summaryrefslogtreecommitdiff
path: root/Documentation/cgroups/memory.txt
blob: e2bc132608fd47c025fda39f7ee488598ad6f5fc (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
Memory Resource Controller

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

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

Benefits and Purpose of the memory controller

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

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

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

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

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

Brief summary of control files.

 tasks				 # attach a task(thread) and show list of threads
 cgroup.procs			 # show list of processes
 cgroup.event_control		 # an interface for event_fd()
 memory.usage_in_bytes		 # show current res_counter usage for memory
				 (See 5.5 for details)
 memory.memsw.usage_in_bytes	 # show current res_counter usage for memory+Swap
				 (See 5.5 for details)
 memory.limit_in_bytes		 # set/show limit of memory usage
 memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage
 memory.failcnt			 # show the number of memory usage hits limits
 memory.memsw.failcnt		 # show the number of memory+Swap hits limits
 memory.max_usage_in_bytes	 # show max memory usage recorded
 memory.memsw.max_usage_in_bytes # show max memory+Swap usage recorded
 memory.soft_limit_in_bytes	 # set/show soft limit of memory usage
 memory.stat			 # show various statistics
 memory.use_hierarchy		 # set/show hierarchical account enabled
 memory.force_empty		 # trigger forced move charge to parent
 memory.pressure_level		 # set memory pressure notifications
 memory.swappiness		 # set/show swappiness parameter of vmscan
				 (See sysctl's vm.swappiness)
 memory.move_charge_at_immigrate # set/show controls of moving charges
 memory.oom_control		 # set/show oom controls.
 memory.numa_stat		 # show the number of memory usage per numa node

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

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

1. History

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

2. Memory Control

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

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

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

The memory controller is the first controller developed.

2.1. Design

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

2.2. Accounting

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

             (Figure 1: Hierarchy of Accounting)


Figure 1 shows the important aspects of the controller

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

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

2.2.1 Accounting details

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

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

An RSS page is unaccounted when it's fully unmapped. A PageCache page is
unaccounted when it's removed from radix-tree. Even if RSS pages are fully
unmapped (by kswapd), they may exist as SwapCache in the system until they
are really freed. Such SwapCaches are also accounted.
A swapped-in page is not accounted until it's mapped.

Note: The kernel does swapin-readahead and reads multiple swaps at once.
This means swapped-in pages may contain pages for other tasks than a task
causing page fault. So, we avoid accounting at swap-in I/O.

At page migration, accounting information is kept.

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

2.3 Shared Page Accounting

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

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

Exception: If CONFIG_MEMCG_SWAP is not used.
When you do swapoff and make swapped-out pages of shmem(tmpfs) to
be backed into memory in force, charges for pages are accounted against the
caller of swapoff rather than the users of shmem.

2.4 Swap Extension (CONFIG_MEMCG_SWAP)

Swap Extension allows you to record charge for swap. A swapped-in page is
charged back to original page allocator if possible.

When swap is accounted, following files are added.
 - memory.memsw.usage_in_bytes.
 - memory.memsw.limit_in_bytes.

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

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

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

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

2.5 Reclaim

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

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

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

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

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

2.6 Locking

   lock_page_cgroup()/unlock_page_cgroup() should not be called under
   mapping->tree_lock.

   Other lock order is following:
   PG_locked.
   mm->page_table_lock
       zone->lru_lock
	  lock_page_cgroup.
  In many cases, just lock_page_cgroup() is called.
  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
  zone->lru_lock, it has no lock of its own.

2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM)

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

Kernel memory won't be accounted at all until limit on a group is set. This
allows for existing setups to continue working without disruption.  The limit
cannot be set if the cgroup have children, or if there are already tasks in the
cgroup. Attempting to set the limit under those conditions will return -EBUSY.
When use_hierarchy == 1 and a group is accounted, its children will
automatically be accounted regardless of their limit value.

