1========================== 2Memory Resource Controller 3========================== 4 5.. caution:: 6 This document is hopelessly outdated and it asks for a complete 7 rewrite. It still contains a useful information so we are keeping it 8 here but make sure to check the current code if you need a deeper 9 understanding. 10 11.. note:: 12 The Memory Resource Controller has generically been referred to as the 13 memory controller in this document. Do not confuse memory controller 14 used here with the memory controller that is used in hardware. 15 16.. hint:: 17 When we mention a cgroup (cgroupfs's directory) with memory controller, 18 we call it "memory cgroup". When you see git-log and source code, you'll 19 see patch's title and function names tend to use "memcg". 20 In this document, we avoid using it. 21 22Benefits and Purpose of the memory controller 23============================================= 24 25The memory controller isolates the memory behaviour of a group of tasks 26from the rest of the system. The article on LWN [12]_ mentions some probable 27uses of the memory controller. The memory controller can be used to 28 29a. Isolate an application or a group of applications 30 Memory-hungry applications can be isolated and limited to a smaller 31 amount of memory. 32b. Create a cgroup with a limited amount of memory; this can be used 33 as a good alternative to booting with mem=XXXX. 34c. Virtualization solutions can control the amount of memory they want 35 to assign to a virtual machine instance. 36d. A CD/DVD burner could control the amount of memory used by the 37 rest of the system to ensure that burning does not fail due to lack 38 of available memory. 39e. There are several other use cases; find one or use the controller just 40 for fun (to learn and hack on the VM subsystem). 41 42Current Status: linux-2.6.34-mmotm(development version of 2010/April) 43 44Features: 45 46 - accounting anonymous pages, file caches, swap caches usage and limiting them. 47 - pages are linked to per-memcg LRU exclusively, and there is no global LRU. 48 - optionally, memory+swap usage can be accounted and limited. 49 - hierarchical accounting 50 - moving (recharging) account at moving a task is selectable. 51 - usage threshold notifier 52 - memory pressure notifier 53 - oom-killer disable knob and oom-notifier 54 - Root cgroup has no limit controls. 55 56 Kernel memory support is a work in progress, and the current version provides 57 basically functionality. (See :ref:`section 2.7 58 <cgroup-v1-memory-kernel-extension>`) 59 60Brief summary of control files. 61 62==================================== ========================================== 63 tasks attach a task(thread) and show list of 64 threads 65 cgroup.procs show list of processes 66 cgroup.event_control an interface for event_fd() 67 This knob is not available on CONFIG_PREEMPT_RT systems. 68 memory.usage_in_bytes show current usage for memory 69 (See 5.5 for details) 70 memory.memsw.usage_in_bytes show current usage for memory+Swap 71 (See 5.5 for details) 72 memory.limit_in_bytes set/show limit of memory usage 73 memory.memsw.limit_in_bytes set/show limit of memory+Swap usage 74 memory.failcnt show the number of memory usage hits limits 75 memory.memsw.failcnt show the number of memory+Swap hits limits 76 memory.max_usage_in_bytes show max memory usage recorded 77 memory.memsw.max_usage_in_bytes show max memory+Swap usage recorded 78 memory.soft_limit_in_bytes This knob is deprecated and has no effect. 79 Writes are ignored and reads always 80 return the maximum value. 81 memory.stat show various statistics 82 memory.use_hierarchy set/show hierarchical account enabled 83 This knob is deprecated and shouldn't be 84 used. 85 memory.force_empty trigger forced page reclaim 86 memory.pressure_level set memory pressure notifications 87 This knob is deprecated and shouldn't be 88 used. 89 memory.swappiness set/show swappiness parameter of vmscan 90 (See sysctl's vm.swappiness) 91 Per memcg knob does not exist in cgroup v2. 92 memory.move_charge_at_immigrate This knob is deprecated. 93 memory.oom_control set/show oom controls. 94 This knob is deprecated and shouldn't be 95 used. 96 memory.numa_stat show the number of memory usage per numa 97 node 98 memory.kmem.limit_in_bytes Deprecated knob to set and read the kernel 99 memory hard limit. Kernel hard limit is not 100 supported since 5.16. Writing any value to 101 do file will not have any effect same as if 102 nokmem kernel parameter was specified. 103 Kernel memory is still charged and reported 104 by memory.kmem.usage_in_bytes. 