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  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  3 NOTE: This document is hopelessly outdated and it asks for a complete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 accounting is enabled for all memory cgroups by default. But
284 it can be disabled system-wide by passing cgroup.memory=nokmem to the kernel
285 at boot time. In this case, kernel memory will not be accounted at all.
287 Kernel memory limits are not imposed for the root cgroup. Usage for the root
288 cgroup may or may not be accounted. The memory used is accumulated into
289 memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
290 (currently only for tcp).
291 The main "kmem" counter is fed into the main counter, so kmem charges will
292 also be visible from the user counter.
294 Currently no soft limit is implemented for kernel memory. It is future work
295 to trigger slab reclaim when those limits are reached.
297 2.7.1 Current Kernel Memory resources accounted
299 * stack pages: every process consumes some stack pages. By accounting into
300 kernel memory, we prevent new processes from being created when the kernel
301 memory usage is too high.
303 * slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy
304 of each kmem_cache is created every time the cache is touched by the first time
305 from inside the memcg. The creation is done lazily, so some objects can still be
306 skipped while the cache is being created. All objects in a slab page should
307 belong to the same memcg. This only fails to hold when a task is migrated to a
308 different memcg during the page allocation by the cache.
310 * sockets memory pressure: some sockets protocols have memory pressure
311 thresholds. The Memory Controller allows them to be controlled individually
312 per cgroup, instead of globally.
314 * tcp memory pressure: sockets memory pressure for the tcp protocol.
316 2.7.2 Common use cases
318 Because the "kmem" counter is fed to the main user counter, kernel memory can
319 never be limited completely independently of user memory. Say "U" is the user
320 limit, and "K" the kernel limit. There are three possible ways limits can be
321 set:
323     U != 0, K = unlimited:
324     This is the standard memcg limitation mechanism already present before kmem
325     accounting. Kernel memory is completely ignored.
327     U != 0, K < U:
328     Kernel memory is a subset of the user memory. This setup is useful in
329     deployments where the total amount of memory per-cgroup is overcommited.
330     Overcommiting kernel memory limits is definitely not recommended, since the
331     box can still run out of non-reclaimable memory.
332     In this case, the admin could set up K so that the sum of all groups is
333     never greater than the total memory, and freely set U at the cost of his
334     QoS.
335     WARNING: In the current implementation, memory reclaim will NOT be
336     triggered for a cgroup when it hits K while staying below U, which makes
337     this setup impractical.
339     U != 0, K >= U:
340     Since kmem charges will also be fed to the user counter and reclaim will be
341     triggered for the cgroup for both kinds of memory. This setup gives the
342     admin a unified view of memory, and it is also useful for people who just
343     want to track kernel memory usage.
345 3. User Interface
347 3.0. Configuration
349 a. Enable CONFIG_CGROUPS
350 b. Enable CONFIG_MEMCG
351 c. Enable CONFIG_MEMCG_SWAP (to use swap extension)
352 d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)
354 3.1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
355 # mount -t tmpfs none /sys/fs/cgroup
356 # mkdir /sys/fs/cgroup/memory
357 # mount -t cgroup none /sys/fs/cgroup/memory -o memory
359 3.2. Make the new group and move bash into it
360 # mkdir /sys/fs/cgroup/memory/0
361 # echo $$ > /sys/fs/cgroup/memory/0/tasks
363 Since now we're in the 0 cgroup, we can alter the memory limit:
364 # echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
366 NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
367 mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
369 NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
370 NOTE: We cannot set limits on the root cgroup any more.
372 # cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes
373 4194304
375 We can check the usage:
376 # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
377 1216512
379 A successful write to this file does not guarantee a successful setting of
380 this limit to the value written into the file. This can be due to a
381 number of factors, such as rounding up to page boundaries or the total
382 availability of memory on the system. The user is required to re-read
383 this file after a write to guarantee the value committed by the kernel.
385 # echo 1 > memory.limit_in_bytes
386 # cat memory.limit_in_bytes
387 4096
389 The memory.failcnt field gives the number of times that the cgroup limit was
390 exceeded.
392 The memory.stat file gives accounting information. Now, the number of
393 caches, RSS and Active pages/Inactive pages are shown.