After a group is first limited, it will be kept being accounted until it
is removed. The memory limitation itself, can of course be removed by writing
-1 to memory.kmem.limit_in_bytes. In this case, kmem will be accounted, but not
limited.

Kernel memory limits are not imposed for the root cgroup. Usage for the root
cgroup may or may not be accounted. The memory used is accumulated into
memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
(currently only for tcp).
The main "kmem" counter is fed into the main counter, so kmem charges will
also be visible from the user counter.

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

2.7.1 Current Kernel Memory resources accounted

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

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

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

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

2.7.3 Common use cases

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

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

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

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

3. User Interface

0. Configuration

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

1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
# mount -t tmpfs none /sys/fs/cgroup
# mkdir /sys/fs/cgroup/memory
# mount -t cgroup none /sys/fs/cgroup/memory -o memory

2. Make the new group and move bash into it
# mkdir /sys/fs/cgroup/memory/0
# echo $$ > /sys/fs/cgroup/memory/0/tasks

Since now we're in the 0 cgroup, we can alter the memory limit:
# echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes

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

NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
NOTE: We cannot set limits on the root cgroup any more.

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

We can check the usage:
# cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
1216512

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

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

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

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

4. Testing

For testing features and implementation, see memcg_test.txt.

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

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

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

4.1 Troubleshooting

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

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

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

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

4.2 Task migration

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

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

4.3 Removing a cgroup

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

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

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

About use_hierarchy, see Section 6.

5. Misc. interfaces.

5.1 force_empty
  memory.force_empty interface is provided to make cgroup's memory usage empty.
  You can use this interface only when the cgroup has no tasks.
  When writing anything to this

  # echo 0 > memory.force_empty

  Almost all pages tracked by this memory cgroup will be unmapped and freed.
  Some pages cannot be freed because they are locked or in-use. Such pages are
  moved to parent (if use_hierarchy==1) or root (if use_hierarchy==0) and this
  cgroup will be empty.

  The typical use case for this interface is before calling rmdir().
  Because rmdir() moves all pages to parent, some out-of-use page caches can be
  moved to the parent. If you want to avoid that, force_empty will be useful.

  Also, note that when memory.kmem.limit_in_bytes is set the charges due to
  kernel pages will still be seen. This is not considered a failure and the
  write will still return success. In this case, it is expected that
  memory.kmem.usage_in_bytes == memory.usage_in_bytes.

  About use_hierarchy, see Section 6.

5.2 stat file

memory.stat file includes following statistics

# per-memory cgroup local status
cache		- # of bytes of page cache memory.
rss		- # of bytes of anonymous and swap cache memory (includes
		transparent hugepages).
rss_huge	- # of bytes of anonymous transparent hugepages.
mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
pgpgin		- # of charging events to the memory cgroup. The charging
		event happens each time a page is accounted as either mapped
		anon page(RSS) or cache page(Page Cache) to the cgroup.
pgpgout		- # of uncharging events to the memory cgroup. The uncharging
		event happens each time a page is unaccounted from the cgroup.
swap		- # of bytes of swap usage
writeback	- # of bytes of file/anon cache that are queued for syncing to
		disk.
inactive_anon	- # of bytes of anonymous and swap cache memory on inactive
		LRU list.
active_anon	- # of bytes of anonymous and swap cache memory on active
		LRU list.
inactive_file	- # of bytes of file-backed memory on inactive LRU list.
active_file	- # of bytes of file-backed memory on active LRU list.
unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).

# status considering hierarchy (see memory.use_hierarchy settings)

hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
			under which the memory cgroup is
hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
			hierarchy under which memory cgroup is.

total_<counter>		- # hierarchical version of <counter>, which in
			addition to the cgroup's own value includes the
			sum of all hierarchical children's values of
			<counter>, i.e. total_cache

# The following additional stats are dependent on CONFIG_DEBUG_VM.

recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
recent_rotated_file	- VM internal parameter. (see mm/vmscan.c)
recent_scanned_anon	- VM internal parameter. (see mm/vmscan.c)
recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)

Memo:
	recent_rotated means recent frequency of LRU rotation.
	recent_scanned means recent # of scans to LRU.
	showing for better debug please see the code for meanings.