105 memory.kmem.usage_in_bytes show current kernel memory allocation 106 memory.kmem.failcnt show the number of kernel memory usage 107 hits limits 108 memory.kmem.max_usage_in_bytes show max kernel memory usage recorded 109 110 memory.kmem.tcp.limit_in_bytes set/show hard limit for tcp buf memory 111 This knob is deprecated and shouldn't be 112 used. 113 memory.kmem.tcp.usage_in_bytes show current tcp buf memory allocation 114 This knob is deprecated and shouldn't be 115 used. 116 memory.kmem.tcp.failcnt show the number of tcp buf memory usage 117 hits limits 118 This knob is deprecated and shouldn't be 119 used. 120 memory.kmem.tcp.max_usage_in_bytes show max tcp buf memory usage recorded 121 This knob is deprecated and shouldn't be 122 used. 123==================================== ========================================== 124 1251. History 126========== 127 128The memory controller has a long history. A request for comments for the memory 129controller was posted by Balbir Singh [1]_. At the time the RFC was posted 130there were several implementations for memory control. The goal of the 131RFC was to build consensus and agreement for the minimal features required 132for memory control. The first RSS controller was posted by Balbir Singh [2]_ 133in Feb 2007. Pavel Emelianov [3]_ [4]_ [5]_ has since posted three versions 134of the RSS controller. At OLS, at the resource management BoF, everyone 135suggested that we handle both page cache and RSS together. Another request was 136raised to allow user space handling of OOM. The current memory controller is 137at version 6; it combines both mapped (RSS) and unmapped Page 138Cache Control [11]_. 139 1402. Memory Control 141================= 142 143Memory is a unique resource in the sense that it is present in a limited 144amount. If a task requires a lot of CPU processing, the task can spread 145its processing over a period of hours, days, months or years, but with 146memory, the same physical memory needs to be reused to accomplish the task. 147 148The memory controller implementation has been divided into phases. These 149are: 150 1511. Memory controller 1522. mlock(2) controller 1533. Kernel user memory accounting and slab control 1544. user mappings length controller 155 156The memory controller is the first controller developed. 157 1582.1. Design 159----------- 160 161The core of the design is a counter called the page_counter. The 162page_counter tracks the current memory usage and limit of the group of 163processes associated with the controller. Each cgroup has a memory controller 164specific data structure (mem_cgroup) associated with it. 165 1662.2. Accounting 167--------------- 168 169.. code-block:: 170 :caption: Figure 1: Hierarchy of Accounting 171 172 +--------------------+ 173 | mem_cgroup | 174 | (page_counter) | 175 +--------------------+ 176 / ^ \ 177 / | \ 178 +---------------+ | +---------------+ 179 | mm_struct | |.... | mm_struct | 180 | | | | | 181 +---------------+ | +---------------+ 182 | 183 + --------------+ 184 | 185 +---------------+ +------+--------+ 186 | page +----------> page_cgroup| 187 | | | | 188 +---------------+ +---------------+ 189 190 191 192Figure 1 shows the important aspects of the controller 193 1941. Accounting happens per cgroup 1952. Each mm_struct knows about which cgroup it belongs to 1963. Each page has a pointer to the page_cgroup, which in turn knows the 197 cgroup it belongs to 198 199The accounting is done as follows: mem_cgroup_charge_common() is invoked to 200set up the necessary data structures and check if the cgroup that is being 201charged is over its limit. If it is, then reclaim is invoked on the cgroup. 202More details can be found in the reclaim section of this document. 203If everything goes well, a page meta-data-structure called page_cgroup is 204updated. page_cgroup has its own LRU on cgroup. 205(*) page_cgroup structure is allocated at boot/memory-hotplug time. 206 2072.2.1 Accounting details 208------------------------ 209 210All mapped anon pages (RSS) and cache pages (Page Cache) are accounted. 211Some pages which are never reclaimable and will not be on the LRU 212are not accounted. We just account pages under usual VM management. 213 214RSS pages are accounted at page_fault unless they've already been accounted 215for earlier. A file page will be accounted for as Page Cache when it's 216inserted into inode (xarray). While it's mapped into the page tables of 217processes, duplicate accounting is carefully avoided. 218 219An RSS page is unaccounted when it's fully unmapped. A PageCache page is 220unaccounted when it's removed from xarray. Even if RSS pages are fully 221unmapped (by kswapd), they may exist as SwapCache in the system until they 222are really freed. Such SwapCaches are also accounted. 223A swapped-in page is accounted after adding into swapcache. 