395 4. Testing
397 For testing features and implementation, see memcg_test.txt.
399 Performance test is also important. To see pure memory controller's overhead,
400 testing on tmpfs will give you good numbers of small overheads.
401 Example: do kernel make on tmpfs.
403 Page-fault scalability is also important. At measuring parallel
404 page fault test, multi-process test may be better than multi-thread
405 test because it has noise of shared objects/status.
407 But the above two are testing extreme situations.
408 Trying usual test under memory controller is always helpful.
410 4.1 Troubleshooting
412 Sometimes a user might find that the application under a cgroup is
413 terminated by the OOM killer. There are several causes for this:
415 1. The cgroup limit is too low (just too low to do anything useful)
416 2. The user is using anonymous memory and swap is turned off or too low
418 A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
419 some of the pages cached in the cgroup (page cache pages).
421 To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
422 seeing what happens will be helpful.
424 4.2 Task migration
426 When a task migrates from one cgroup to another, its charge is not
427 carried forward by default. The pages allocated from the original cgroup still
428 remain charged to it, the charge is dropped when the page is freed or
429 reclaimed.
431 You can move charges of a task along with task migration.
432 See 8. "Move charges at task migration"
434 4.3 Removing a cgroup
436 A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
437 cgroup might have some charge associated with it, even though all
438 tasks have migrated away from it. (because we charge against pages, not
439 against tasks.)
441 We move the stats to root (if use_hierarchy==0) or parent (if
442 use_hierarchy==1), and no change on the charge except uncharging
443 from the child.
445 Charges recorded in swap information is not updated at removal of cgroup.
446 Recorded information is discarded and a cgroup which uses swap (swapcache)
447 will be charged as a new owner of it.
449 About use_hierarchy, see Section 6.
451 5. Misc. interfaces.
453 5.1 force_empty
454   memory.force_empty interface is provided to make cgroup's memory usage empty.
455   When writing anything to this
457   # echo 0 > memory.force_empty
459   the cgroup will be reclaimed and as many pages reclaimed as possible.
461   The typical use case for this interface is before calling rmdir().
462   Because rmdir() moves all pages to parent, some out-of-use page caches can be
463   moved to the parent. If you want to avoid that, force_empty will be useful.
465   Also, note that when memory.kmem.limit_in_bytes is set the charges due to
466   kernel pages will still be seen. This is not considered a failure and the
467   write will still return success. In this case, it is expected that
468   memory.kmem.usage_in_bytes == memory.usage_in_bytes.
470   About use_hierarchy, see Section 6.
472 5.2 stat file
474 memory.stat file includes following statistics
476 # per-memory cgroup local status
477 cache           - # of bytes of page cache memory.
478 rss             - # of bytes of anonymous and swap cache memory (includes
479                 transparent hugepages).
480 rss_huge        - # of bytes of anonymous transparent hugepages.
481 mapped_file     - # of bytes of mapped file (includes tmpfs/shmem)
482 pgpgin          - # of charging events to the memory cgroup. The charging
483                 event happens each time a page is accounted as either mapped
484                 anon page(RSS) or cache page(Page Cache) to the cgroup.
485 pgpgout         - # of uncharging events to the memory cgroup. The uncharging
486                 event happens each time a page is unaccounted from the cgroup.
487 swap            - # of bytes of swap usage
488 dirty           - # of bytes that are waiting to get written back to the disk.
489 writeback       - # of bytes of file/anon cache that are queued for syncing to
490                 disk.
491 inactive_anon   - # of bytes of anonymous and swap cache memory on inactive
492                 LRU list.
493 active_anon     - # of bytes of anonymous and swap cache memory on active
494                 LRU list.
495 inactive_file   - # of bytes of file-backed memory on inactive LRU list.
496 active_file     - # of bytes of file-backed memory on active LRU list.
497 unevictable     - # of bytes of memory that cannot be reclaimed (mlocked etc).
499 # status considering hierarchy (see memory.use_hierarchy settings)
501 hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
502                         under which the memory cgroup is
503 hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
504                         hierarchy under which memory cgroup is.
506 total_<counter>         - # hierarchical version of <counter>, which in
507                         addition to the cgroup's own value includes the
508                         sum of all hierarchical children's values of
509                         <counter>, i.e. total_cache
511 # The following additional stats are dependent on CONFIG_DEBUG_VM.