Note:
	Only anonymous and swap cache memory is listed as part of 'rss' stat.
	This should not be confused with the true 'resident set size' or the
	amount of physical memory used by the cgroup.
	'rss + file_mapped" will give you resident set size of cgroup.
	(Note: file and shmem may be shared among other cgroups. In that case,
	 file_mapped is accounted only when the memory cgroup is owner of page
	 cache.)

5.3 swappiness

Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
Please note that unlike the global swappiness, memcg knob set to 0
really prevents from any swapping even if there is a swap storage
available. This might lead to memcg OOM killer if there are no file
pages to reclaim.

Following cgroups' swappiness can't be changed.
- root cgroup (uses /proc/sys/vm/swappiness).
- a cgroup which uses hierarchy and it has other cgroup(s) below it.
- a cgroup which uses hierarchy and not the root of hierarchy.

5.4 failcnt

A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
This failcnt(== failure count) shows the number of times that a usage counter
hit its limit. When a memory cgroup hits a limit, failcnt increases and
memory under it will be reclaimed.

You can reset failcnt by writing 0 to failcnt file.
# echo 0 > .../memory.failcnt

5.5 usage_in_bytes

For efficiency, as other kernel components, memory cgroup uses some optimization
to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
value for efficient access. (Of course, when necessary, it's synchronized.)
If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
value in memory.stat(see 5.2).

5.6 numa_stat

This is similar to numa_maps but operates on a per-memcg basis.  This is
useful for providing visibility into the numa locality information within
an memcg since the pages are allowed to be allocated from any physical
node.  One of the use cases is evaluating application performance by
combining this information with the application's CPU allocation.

Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
per-node page counts including "hierarchical_<counter>" which sums up all
hierarchical children's values in addition to the memcg's own value.

The ouput format of memory.numa_stat is:

total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...

The "total" count is sum of file + anon + unevictable.

6. Hierarchy support

The memory controller supports a deep hierarchy and hierarchical accounting.
The hierarchy is created by creating the appropriate cgroups in the
cgroup filesystem. Consider for example, the following cgroup filesystem
hierarchy

	       root
	     /  |   \
            /	|    \
	   a	b     c
		      | \
		      |  \
		      d   e

In the diagram above, with hierarchical accounting enabled, all memory
usage of e, is accounted to its ancestors up until the root (i.e, c and root),
that has memory.use_hierarchy enabled. If one of the ancestors goes over its
limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
children of the ancestor.

6.1 Enabling hierarchical accounting and reclaim

A memory cgroup by default disables the hierarchy feature. Support
can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup

# echo 1 > memory.use_hierarchy

The feature can be disabled by

# echo 0 > memory.use_hierarchy

NOTE1: Enabling/disabling will fail if either the cgroup already has other
       cgroups created below it, or if the parent cgroup has use_hierarchy
       enabled.

NOTE2: When panic_on_oom is set to "2", the whole system will panic in
       case of an OOM event in any cgroup.

7. Soft limits

Soft limits allow for greater sharing of memory. The idea behind soft limits
is to allow control groups to use as much of the memory as needed, provided

a. There is no memory contention
b. They do not exceed their hard limit

When the system detects memory contention or low memory, control groups
are pushed back to their soft limits. If the soft limit of each control
group is very high, they are pushed back as much as possible to make
sure that one control group does not starve the others of memory.

Please note that soft limits is a best-effort feature; it comes with
no guarantees, but it does its best to make sure that when memory is
heavily contended for, memory is allocated based on the soft limit
hints/setup. Currently soft limit based reclaim is set up such that
it gets invoked from balance_pgdat (kswapd).