224 225Note: The kernel does swapin-readahead and reads multiple swaps at once. 226Since page's memcg recorded into swap whatever memsw enabled, the page will 227be accounted after swapin. 228 229At page migration, accounting information is kept. 230 231Note: we just account pages-on-LRU because our purpose is to control amount 232of used pages; not-on-LRU pages tend to be out-of-control from VM view. 233 2342.3 Shared Page Accounting 235-------------------------- 236 237Shared pages are accounted on the basis of the first touch approach. The 238cgroup that first touches a page is accounted for the page. The principle 239behind this approach is that a cgroup that aggressively uses a shared 240page will eventually get charged for it (once it is uncharged from 241the cgroup that brought it in -- this will happen on memory pressure). 242 2432.4 Swap Extension 244-------------------------------------- 245 246Swap usage is always recorded for each of cgroup. Swap Extension allows you to 247read and limit it. 248 249When CONFIG_SWAP is enabled, following files are added. 250 251 - memory.memsw.usage_in_bytes. 252 - memory.memsw.limit_in_bytes. 253 254memsw means memory+swap. Usage of memory+swap is limited by 255memsw.limit_in_bytes. 256 257Example: Assume a system with 4G of swap. A task which allocates 6G of memory 258(by mistake) under 2G memory limitation will use all swap. 259In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap. 260By using the memsw limit, you can avoid system OOM which can be caused by swap 261shortage. 262 2632.4.1 why 'memory+swap' rather than swap 264~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 265 266The global LRU(kswapd) can swap out arbitrary pages. Swap-out means 267to move account from memory to swap...there is no change in usage of 268memory+swap. In other words, when we want to limit the usage of swap without 269affecting global LRU, memory+swap limit is better than just limiting swap from 270an OS point of view. 271 2722.4.2. What happens when a cgroup hits memory.memsw.limit_in_bytes 273~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 274 275When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out 276in this cgroup. Then, swap-out will not be done by cgroup routine and file 277caches are dropped. But as mentioned above, global LRU can do swapout memory 278from it for sanity of the system's memory management state. You can't forbid 279it by cgroup. 280 2812.5 Reclaim 282----------- 283 284Each cgroup maintains a per cgroup LRU which has the same structure as 285global VM. When a cgroup goes over its limit, we first try 286to reclaim memory from the cgroup so as to make space for the new 287pages that the cgroup has touched. If the reclaim is unsuccessful, 288an OOM routine is invoked to select and kill the bulkiest task in the 289cgroup. (See :ref:`10. OOM Control <cgroup-v1-memory-oom-control>` below.) 290 291The reclaim algorithm has not been modified for cgroups, except that 292pages that are selected for reclaiming come from the per-cgroup LRU 293list. 294 295.. note:: 296 Reclaim does not work for the root cgroup, since we cannot set any 297 limits on the root cgroup. 298 299.. note:: 300 When panic_on_oom is set to "2", the whole system will panic. 301 302When oom event notifier is registered, event will be delivered. 303(See :ref:`oom_control <cgroup-v1-memory-oom-control>` section) 304 3052.6 Locking 306----------- 307 308Lock order is as follows:: 309 310 folio_lock 311 mm->page_table_lock or split pte_lock 312 mapping->i_pages lock 313 lruvec->lru_lock. 314 315Per-node-per-memcgroup LRU (cgroup's private LRU) is guarded by 316lruvec->lru_lock; the folio LRU flag is cleared before 317isolating a page from its LRU under lruvec->lru_lock. 318 319.. _cgroup-v1-memory-kernel-extension: 320 3212.7 Kernel Memory Extension 322----------------------------------------------- 323 324With the Kernel memory extension, the Memory Controller is able to limit 325the amount of kernel memory used by the system. Kernel memory is fundamentally 326different than user memory, since it can't be swapped out, which makes it 327possible to DoS the system by consuming too much of this precious resource. 328 329Kernel memory accounting is enabled for all memory cgroups by default. But 330it can be disabled system-wide by passing cgroup.memory=nokmem to the kernel 331at boot time. In this case, kernel memory will not be accounted at all. 332 333Kernel memory limits are not imposed for the root cgroup. Usage for the root 334cgroup may or may not be accounted. The memory used is accumulated into 335memory.kmem.usage_in_bytes, or in a separate counter when it makes sense. 336(currently only for tcp). 337 338The main "kmem" counter is fed into the main counter, so kmem charges will 339also be visible from the user counter. 