375	mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
377	NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
378	NOTE: We cannot set limits on the root cgroup any more.
380	# cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
381	4194304
383	We can check the usage:
384	# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
385	1216512
387	A successful write to this file does not guarantee a successful setting of
388	this limit to the value written into the file. This can be due to a
389	number of factors, such as rounding up to page boundaries or the total
390	availability of memory on the system. The user is required to re-read
391	this file after a write to guarantee the value committed by the kernel.
393	# echo 1 > memory.limit_in_bytes
394	# cat memory.limit_in_bytes
395	4096
397	The memory.failcnt field gives the number of times that the cgroup limit was
398	exceeded.
400	The memory.stat file gives accounting information. Now, the number of
401	caches, RSS and Active pages/Inactive pages are shown.
403	4. Testing
405	For testing features and implementation, see memcg_test.txt.
407	Performance test is also important. To see pure memory controller's overhead,
408	testing on tmpfs will give you good numbers of small overheads.
409	Example: do kernel make on tmpfs.
411	Page-fault scalability is also important. At measuring parallel
412	page fault test, multi-process test may be better than multi-thread
413	test because it has noise of shared objects/status.
415	But the above two are testing extreme situations.
416	Trying usual test under memory controller is always helpful.
418	4.1 Troubleshooting
420	Sometimes a user might find that the application under a cgroup is
421	terminated by the OOM killer. There are several causes for this:
423	1. The cgroup limit is too low (just too low to do anything useful)
424	2. The user is using anonymous memory and swap is turned off or too low
426	A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
427	some of the pages cached in the cgroup (page cache pages).
429	To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
430	seeing what happens will be helpful.
432	4.2 Task migration
434	When a task migrates from one cgroup to another, its charge is not
435	carried forward by default. The pages allocated from the original cgroup still
436	remain charged to it, the charge is dropped when the page is freed or
437	reclaimed.
439	You can move charges of a task along with task migration.
440	See 8. "Move charges at task migration"
442	4.3 Removing a cgroup
444	A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
445	cgroup might have some charge associated with it, even though all
446	tasks have migrated away from it. (because we charge against pages, not
447	against tasks.)
449	We move the stats to root (if use_hierarchy==0) or parent (if
450	use_hierarchy==1), and no change on the charge except uncharging
451	from the child.
453	Charges recorded in swap information is not updated at removal of cgroup.
454	Recorded information is discarded and a cgroup which uses swap (swapcache)
455	will be charged as a new owner of it.
457	About use_hierarchy, see Section 6.
459	5. Misc. interfaces.
461	5.1 force_empty
462	  memory.force_empty interface is provided to make cgroup's memory usage empty.
463	  When writing anything to this
465	  # echo 0 > memory.force_empty
467	  the cgroup will be reclaimed and as many pages reclaimed as possible.
469	  The typical use case for this interface is before calling rmdir().
470	  Because rmdir() moves all pages to parent, some out-of-use page caches can be
471	  moved to the parent. If you want to avoid that, force_empty will be useful.
473	  Also, note that when memory.kmem.limit_in_bytes is set the charges due to
474	  kernel pages will still be seen. This is not considered a failure and the
475	  write will still return success. In this case, it is expected that
476	  memory.kmem.usage_in_bytes == memory.usage_in_bytes.
478	  About use_hierarchy, see Section 6.
480	5.2 stat file
482	memory.stat file includes following statistics
484	# per-memory cgroup local status
485	cache		- # of bytes of page cache memory.
486	rss		- # of bytes of anonymous and swap cache memory (includes
487			transparent hugepages).
488	rss_huge	- # of bytes of anonymous transparent hugepages.
489	mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
490	pgpgin		- # of charging events to the memory cgroup. The charging
491			event happens each time a page is accounted as either mapped
492			anon page(RSS) or cache page(Page Cache) to the cgroup.
493	pgpgout		- # of uncharging events to the memory cgroup. The uncharging
494			event happens each time a page is unaccounted from the cgroup.
495	swap		- # of bytes of swap usage
496	writeback	- # of bytes of file/anon cache that are queued for syncing to
497			disk.
498	inactive_anon	- # of bytes of anonymous and swap cache memory on inactive
499			LRU list.
500	active_anon	- # of bytes of anonymous and swap cache memory on active
501			LRU list.
502	inactive_file	- # of bytes of file-backed memory on inactive LRU list.
503	active_file	- # of bytes of file-backed memory on active LRU list.