7.1 Interface

Soft limits can be setup by using the following commands (in this example we
assume a soft limit of 256 MiB)

# echo 256M > memory.soft_limit_in_bytes

If we want to change this to 1G, we can at any time use

# echo 1G > memory.soft_limit_in_bytes

NOTE1: Soft limits take effect over a long period of time, since they involve
       reclaiming memory for balancing between memory cgroups
NOTE2: It is recommended to set the soft limit always below the hard limit,
       otherwise the hard limit will take precedence.

8. Move charges at task migration

Users can move charges associated with a task along with task migration, that
is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
This feature is not supported in !CONFIG_MMU environments because of lack of
page tables.

8.1 Interface

This feature is disabled by default. It can be enabledi (and disabled again) by
writing to memory.move_charge_at_immigrate of the destination cgroup.

If you want to enable it:

# echo (some positive value) > memory.move_charge_at_immigrate

Note: Each bits of move_charge_at_immigrate has its own meaning about what type
      of charges should be moved. See 8.2 for details.
Note: Charges are moved only when you move mm->owner, in other words,
      a leader of a thread group.
Note: If we cannot find enough space for the task in the destination cgroup, we
      try to make space by reclaiming memory. Task migration may fail if we
      cannot make enough space.
Note: It can take several seconds if you move charges much.

And if you want disable it again:

# echo 0 > memory.move_charge_at_immigrate

8.2 Type of charges which can be moved

Each bit in move_charge_at_immigrate has its own meaning about what type of
charges should be moved. But in any case, it must be noted that an account of
a page or a swap can be moved only when it is charged to the task's current
(old) memory cgroup.

  bit | what type of charges would be moved ?
 -----+------------------------------------------------------------------------
   0  | A charge of an anonymous page (or swap of it) used by the target task.
      | You must enable Swap Extension (see 2.4) to enable move of swap charges.
 -----+------------------------------------------------------------------------
   1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
      | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
      | anonymous pages, file pages (and swaps) in the range mmapped by the task
      | will be moved even if the task hasn't done page fault, i.e. they might
      | not be the task's "RSS", but other task's "RSS" that maps the same file.
      | And mapcount of the page is ignored (the page can be moved even if
      | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
      | enable move of swap charges.

8.3 TODO

- All of moving charge operations are done under cgroup_mutex. It's not good
  behavior to hold the mutex too long, so we may need some trick.

9. Memory thresholds

Memory cgroup implements memory thresholds using the cgroups notification
API (see cgroups.txt). It allows to register multiple memory and memsw
thresholds and gets notifications when it crosses.

To register a threshold, an application must:
- create an eventfd using eventfd(2);
- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
  cgroup.event_control.

Application will be notified through eventfd when memory usage crosses
threshold in any direction.

It's applicable for root and non-root cgroup.

10. OOM Control

memory.oom_control file is for OOM notification and other controls.

Memory cgroup implements OOM notifier using the cgroup notification
API (See cgroups.txt). It allows to register multiple OOM notification
delivery and gets notification when OOM happens.

To register a notifier, an application must:
 - create an eventfd using eventfd(2)
 - open memory.oom_control file
 - write string like "<event_fd> <fd of memory.oom_control>" to
   cgroup.event_control

The application will be notified through eventfd when OOM happens.
OOM notification doesn't work for the root cgroup.

You can disable the OOM-killer by writing "1" to memory.oom_control file, as:

	#echo 1 > memory.oom_control

This operation is only allowed to the top cgroup of a sub-hierarchy.
If OOM-killer is disabled, tasks under cgroup will hang/sleep
in memory cgroup's OOM-waitqueue when they request accountable memory.

For running them, you have to relax the memory cgroup's OOM status by
	* enlarge limit or reduce usage.
To reduce usage,
	* kill some tasks.
	* move some tasks to other group with account migration.
	* remove some files (on tmpfs?)

Then, stopped tasks will work again.