340 3412.7.1 Current Kernel Memory resources accounted 342----------------------------------------------- 343 344stack pages: 345 every process consumes some stack pages. By accounting into 346 kernel memory, we prevent new processes from being created when the kernel 347 memory usage is too high. 348 349slab pages: 350 pages allocated by the SLAB or SLUB allocator are tracked. A copy 351 of each kmem_cache is created every time the cache is touched by the first time 352 from inside the memcg. The creation is done lazily, so some objects can still be 353 skipped while the cache is being created. All objects in a slab page should 354 belong to the same memcg. This only fails to hold when a task is migrated to a 355 different memcg during the page allocation by the cache. 356 357sockets memory pressure: 358 some sockets protocols have memory pressure 359 thresholds. The Memory Controller allows them to be controlled individually 360 per cgroup, instead of globally. 361 362tcp memory pressure: 363 sockets memory pressure for the tcp protocol. 364 3652.7.2 Common use cases 366---------------------- 367 368Because the "kmem" counter is fed to the main user counter, kernel memory can 369never be limited completely independently of user memory. Say "U" is the user 370limit, and "K" the kernel limit. There are three possible ways limits can be 371set: 372 373U != 0, K = unlimited: 374 This is the standard memcg limitation mechanism already present before kmem 375 accounting. Kernel memory is completely ignored. 376 377U != 0, K < U: 378 Kernel memory is a subset of the user memory. This setup is useful in 379 deployments where the total amount of memory per-cgroup is overcommitted. 380 Overcommitting kernel memory limits is definitely not recommended, since the 381 box can still run out of non-reclaimable memory. 382 In this case, the admin could set up K so that the sum of all groups is 383 never greater than the total memory, and freely set U at the cost of his 384 QoS. 385 386 .. warning:: 387 In the current implementation, memory reclaim will NOT be triggered for 388 a cgroup when it hits K while staying below U, which makes this setup 389 impractical. 390 391U != 0, K >= U: 392 Since kmem charges will also be fed to the user counter and reclaim will be 393 triggered for the cgroup for both kinds of memory. This setup gives the 394 admin a unified view of memory, and it is also useful for people who just 395 want to track kernel memory usage. 396 3973. User Interface 398================= 399 400To use the user interface: 401 4021. Enable CONFIG_CGROUPS and CONFIG_MEMCG options 4032. Prepare the cgroups (see :ref:`Why are cgroups needed? 404 <cgroups-why-needed>` for the background information):: 405 406 # mount -t tmpfs none /sys/fs/cgroup 407 # mkdir /sys/fs/cgroup/memory 408 # mount -t cgroup none /sys/fs/cgroup/memory -o memory 409 4103. Make the new group and move bash into it:: 411 412 # mkdir /sys/fs/cgroup/memory/0 413 # echo $$ > /sys/fs/cgroup/memory/0/tasks 414 4154. Since now we're in the 0 cgroup, we can alter the memory limit:: 416 417 # echo 4M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes 418 419 The limit can now be queried:: 420 421 # cat /sys/fs/cgroup/memory/0/memory.limit_in_bytes 422 4194304 423 424.. note:: 425 We can use a suffix (k, K, m, M, g or G) to indicate values in kilo, 426 mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, 427 Gibibytes.) 428 429.. note:: 430 We can write "-1" to reset the ``*.limit_in_bytes(unlimited)``. 431 432.. note:: 433 We cannot set limits on the root cgroup any more. 434 435 436We can check the usage:: 437 438 # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes 439 1216512 440 441A successful write to this file does not guarantee a successful setting of 442this limit to the value written into the file. This can be due to a 443number of factors, such as rounding up to page boundaries or the total 444availability of memory on the system. The user is required to re-read 445this file after a write to guarantee the value committed by the kernel:: 446 447 # echo 1 > memory.limit_in_bytes 448 # cat memory.limit_in_bytes 449 4096 450 451The memory.failcnt field gives the number of times that the cgroup limit was 452exceeded. 453 454The memory.stat file gives accounting information. Now, the number of 455caches, RSS and Active pages/Inactive pages are shown. 456 4574. Testing 458========== 459 460For testing features and implementation, see memcg_test.txt. 461 462Performance test is also important. To see pure memory controller's overhead, 463testing on tmpfs will give you good numbers of small overheads. 464Example: do kernel make on tmpfs. 465 466Page-fault scalability is also important. At measuring parallel 467page fault test, multi-process test may be better than multi-thread 468test because it has noise of shared objects/status. 