504	unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).
506	# status considering hierarchy (see memory.use_hierarchy settings)
508	hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
509				under which the memory cgroup is
510	hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
511				hierarchy under which memory cgroup is.
513	total_<counter>		- # hierarchical version of <counter>, which in
514				addition to the cgroup's own value includes the
515				sum of all hierarchical children's values of
516				<counter>, i.e. total_cache
518	# The following additional stats are dependent on CONFIG_DEBUG_VM.
520	recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
521	recent_rotated_file	- VM internal parameter. (see mm/vmscan.c)
522	recent_scanned_anon	- VM internal parameter. (see mm/vmscan.c)
523	recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)
525	Memo:
526		recent_rotated means recent frequency of LRU rotation.
527		recent_scanned means recent # of scans to LRU.
528		showing for better debug please see the code for meanings.
530	Note:
531		Only anonymous and swap cache memory is listed as part of 'rss' stat.
532		This should not be confused with the true 'resident set size' or the
533		amount of physical memory used by the cgroup.
534		'rss + file_mapped" will give you resident set size of cgroup.
535		(Note: file and shmem may be shared among other cgroups. In that case,
536		 file_mapped is accounted only when the memory cgroup is owner of page
537		 cache.)
539	5.3 swappiness
541	Overrides /proc/sys/vm/swappiness for the particular group. The tunable
542	in the root cgroup corresponds to the global swappiness setting.
544	Please note that unlike during the global reclaim, limit reclaim
545	enforces that 0 swappiness really prevents from any swapping even if
546	there is a swap storage available. This might lead to memcg OOM killer
547	if there are no file pages to reclaim.
549	5.4 failcnt
551	A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
552	This failcnt(== failure count) shows the number of times that a usage counter
553	hit its limit. When a memory cgroup hits a limit, failcnt increases and
554	memory under it will be reclaimed.
556	You can reset failcnt by writing 0 to failcnt file.
557	# echo 0 > .../memory.failcnt
559	5.5 usage_in_bytes
561	For efficiency, as other kernel components, memory cgroup uses some optimization
562	to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
563	method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
564	value for efficient access. (Of course, when necessary, it's synchronized.)
565	If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
566	value in memory.stat(see 5.2).
568	5.6 numa_stat
570	This is similar to numa_maps but operates on a per-memcg basis.  This is
571	useful for providing visibility into the numa locality information within
572	an memcg since the pages are allowed to be allocated from any physical
573	node.  One of the use cases is evaluating application performance by
574	combining this information with the application's CPU allocation.
576	Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
577	per-node page counts including "hierarchical_<counter>" which sums up all
578	hierarchical children's values in addition to the memcg's own value.
580	The output format of memory.numa_stat is:
582	total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
583	file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
584	anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
585	unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
586	hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
588	The "total" count is sum of file + anon + unevictable.
590	6. Hierarchy support
592	The memory controller supports a deep hierarchy and hierarchical accounting.
593	The hierarchy is created by creating the appropriate cgroups in the
594	cgroup filesystem. Consider for example, the following cgroup filesystem
595	hierarchy
597		       root
598		     /  |   \
599	            /	|    \
600		   a	b     c
601			      | \
602			      |  \
603			      d   e
605	In the diagram above, with hierarchical accounting enabled, all memory
606	usage of e, is accounted to its ancestors up until the root (i.e, c and root),
607	that has memory.use_hierarchy enabled. If one of the ancestors goes over its
608	limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
609	children of the ancestor.
611	6.1 Enabling hierarchical accounting and reclaim
613	A memory cgroup by default disables the hierarchy feature. Support
614	can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
616	# echo 1 > memory.use_hierarchy
618	The feature can be disabled by
620	# echo 0 > memory.use_hierarchy
622	NOTE1: Enabling/disabling will fail if either the cgroup already has other
623	       cgroups created below it, or if the parent cgroup has use_hierarchy
624	       enabled.
626	NOTE2: When panic_on_oom is set to "2", the whole system will panic in
627	       case of an OOM event in any cgroup.
629	7. Soft limits
631	Soft limits allow for greater sharing of memory. The idea behind soft limits
632	is to allow control groups to use as much of the memory as needed, provided
634	a. There is no memory contention
635	b. They do not exceed their hard limit
637	When the system detects memory contention or low memory, control groups
638	are pushed back to their soft limits. If the soft limit of each control
639	group is very high, they are pushed back as much as possible to make
640	sure that one control group does not starve the others of memory.