513 recent_rotated_anon     - VM internal parameter. (see mm/vmscan.c)
514 recent_rotated_file     - VM internal parameter. (see mm/vmscan.c)
515 recent_scanned_anon     - VM internal parameter. (see mm/vmscan.c)
516 recent_scanned_file     - VM internal parameter. (see mm/vmscan.c)
518 Memo:
519         recent_rotated means recent frequency of LRU rotation.
520         recent_scanned means recent # of scans to LRU.
521         showing for better debug please see the code for meanings.
523 Note:
524         Only anonymous and swap cache memory is listed as part of 'rss' stat.
525         This should not be confused with the true 'resident set size' or the
526         amount of physical memory used by the cgroup.
527         'rss + file_mapped" will give you resident set size of cgroup.
528         (Note: file and shmem may be shared among other cgroups. In that case,
529          file_mapped is accounted only when the memory cgroup is owner of page
530          cache.)
532 5.3 swappiness
534 Overrides /proc/sys/vm/swappiness for the particular group. The tunable
535 in the root cgroup corresponds to the global swappiness setting.
537 Please note that unlike during the global reclaim, limit reclaim
538 enforces that 0 swappiness really prevents from any swapping even if
539 there is a swap storage available. This might lead to memcg OOM killer
540 if there are no file pages to reclaim.
542 5.4 failcnt
544 A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
545 This failcnt(== failure count) shows the number of times that a usage counter
546 hit its limit. When a memory cgroup hits a limit, failcnt increases and
547 memory under it will be reclaimed.
549 You can reset failcnt by writing 0 to failcnt file.
550 # echo 0 > .../memory.failcnt
552 5.5 usage_in_bytes
554 For efficiency, as other kernel components, memory cgroup uses some optimization
555 to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
556 method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
557 value for efficient access. (Of course, when necessary, it's synchronized.)
558 If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
559 value in memory.stat(see 5.2).
561 5.6 numa_stat
563 This is similar to numa_maps but operates on a per-memcg basis.  This is
564 useful for providing visibility into the numa locality information within
565 an memcg since the pages are allowed to be allocated from any physical
566 node.  One of the use cases is evaluating application performance by
567 combining this information with the application's CPU allocation.
569 Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable"
570 per-node page counts including "hierarchical_<counter>" which sums up all
571 hierarchical children's values in addition to the memcg's own value.
573 The output format of memory.numa_stat is:
575 total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ...
576 file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ...
577 anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
578 unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ...
579 hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ...
581 The "total" count is sum of file + anon + unevictable.
583 6. Hierarchy support
585 The memory controller supports a deep hierarchy and hierarchical accounting.
586 The hierarchy is created by creating the appropriate cgroups in the
587 cgroup filesystem. Consider for example, the following cgroup filesystem
588 hierarchy
590                root
591              /  |   \
592             /   |    \
593            a    b     c
594                       | \
595                       |  \
596                       d   e
598 In the diagram above, with hierarchical accounting enabled, all memory
599 usage of e, is accounted to its ancestors up until the root (i.e, c and root),
600 that has memory.use_hierarchy enabled. If one of the ancestors goes over its
601 limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
602 children of the ancestor.
604 6.1 Enabling hierarchical accounting and reclaim
606 A memory cgroup by default disables the hierarchy feature. Support
607 can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
609 # echo 1 > memory.use_hierarchy
611 The feature can be disabled by
613 # echo 0 > memory.use_hierarchy
615 NOTE1: Enabling/disabling will fail if either the cgroup already has other
616        cgroups created below it, or if the parent cgroup has use_hierarchy
617        enabled.
619 NOTE2: When panic_on_oom is set to "2", the whole system will panic in
620        case of an OOM event in any cgroup.
622 7. Soft limits
624 Soft limits allow for greater sharing of memory. The idea behind soft limits
625 is to allow control groups to use as much of the memory as needed, provided
627 a. There is no memory contention
628 b. They do not exceed their hard limit
630 When the system detects memory contention or low memory, control groups
631 are pushed back to their soft limits. If the soft limit of each control
632 group is very high, they are pushed back as much as possible to make
633 sure that one control group does not starve the others of memory.