469 470But the above two are testing extreme situations. 471Trying usual test under memory controller is always helpful. 472 473.. _cgroup-v1-memory-test-troubleshoot: 474 4754.1 Troubleshooting 476------------------- 477 478Sometimes a user might find that the application under a cgroup is 479terminated by the OOM killer. There are several causes for this: 480 4811. The cgroup limit is too low (just too low to do anything useful) 4822. The user is using anonymous memory and swap is turned off or too low 483 484A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of 485some of the pages cached in the cgroup (page cache pages). 486 487To know what happens, disabling OOM_Kill as per :ref:`"10. OOM Control" 488<cgroup-v1-memory-oom-control>` (below) and seeing what happens will be 489helpful. 490 491.. _cgroup-v1-memory-test-task-migration: 492 4934.2 Task migration 494------------------ 495 496When a task migrates from one cgroup to another, its charge is not 497carried forward by default. The pages allocated from the original cgroup still 498remain charged to it, the charge is dropped when the page is freed or 499reclaimed. 500 501You can move charges of a task along with task migration. 502See :ref:`8. "Move charges at task migration" <cgroup-v1-memory-move-charges>` 503 5044.3 Removing a cgroup 505--------------------- 506 507A cgroup can be removed by rmdir, but as discussed in :ref:`sections 4.1 508<cgroup-v1-memory-test-troubleshoot>` and :ref:`4.2 509<cgroup-v1-memory-test-task-migration>`, a cgroup might have some charge 510associated with it, even though all tasks have migrated away from it. (because 511we charge against pages, not against tasks.) 512 513We move the stats to parent, and no change on the charge except uncharging 514from the child. 515 516Charges recorded in swap information is not updated at removal of cgroup. 517Recorded information is discarded and a cgroup which uses swap (swapcache) 518will be charged as a new owner of it. 519 5205. Misc. interfaces 521=================== 522 5235.1 force_empty 524--------------- 525 memory.force_empty interface is provided to make cgroup's memory usage empty. 526 When writing anything to this:: 527 528 # echo 0 > memory.force_empty 529 530 the cgroup will be reclaimed and as many pages reclaimed as possible. 531 532 The typical use case for this interface is before calling rmdir(). 533 Though rmdir() offlines memcg, but the memcg may still stay there due to 534 charged file caches. Some out-of-use page caches may keep charged until 535 memory pressure happens. If you want to avoid that, force_empty will be useful. 536 5375.2 stat file 538------------- 539 540memory.stat file includes following statistics: 541 542 * per-memory cgroup local status 543 544 =============== =============================================================== 545 cache # of bytes of page cache memory. 546 rss # of bytes of anonymous and swap cache memory (includes 547 transparent hugepages). 548 rss_huge # of bytes of anonymous transparent hugepages. 549 mapped_file # of bytes of mapped file (includes tmpfs/shmem) 550 pgpgin # of charging events to the memory cgroup. The charging 551 event happens each time a page is accounted as either mapped 552 anon page(RSS) or cache page(Page Cache) to the cgroup. 553 pgpgout # of uncharging events to the memory cgroup. The uncharging 554 event happens each time a page is unaccounted from the 555 cgroup. 556 swap # of bytes of swap usage 557 swapcached # of bytes of swap cached in memory 558 dirty # of bytes that are waiting to get written back to the disk. 559 writeback # of bytes of file/anon cache that are queued for syncing to 560 disk. 561 inactive_anon # of bytes of anonymous and swap cache memory on inactive 562 LRU list. 563 active_anon # of bytes of anonymous and swap cache memory on active 564 LRU list. 565 inactive_file # of bytes of file-backed memory and MADV_FREE anonymous 566 memory (LazyFree pages) on inactive LRU list. 567 active_file # of bytes of file-backed memory on active LRU list. 568 unevictable # of bytes of memory that cannot be reclaimed (mlocked etc). 