642	Please note that soft limits is a best-effort feature; it comes with
643	no guarantees, but it does its best to make sure that when memory is
644	heavily contended for, memory is allocated based on the soft limit
645	hints/setup. Currently soft limit based reclaim is set up such that
646	it gets invoked from balance_pgdat (kswapd).
648	7.1 Interface
650	Soft limits can be setup by using the following commands (in this example we
651	assume a soft limit of 256 MiB)
653	# echo 256M > memory.soft_limit_in_bytes
655	If we want to change this to 1G, we can at any time use
657	# echo 1G > memory.soft_limit_in_bytes
659	NOTE1: Soft limits take effect over a long period of time, since they involve
660	       reclaiming memory for balancing between memory cgroups
661	NOTE2: It is recommended to set the soft limit always below the hard limit,
662	       otherwise the hard limit will take precedence.
664	8. Move charges at task migration
666	Users can move charges associated with a task along with task migration, that
667	is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
668	This feature is not supported in !CONFIG_MMU environments because of lack of
669	page tables.
671	8.1 Interface
673	This feature is disabled by default. It can be enabled (and disabled again) by
674	writing to memory.move_charge_at_immigrate of the destination cgroup.
676	If you want to enable it:
678	# echo (some positive value) > memory.move_charge_at_immigrate
680	Note: Each bits of move_charge_at_immigrate has its own meaning about what type
681	      of charges should be moved. See 8.2 for details.
682	Note: Charges are moved only when you move mm->owner, in other words,
683	      a leader of a thread group.
684	Note: If we cannot find enough space for the task in the destination cgroup, we
685	      try to make space by reclaiming memory. Task migration may fail if we
686	      cannot make enough space.
687	Note: It can take several seconds if you move charges much.
689	And if you want disable it again:
691	# echo 0 > memory.move_charge_at_immigrate
693	8.2 Type of charges which can be moved
695	Each bit in move_charge_at_immigrate has its own meaning about what type of
696	charges should be moved. But in any case, it must be noted that an account of
697	a page or a swap can be moved only when it is charged to the task's current
698	(old) memory cgroup.
700	  bit | what type of charges would be moved ?
701	 -----+------------------------------------------------------------------------
702	   0  | A charge of an anonymous page (or swap of it) used by the target task.
703	      | You must enable Swap Extension (see 2.4) to enable move of swap charges.
704	 -----+------------------------------------------------------------------------
705	   1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
706	      | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
707	      | anonymous pages, file pages (and swaps) in the range mmapped by the task
708	      | will be moved even if the task hasn't done page fault, i.e. they might
709	      | not be the task's "RSS", but other task's "RSS" that maps the same file.
710	      | And mapcount of the page is ignored (the page can be moved even if
711	      | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
712	      | enable move of swap charges.
714	8.3 TODO
716	- All of moving charge operations are done under cgroup_mutex. It's not good
717	  behavior to hold the mutex too long, so we may need some trick.
719	9. Memory thresholds
721	Memory cgroup implements memory thresholds using the cgroups notification
722	API (see cgroups.txt). It allows to register multiple memory and memsw
723	thresholds and gets notifications when it crosses.
725	To register a threshold, an application must:
726	- create an eventfd using eventfd(2);
727	- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
728	- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
729	  cgroup.event_control.
731	Application will be notified through eventfd when memory usage crosses
732	threshold in any direction.
734	It's applicable for root and non-root cgroup.
736	10. OOM Control
738	memory.oom_control file is for OOM notification and other controls.
740	Memory cgroup implements OOM notifier using the cgroup notification
741	API (See cgroups.txt). It allows to register multiple OOM notification
742	delivery and gets notification when OOM happens.
744	To register a notifier, an application must:
745	 - create an eventfd using eventfd(2)
746	 - open memory.oom_control file
747	 - write string like "<event_fd> <fd of memory.oom_control>" to
748	   cgroup.event_control
750	The application will be notified through eventfd when OOM happens.
751	OOM notification doesn't work for the root cgroup.
753	You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
755		#echo 1 > memory.oom_control
757	If OOM-killer is disabled, tasks under cgroup will hang/sleep
758	in memory cgroup's OOM-waitqueue when they request accountable memory.
760	For running them, you have to relax the memory cgroup's OOM status by
761		* enlarge limit or reduce usage.
762	To reduce usage,
763		* kill some tasks.