635 Please note that soft limits is a best-effort feature; it comes with
636 no guarantees, but it does its best to make sure that when memory is
637 heavily contended for, memory is allocated based on the soft limit
638 hints/setup. Currently soft limit based reclaim is set up such that
639 it gets invoked from balance_pgdat (kswapd).
641 7.1 Interface
643 Soft limits can be setup by using the following commands (in this example we
644 assume a soft limit of 256 MiB)
646 # echo 256M > memory.soft_limit_in_bytes
648 If we want to change this to 1G, we can at any time use
650 # echo 1G > memory.soft_limit_in_bytes
652 NOTE1: Soft limits take effect over a long period of time, since they involve
653        reclaiming memory for balancing between memory cgroups
654 NOTE2: It is recommended to set the soft limit always below the hard limit,
655        otherwise the hard limit will take precedence.
657 8. Move charges at task migration
659 Users can move charges associated with a task along with task migration, that
660 is, uncharge task's pages from the old cgroup and charge them to the new cgroup.
661 This feature is not supported in !CONFIG_MMU environments because of lack of
662 page tables.
664 8.1 Interface
666 This feature is disabled by default. It can be enabled (and disabled again) by
667 writing to memory.move_charge_at_immigrate of the destination cgroup.
669 If you want to enable it:
671 # echo (some positive value) > memory.move_charge_at_immigrate
673 Note: Each bits of move_charge_at_immigrate has its own meaning about what type
674       of charges should be moved. See 8.2 for details.
675 Note: Charges are moved only when you move mm->owner, in other words,
676       a leader of a thread group.
677 Note: If we cannot find enough space for the task in the destination cgroup, we
678       try to make space by reclaiming memory. Task migration may fail if we
679       cannot make enough space.
680 Note: It can take several seconds if you move charges much.
682 And if you want disable it again:
684 # echo 0 > memory.move_charge_at_immigrate
686 8.2 Type of charges which can be moved
688 Each bit in move_charge_at_immigrate has its own meaning about what type of
689 charges should be moved. But in any case, it must be noted that an account of
690 a page or a swap can be moved only when it is charged to the task's current
691 (old) memory cgroup.
693   bit | what type of charges would be moved ?
694  -----+------------------------------------------------------------------------
695    0  | A charge of an anonymous page (or swap of it) used by the target task.
696       | You must enable Swap Extension (see 2.4) to enable move of swap charges.
697  -----+------------------------------------------------------------------------
698    1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
699       | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
700       | anonymous pages, file pages (and swaps) in the range mmapped by the task
701       | will be moved even if the task hasn't done page fault, i.e. they might
702       | not be the task's "RSS", but other task's "RSS" that maps the same file.
703       | And mapcount of the page is ignored (the page can be moved even if
704       | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
705       | enable move of swap charges.
707 8.3 TODO
709 - All of moving charge operations are done under cgroup_mutex. It's not good
710   behavior to hold the mutex too long, so we may need some trick.
712 9. Memory thresholds
714 Memory cgroup implements memory thresholds using the cgroups notification
715 API (see cgroups.txt). It allows to register multiple memory and memsw
716 thresholds and gets notifications when it crosses.
718 To register a threshold, an application must:
719 - create an eventfd using eventfd(2);
720 - open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
721 - write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
722   cgroup.event_control.
724 Application will be notified through eventfd when memory usage crosses
725 threshold in any direction.
727 It's applicable for root and non-root cgroup.
729 10. OOM Control
731 memory.oom_control file is for OOM notification and other controls.
733 Memory cgroup implements OOM notifier using the cgroup notification
734 API (See cgroups.txt). It allows to register multiple OOM notification
735 delivery and gets notification when OOM happens.
737 To register a notifier, an application must:
738  - create an eventfd using eventfd(2)
739  - open memory.oom_control file
740  - write string like "<event_fd> <fd of memory.oom_control>" to
741    cgroup.event_control
743 The application will be notified through eventfd when OOM happens.
744 OOM notification doesn't work for the root cgroup.
746 You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
748         #echo 1 > memory.oom_control
750 If OOM-killer is disabled, tasks under cgroup will hang/sleep
751 in memory cgroup's OOM-waitqueue when they request accountable memory.
753 For running them, you have to relax the memory cgroup's OOM status by
754         * enlarge limit or reduce usage.