569 =============== =============================================================== 570 571 * status considering hierarchy (see memory.use_hierarchy settings): 572 573 ========================= =================================================== 574 hierarchical_memory_limit # of bytes of memory limit with regard to 575 hierarchy 576 under which the memory cgroup is 577 hierarchical_memsw_limit # of bytes of memory+swap limit with regard to 578 hierarchy under which memory cgroup is. 579 580 total_<counter> # hierarchical version of <counter>, which in 581 addition to the cgroup's own value includes the 582 sum of all hierarchical children's values of 583 <counter>, i.e. total_cache 584 ========================= =================================================== 585 586 * additional vm parameters (depends on CONFIG_DEBUG_VM): 587 588 ========================= ======================================== 589 recent_rotated_anon VM internal parameter. (see mm/vmscan.c) 590 recent_rotated_file VM internal parameter. (see mm/vmscan.c) 591 recent_scanned_anon VM internal parameter. (see mm/vmscan.c) 592 recent_scanned_file VM internal parameter. (see mm/vmscan.c) 593 ========================= ======================================== 594 595.. hint:: 596 recent_rotated means recent frequency of LRU rotation. 597 recent_scanned means recent # of scans to LRU. 598 showing for better debug please see the code for meanings. 599 600.. note:: 601 Only anonymous and swap cache memory is listed as part of 'rss' stat. 602 This should not be confused with the true 'resident set size' or the 603 amount of physical memory used by the cgroup. 604 605 'rss + mapped_file" will give you resident set size of cgroup. 606 607 Note that some kernel configurations might account complete larger 608 allocations (e.g., THP) towards 'rss' and 'mapped_file', even if 609 only some, but not all that memory is mapped. 610 611 (Note: file and shmem may be shared among other cgroups. In that case, 612 mapped_file is accounted only when the memory cgroup is owner of page 613 cache.) 614 6155.3 swappiness 616-------------- 617 618Overrides /proc/sys/vm/swappiness for the particular group. The tunable 619in the root cgroup corresponds to the global swappiness setting. 620 621Please note that unlike during the global reclaim, limit reclaim 622enforces that 0 swappiness really prevents from any swapping even if 623there is a swap storage available. This might lead to memcg OOM killer 624if there are no file pages to reclaim. 625 6265.4 failcnt 627----------- 628 629A memory cgroup provides memory.failcnt and memory.memsw.failcnt files. 630This failcnt(== failure count) shows the number of times that a usage counter 631hit its limit. When a memory cgroup hits a limit, failcnt increases and 632memory under it will be reclaimed. 633 634You can reset failcnt by writing 0 to failcnt file:: 635 636 # echo 0 > .../memory.failcnt 637 6385.5 usage_in_bytes 639------------------ 640 641For efficiency, as other kernel components, memory cgroup uses some optimization 642to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the 643method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz 644value for efficient access. (Of course, when necessary, it's synchronized.) 645If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP) 646value in memory.stat(see 5.2). 647 6485.6 numa_stat 649------------- 650 651This is similar to numa_maps but operates on a per-memcg basis. This is 652useful for providing visibility into the numa locality information within 653an memcg since the pages are allowed to be allocated from any physical 654node. One of the use cases is evaluating application performance by 655combining this information with the application's CPU allocation. 656 657Each memcg's numa_stat file includes "total", "file", "anon" and "unevictable" 658per-node page counts including "hierarchical_<counter>" which sums up all 659hierarchical children's values in addition to the memcg's own value. 660 661The output format of memory.numa_stat is:: 662 663 total=<total pages> N0=<node 0 pages> N1=<node 1 pages> ... 664 file=<total file pages> N0=<node 0 pages> N1=<node 1 pages> ... 665 anon=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ... 666 unevictable=<total anon pages> N0=<node 0 pages> N1=<node 1 pages> ... 667 hierarchical_<counter>=<counter pages> N0=<node 0 pages> N1=<node 1 pages> ... 668 669The "total" count is sum of file + anon + unevictable. 670 6716. Hierarchy support 672==================== 673 674The memory controller supports a deep hierarchy and hierarchical accounting. 675The hierarchy is created by creating the appropriate cgroups in the 676cgroup filesystem. Consider for example, the following cgroup filesystem 677hierarchy:: 678 679 root 680 / | \ 681 / | \ 682 a b c 683 | \ 684 | \ 685 d e 686 687In the diagram above, with hierarchical accounting enabled, all memory 688usage of e, is accounted to its ancestors up until the root (i.e, c and root). 689If one of the ancestors goes over its limit, the reclaim algorithm reclaims 690from the tasks in the ancestor and the children of the ancestor. 691 6926.1 Hierarchical accounting and reclaim 693--------------------------------------- 694 695Hierarchical accounting is enabled by default. Disabling the hierarchical 696accounting is deprecated. An attempt to do it will result in a failure 697and a warning printed to dmesg. 