At reading, current status of OOM is shown.
	oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
	under_oom	 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
				 be stopped.)

11. Memory Pressure

The pressure level notifications can be used to monitor the memory
allocation cost; based on the pressure, applications can implement
different strategies of managing their memory resources. The pressure
levels are defined as following:

The "low" level means that the system is reclaiming memory for new
allocations. Monitoring this reclaiming activity might be useful for
maintaining cache level. Upon notification, the program (typically
"Activity Manager") might analyze vmstat and act in advance (i.e.
prematurely shutdown unimportant services).

The "medium" level means that the system is experiencing medium memory
pressure, the system might be making swap, paging out active file caches,
etc. Upon this event applications may decide to further analyze
vmstat/zoneinfo/memcg or internal memory usage statistics and free any
resources that can be easily reconstructed or re-read from a disk.

The "critical" level means that the system is actively thrashing, it is
about to out of memory (OOM) or even the in-kernel OOM killer is on its
way to trigger. Applications should do whatever they can to help the
system. It might be too late to consult with vmstat or any other
statistics, so it's advisable to take an immediate action.

The events are propagated upward until the event is handled, i.e. the
events are not pass-through. Here is what this means: for example you have
three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
and C, and suppose group C experiences some pressure. In this situation,
only group C will receive the notification, i.e. groups A and B will not
receive it. This is done to avoid excessive "broadcasting" of messages,
which disturbs the system and which is especially bad if we are low on
memory or thrashing. So, organize the cgroups wisely, or propagate the
events manually (or, ask us to implement the pass-through events,
explaining why would you need them.)

The file memory.pressure_level is only used to setup an eventfd. To
register a notification, an application must:

- create an eventfd using eventfd(2);
- open memory.pressure_level;
- write string like "<event_fd> <fd of memory.pressure_level> <level>"
  to cgroup.event_control.

Application will be notified through eventfd when memory pressure is at
the specific level (or higher). Read/write operations to
memory.pressure_level are no implemented.

Test:

   Here is a small script example that makes a new cgroup, sets up a
   memory limit, sets up a notification in the cgroup and then makes child
   cgroup experience a critical pressure:

   # cd /sys/fs/cgroup/memory/
   # mkdir foo
   # cd foo
   # cgroup_event_listener memory.pressure_level low &
   # echo 8000000 > memory.limit_in_bytes
   # echo 8000000 > memory.memsw.limit_in_bytes
   # echo $$ > tasks
   # dd if=/dev/zero | read x

   (Expect a bunch of notifications, and eventually, the oom-killer will
   trigger.)

12. TODO

1. Make per-cgroup scanner reclaim not-shared pages first
2. Teach controller to account for shared-pages
3. Start reclamation in the background when the limit is
   not yet hit but the usage is getting closer

Summary

Overall, the memory controller has been a stable controller and has been
commented and discussed quite extensively in the community.

References

1. Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/
2. Singh, Balbir. Memory Controller (RSS Control),
   http://lwn.net/Articles/222762/
3. Emelianov, Pavel. Resource controllers based on process cgroups
   http://lkml.org/lkml/2007/3/6/198
4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
   http://lkml.org/lkml/2007/4/9/78
5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
   http://lkml.org/lkml/2007/5/30/244
6. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/
7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
   subsystem (v3), http://lwn.net/Articles/235534/
8. Singh, Balbir. RSS controller v2 test results (lmbench),
   http://lkml.org/lkml/2007/5/17/232
9. Singh, Balbir. RSS controller v2 AIM9 results
   http://lkml.org/lkml/2007/5/18/1
10. Singh, Balbir. Memory controller v6 test results,
    http://lkml.org/lkml/2007/8/19/36
11. Singh, Balbir. Memory controller introduction (v6),
    http://lkml.org/lkml/2007/8/17/69
12. Corbet, Jonathan, Controlling memory use in cgroups,
    http://lwn.net/Articles/243795/