755 To reduce usage,
756         * kill some tasks.
757         * move some tasks to other group with account migration.
758         * remove some files (on tmpfs?)
760 Then, stopped tasks will work again.
762 At reading, current status of OOM is shown.
763         oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
764         under_oom        0 or 1 (if 1, the memory cgroup is under OOM, tasks may
765                                  be stopped.)
767 11. Memory Pressure
769 The pressure level notifications can be used to monitor the memory
770 allocation cost; based on the pressure, applications can implement
771 different strategies of managing their memory resources. The pressure
772 levels are defined as following:
774 The "low" level means that the system is reclaiming memory for new
775 allocations. Monitoring this reclaiming activity might be useful for
776 maintaining cache level. Upon notification, the program (typically
777 "Activity Manager") might analyze vmstat and act in advance (i.e.
778 prematurely shutdown unimportant services).
780 The "medium" level means that the system is experiencing medium memory
781 pressure, the system might be making swap, paging out active file caches,
782 etc. Upon this event applications may decide to further analyze
783 vmstat/zoneinfo/memcg or internal memory usage statistics and free any
784 resources that can be easily reconstructed or re-read from a disk.
786 The "critical" level means that the system is actively thrashing, it is
787 about to out of memory (OOM) or even the in-kernel OOM killer is on its
788 way to trigger. Applications should do whatever they can to help the
789 system. It might be too late to consult with vmstat or any other
790 statistics, so it's advisable to take an immediate action.
792 The events are propagated upward until the event is handled, i.e. the
793 events are not pass-through. Here is what this means: for example you have
794 three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
795 and C, and suppose group C experiences some pressure. In this situation,
796 only group C will receive the notification, i.e. groups A and B will not
797 receive it. This is done to avoid excessive "broadcasting" of messages,
798 which disturbs the system and which is especially bad if we are low on
799 memory or thrashing. So, organize the cgroups wisely, or propagate the
800 events manually (or, ask us to implement the pass-through events,
801 explaining why would you need them.)
803 The file memory.pressure_level is only used to setup an eventfd. To
804 register a notification, an application must:
806 - create an eventfd using eventfd(2);
807 - open memory.pressure_level;
808 - write string like "<event_fd> <fd of memory.pressure_level> <level>"
809   to cgroup.event_control.
811 Application will be notified through eventfd when memory pressure is at
812 the specific level (or higher). Read/write operations to
813 memory.pressure_level are no implemented.
815 Test:
817    Here is a small script example that makes a new cgroup, sets up a
818    memory limit, sets up a notification in the cgroup and then makes child
819    cgroup experience a critical pressure:
821    # cd /sys/fs/cgroup/memory/
822    # mkdir foo
823    # cd foo
824    # cgroup_event_listener memory.pressure_level low &
825    # echo 8000000 > memory.limit_in_bytes
826    # echo 8000000 > memory.memsw.limit_in_bytes
827    # echo $$ > tasks
828    # dd if=/dev/zero | read x
830    (Expect a bunch of notifications, and eventually, the oom-killer will
831    trigger.)
833 12. TODO
835 1. Make per-cgroup scanner reclaim not-shared pages first
836 2. Teach controller to account for shared-pages
837 3. Start reclamation in the background when the limit is
838    not yet hit but the usage is getting closer
840 Summary
842 Overall, the memory controller has been a stable controller and has been
843 commented and discussed quite extensively in the community.
845 References
847 1. Singh, Balbir. RFC: Memory Controller,
848 2. Singh, Balbir. Memory Controller (RSS Control),
850 3. Emelianov, Pavel. Resource controllers based on process cgroups
852 4. Emelianov, Pavel. RSS controller based on process cgroups (v2)
854 5. Emelianov, Pavel. RSS controller based on process cgroups (v3)
856 6. Menage, Paul. Control Groups v10,
857 7. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control
858    subsystem (v3),
859 8. Singh, Balbir. RSS controller v2 test results (lmbench),
861 9. Singh, Balbir. RSS controller v2 AIM9 results
863 10. Singh, Balbir. Memory controller v6 test results,
865 11. Singh, Balbir. Memory controller introduction (v6),
867 12. Corbet, Jonathan, Controlling memory use in cgroups,

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