698 699For compatibility reasons writing 1 to memory.use_hierarchy will always pass:: 700 701 # echo 1 > memory.use_hierarchy 702 7037. Soft limits (DEPRECATED) 704=========================== 705 706THIS IS DEPRECATED! 707 708Writing to memory.soft_limit_in_bytes has no effect and reading it will 709always return the maximum value. 710 711Use memory.low and memory.min in cgroup v2 instead. 712 713.. _cgroup-v1-memory-move-charges: 714 7158. Move charges at task migration (DEPRECATED!) 716=============================================== 717 718THIS IS DEPRECATED! 719 720Reading memory.move_charge_at_immigrate will always return 0 and writing 721to it will always return -EINVAL. 722 7239. Memory thresholds 724==================== 725 726Memory cgroup implements memory thresholds using the cgroups notification 727API (see cgroups.txt). It allows to register multiple memory and memsw 728thresholds and gets notifications when it crosses. 729 730To register a threshold, an application must: 731 732- create an eventfd using eventfd(2); 733- open memory.usage_in_bytes or memory.memsw.usage_in_bytes; 734- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to 735 cgroup.event_control. 736 737Application will be notified through eventfd when memory usage crosses 738threshold in any direction. 739 740It's applicable for root and non-root cgroup. 741 742.. _cgroup-v1-memory-oom-control: 743 74410. OOM Control (DEPRECATED) 745============================ 746 747THIS IS DEPRECATED! 748 749memory.oom_control file is for OOM notification and other controls. 750 751Memory cgroup implements OOM notifier using the cgroup notification 752API (See cgroups.txt). It allows to register multiple OOM notification 753delivery and gets notification when OOM happens. 754 755To register a notifier, an application must: 756 757 - create an eventfd using eventfd(2) 758 - open memory.oom_control file 759 - write string like "<event_fd> <fd of memory.oom_control>" to 760 cgroup.event_control 761 762The application will be notified through eventfd when OOM happens. 763OOM notification doesn't work for the root cgroup. 764 765You can disable the OOM-killer by writing "1" to memory.oom_control file, as: 766 767 #echo 1 > memory.oom_control 768 769If OOM-killer is disabled, tasks under cgroup will hang/sleep 770in memory cgroup's OOM-waitqueue when they request accountable memory. 771 772For running them, you have to relax the memory cgroup's OOM status by 773 774 * enlarge limit or reduce usage. 775 776To reduce usage, 777 778 * kill some tasks. 779 * move some tasks to other group with account migration. 780 * remove some files (on tmpfs?) 781 782Then, stopped tasks will work again. 783 784At reading, current status of OOM is shown. 785 786 - oom_kill_disable 0 or 1 787 (if 1, oom-killer is disabled) 788 - under_oom 0 or 1 789 (if 1, the memory cgroup is under OOM, tasks may be stopped.) 790 - oom_kill integer counter 791 The number of processes belonging to this cgroup killed by any 792 kind of OOM killer. 793 79411. Memory Pressure (DEPRECATED) 795================================ 796 797THIS IS DEPRECATED! 798 799The pressure level notifications can be used to monitor the memory 800allocation cost; based on the pressure, applications can implement 801different strategies of managing their memory resources. The pressure 802levels are defined as following: 803 804The "low" level means that the system is reclaiming memory for new 805allocations. Monitoring this reclaiming activity might be useful for 806maintaining cache level. Upon notification, the program (typically 807"Activity Manager") might analyze vmstat and act in advance (i.e. 808prematurely shutdown unimportant services). 809 810The "medium" level means that the system is experiencing medium memory 811pressure, the system might be making swap, paging out active file caches, 812etc. Upon this event applications may decide to further analyze 813vmstat/zoneinfo/memcg or internal memory usage statistics and free any 814resources that can be easily reconstructed or re-read from a disk. 815 816The "critical" level means that the system is actively thrashing, it is 817about to out of memory (OOM) or even the in-kernel OOM killer is on its 818way to trigger. Applications should do whatever they can to help the 819system. It might be too late to consult with vmstat or any other 820statistics, so it's advisable to take an immediate action. 