764		* move some tasks to other group with account migration.
765		* remove some files (on tmpfs?)
767	Then, stopped tasks will work again.
769	At reading, current status of OOM is shown.
770		oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
771		under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
772					 be stopped.)
774	11. Memory Pressure
776	The pressure level notifications can be used to monitor the memory
777	allocation cost; based on the pressure, applications can implement
778	different strategies of managing their memory resources. The pressure
779	levels are defined as following:
781	The "low" level means that the system is reclaiming memory for new
782	allocations. Monitoring this reclaiming activity might be useful for
783	maintaining cache level. Upon notification, the program (typically
784	"Activity Manager") might analyze vmstat and act in advance (i.e.
785	prematurely shutdown unimportant services).
787	The "medium" level means that the system is experiencing medium memory
788	pressure, the system might be making swap, paging out active file caches,
789	etc. Upon this event applications may decide to further analyze
790	vmstat/zoneinfo/memcg or internal memory usage statistics and free any
791	resources that can be easily reconstructed or re-read from a disk.
793	The "critical" level means that the system is actively thrashing, it is
794	about to out of memory (OOM) or even the in-kernel OOM killer is on its
795	way to trigger. Applications should do whatever they can to help the
796	system. It might be too late to consult with vmstat or any other
797	statistics, so it's advisable to take an immediate action.
799	The events are propagated upward until the event is handled, i.e. the
800	events are not pass-through. Here is what this means: for example you have
801	three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
802	and C, and suppose group C experiences some pressure. In this situation,
803	only group C will receive the notification, i.e. groups A and B will not
804	receive it. This is done to avoid excessive "broadcasting" of messages,
805	which disturbs the system and which is especially bad if we are low on
806	memory or thrashing. So, organize the cgroups wisely, or propagate the
807	events manually (or, ask us to implement the pass-through events,
808	explaining why would you need them.)
810	The file memory.pressure_level is only used to setup an eventfd. To
811	register a notification, an application must:
813	- create an eventfd using eventfd(2);
814	- open memory.pressure_level;
815	- write string like "<event_fd> <fd of memory.pressure_level> <level>"
816	  to cgroup.event_control.
818	Application will be notified through eventfd when memory pressure is at
819	the specific level (or higher). Read/write operations to
820	memory.pressure_level are no implemented.
822	Test:
824	   Here is a small script example that makes a new cgroup, sets up a
825	   memory limit, sets up a notification in the cgroup and then makes child
826	   cgroup experience a critical pressure:
828	   # cd /sys/fs/cgroup/memory/
829	   # mkdir foo
830	   # cd foo
831	   # cgroup_event_listener memory.pressure_level low &
832	   # echo 8000000 > memory.limit_in_bytes
833	   # echo 8000000 > memory.memsw.limit_in_bytes
834	   # echo $$ > tasks
835	   # dd if=/dev/zero | read x
837	   (Expect a bunch of notifications, and eventually, the oom-killer will
838	   trigger.)
840	12. TODO
842	1. Make per-cgroup scanner reclaim not-shared pages first
843	2. Teach controller to account for shared-pages
844	3. Start reclamation in the background when the limit is
845	   not yet hit but the usage is getting closer
847	Summary
849	Overall, the memory controller has been a stable controller and has been
850	commented and discussed quite extensively in the community.
852	References
854	1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
855	2. Singh, Balbir. Memory Controller (RSS Control),
856	   http://lwn.net/Articles/222762/
857	3. Emelianov, Pavel. Resource controllers based on process cgroups
858	   http://lkml.org/lkml/2007/3/6/198
859	4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
860	   http://lkml.org/lkml/2007/4/9/78
861	5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
862	   http://lkml.org/lkml/2007/5/30/244
863	6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
864	7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
865	   subsystem (v3), http://lwn.net/Articles/235534/
866	8. Singh, Balbir. RSS controller v2 test results (lmbench),
867	   http://lkml.org/lkml/2007/5/17/232
868	9. Singh, Balbir. RSS controller v2 AIM9 results
869	   http://lkml.org/lkml/2007/5/18/1
870	10. Singh, Balbir. Memory controller v6 test results,
871	    http://lkml.org/lkml/2007/8/19/36
872	11. Singh, Balbir. Memory controller introduction (v6),
873	    http://lkml.org/lkml/2007/8/17/69
874	12. Corbet, Jonathan, Controlling memory use in cgroups,
875	    http://lwn.net/Articles/243795/
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