821 822By default, events are propagated upward until the event is handled, i.e. the 823events are not pass-through. For example, you have three cgroups: A->B->C. Now 824you set up an event listener on cgroups A, B and C, and suppose group C 825experiences some pressure. In this situation, only group C will receive the 826notification, i.e. groups A and B will not receive it. This is done to avoid 827excessive "broadcasting" of messages, which disturbs the system and which is 828especially bad if we are low on memory or thrashing. Group B, will receive 829notification only if there are no event listeners for group C. 830 831There are three optional modes that specify different propagation behavior: 832 833 - "default": this is the default behavior specified above. This mode is the 834 same as omitting the optional mode parameter, preserved by backwards 835 compatibility. 836 837 - "hierarchy": events always propagate up to the root, similar to the default 838 behavior, except that propagation continues regardless of whether there are 839 event listeners at each level, with the "hierarchy" mode. In the above 840 example, groups A, B, and C will receive notification of memory pressure. 841 842 - "local": events are pass-through, i.e. they only receive notifications when 843 memory pressure is experienced in the memcg for which the notification is 844 registered. In the above example, group C will receive notification if 845 registered for "local" notification and the group experiences memory 846 pressure. However, group B will never receive notification, regardless if 847 there is an event listener for group C or not, if group B is registered for 848 local notification. 849 850The level and event notification mode ("hierarchy" or "local", if necessary) are 851specified by a comma-delimited string, i.e. "low,hierarchy" specifies 852hierarchical, pass-through, notification for all ancestor memcgs. Notification 853that is the default, non pass-through behavior, does not specify a mode. 854"medium,local" specifies pass-through notification for the medium level. 855 856The file memory.pressure_level is only used to setup an eventfd. To 857register a notification, an application must: 858 859- create an eventfd using eventfd(2); 860- open memory.pressure_level; 861- write string as "<event_fd> <fd of memory.pressure_level> <level[,mode]>" 862 to cgroup.event_control. 863 864Application will be notified through eventfd when memory pressure is at 865the specific level (or higher). Read/write operations to 866memory.pressure_level are no implemented. 867 868Test: 869 870 Here is a small script example that makes a new cgroup, sets up a 871 memory limit, sets up a notification in the cgroup and then makes child 872 cgroup experience a critical pressure:: 873 874 # cd /sys/fs/cgroup/memory/ 875 # mkdir foo 876 # cd foo 877 # cgroup_event_listener memory.pressure_level low,hierarchy & 878 # echo 8000000 > memory.limit_in_bytes 879 # echo 8000000 > memory.memsw.limit_in_bytes 880 # echo $$ > tasks 881 # dd if=/dev/zero | read x 882 883 (Expect a bunch of notifications, and eventually, the oom-killer will 884 trigger.) 885 88612. TODO 887======== 888 8891. Make per-cgroup scanner reclaim not-shared pages first 8902. Teach controller to account for shared-pages 8913. Start reclamation in the background when the limit is 892 not yet hit but the usage is getting closer 893 894Summary 895======= 896 897Overall, the memory controller has been a stable controller and has been 898commented and discussed quite extensively in the community. 899 900References 901========== 902 903.. [1] Singh, Balbir. RFC: Memory Controller, http://lwn.net/Articles/206697/ 904.. [2] Singh, Balbir. Memory Controller (RSS Control), 905 http://lwn.net/Articles/222762/ 906.. [3] Emelianov, Pavel. Resource controllers based on process cgroups 907 https://lore.kernel.org/r/45ED7DEC.7010403@sw.ru 908.. [4] Emelianov, Pavel. RSS controller based on process cgroups (v2) 909 https://lore.kernel.org/r/461A3010.90403@sw.ru 910.. [5] Emelianov, Pavel. RSS controller based on process cgroups (v3) 911 https://lore.kernel.org/r/465D9739.8070209@openvz.org 912 9136. Menage, Paul. Control Groups v10, http://lwn.net/Articles/236032/ 9147. Vaidyanathan, Srinivasan, Control Groups: Pagecache accounting and control 915 subsystem (v3), http://lwn.net/Articles/235534/ 9168. Singh, Balbir. RSS controller v2 test results (lmbench), 917 https://lore.kernel.org/r/464C95D4.7070806@linux.vnet.ibm.com 9189. Singh, Balbir. RSS controller v2 AIM9 results 919 https://lore.kernel.org/r/464D267A.50107@linux.vnet.ibm.com 92010. Singh, Balbir. Memory controller v6 test results, 921 https://lore.kernel.org/r/20070819094658.654.84837.sendpatchset@balbir-laptop 922 923.. [11] Singh, Balbir. Memory controller introduction (v6), 924 https://lore.kernel.org/r/20070817084228.26003.12568.sendpatchset@balbir-laptop 925.. [12] Corbet, Jonathan, Controlling memory use in cgroups, 926 http://lwn.net/Articles/243795/ 927