1 2 Control Group v2 3 4 October, 2015 Tejun Heo <email@example.com> 5 6 This is the authoritative documentation on the design, interface and 7 conventions of cgroup v2. It describes all userland-visible aspects 8 of cgroup including core and specific controller behaviors. All 9 future changes must be reflected in this document. Documentation for 10 v1 is available under Documentation/cgroup-v1/. 11 12 CONTENTS 13 14 1. Introduction 15 1-1. Terminology 16 1-2. What is cgroup? 17 2. Basic Operations 18 2-1. Mounting 19 2-2. Organizing Processes 20 2-3. [Un]populated Notification 21 2-4. Controlling Controllers 22 2-4-1. Enabling and Disabling 23 2-4-2. Top-down Constraint 24 2-4-3. No Internal Process Constraint 25 2-5. Delegation 26 2-5-1. Model of Delegation 27 2-5-2. Delegation Containment 28 2-6. Guidelines 29 2-6-1. Organize Once and Control 30 2-6-2. Avoid Name Collisions 31 3. Resource Distribution Models 32 3-1. Weights 33 3-2. Limits 34 3-3. Protections 35 3-4. Allocations 36 4. Interface Files 37 4-1. Format 38 4-2. Conventions 39 4-3. Core Interface Files 40 5. Controllers 41 5-1. CPU 42 5-1-1. CPU Interface Files 43 5-2. Memory 44 5-2-1. Memory Interface Files 45 5-2-2. Usage Guidelines 46 5-2-3. Memory Ownership 47 5-3. IO 48 5-3-1. IO Interface Files 49 5-3-2. Writeback 50 6. Namespace 51 6-1. Basics 52 6-2. The Root and Views 53 6-3. Migration and setns(2) 54 6-4. Interaction with Other Namespaces 55 P. Information on Kernel Programming 56 P-1. Filesystem Support for Writeback 57 D. Deprecated v1 Core Features 58 R. Issues with v1 and Rationales for v2 59 R-1. Multiple Hierarchies 60 R-2. Thread Granularity 61 R-3. Competition Between Inner Nodes and Threads 62 R-4. Other Interface Issues 63 R-5. Controller Issues and Remedies 64 R-5-1. Memory 65 66 67 1. Introduction 68 69 1-1. Terminology 70 71 "cgroup" stands for "control group" and is never capitalized. The 72 singular form is used to designate the whole feature and also as a 73 qualifier as in "cgroup controllers". When explicitly referring to 74 multiple individual control groups, the plural form "cgroups" is used. 75 76 77 1-2. What is cgroup? 78 79 cgroup is a mechanism to organize processes hierarchically and 80 distribute system resources along the hierarchy in a controlled and 81 configurable manner. 82 83 cgroup is largely composed of two parts - the core and controllers. 84 cgroup core is primarily responsible for hierarchically organizing 85 processes. A cgroup controller is usually responsible for 86 distributing a specific type of system resource along the hierarchy 87 although there are utility controllers which serve purposes other than 88 resource distribution. 89 90 cgroups form a tree structure and every process in the system belongs 91 to one and only one cgroup. All threads of a process belong to the 92 same cgroup. On creation, all processes are put in the cgroup that 93 the parent process belongs to at the time. A process can be migrated 94 to another cgroup. Migration of a process doesn't affect already 95 existing descendant processes. 96 97 Following certain structural constraints, controllers may be enabled or 98 disabled selectively on a cgroup. All controller behaviors are 99 hierarchical - if a controller is enabled on a cgroup, it affects all 100 processes which belong to the cgroups consisting the inclusive 101 sub-hierarchy of the cgroup. When a controller is enabled on a nested 102 cgroup, it always restricts the resource distribution further. The 103 restrictions set closer to the root in the hierarchy can not be 104 overridden from further away. 105 106 107 2. Basic Operations 108 109 2-1. Mounting 110 111 Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2 112 hierarchy can be mounted with the following mount command. 113 114 # mount -t cgroup2 none $MOUNT_POINT 115 116 cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All 117 controllers which support v2 and are not bound to a v1 hierarchy are 118 automatically bound to the v2 hierarchy and show up at the root. 119 Controllers which are not in active use in the v2 hierarchy can be 120 bound to other hierarchies. This allows mixing v2 hierarchy with the 121 legacy v1 multiple hierarchies in a fully backward compatible way. 122 123 A controller can be moved across hierarchies only after the controller 124 is no longer referenced in its current hierarchy. Because per-cgroup 125 controller states are destroyed asynchronously and controllers may 126 have lingering references, a controller may not show up immediately on 127 the v2 hierarchy after the final umount of the previous hierarchy. 128 Similarly, a controller should be fully disabled to be moved out of 129 the unified hierarchy and it may take some time for the disabled 130 controller to become available for other hierarchies; furthermore, due 131 to inter-controller dependencies, other controllers may need to be 132 disabled too. 133 134 While useful for development and manual configurations, moving 135 controllers dynamically between the v2 and other hierarchies is 136 strongly discouraged for production use. It is recommended to decide 137 the hierarchies and controller associations before starting using the 138 controllers after system boot. 139 140 During transition to v2, system management software might still 141 automount the v1 cgroup filesystem and so hijack all controllers 142 during boot, before manual intervention is possible. To make testing 143 and experimenting easier, the kernel parameter cgroup_no_v1= allows 144 disabling controllers in v1 and make them always available in v2. 145 146 147 2-2. Organizing Processes 148 149 Initially, only the root cgroup exists to which all processes belong. 150 A child cgroup can be created by creating a sub-directory. 151 152 # mkdir $CGROUP_NAME 153 154 A given cgroup may have multiple child cgroups forming a tree 155 structure. Each cgroup has a read-writable interface file 156 "cgroup.procs". When read, it lists the PIDs of all processes which 157 belong to the cgroup one-per-line. The PIDs are not ordered and the 158 same PID may show up more than once if the process got moved to 159 another cgroup and then back or the PID got recycled while reading. 160 161 A process can be migrated into a cgroup by writing its PID to the 162 target cgroup's "cgroup.procs" file. Only one process can be migrated 163 on a single write(2) call. If a process is composed of multiple 164 threads, writing the PID of any thread migrates all threads of the 165 process. 166 167 When a process forks a child process, the new process is born into the 168 cgroup that the forking process belongs to at the time of the 169 operation. After exit, a process stays associated with the cgroup 170 that it belonged to at the time of exit until it's reaped; however, a 171 zombie process does not appear in "cgroup.procs" and thus can't be 172 moved to another cgroup. 173 174 A cgroup which doesn't have any children or live processes can be 175 destroyed by removing the directory. Note that a cgroup which doesn't 176 have any children and is associated only with zombie processes is 177 considered empty and can be removed. 178 179 # rmdir $CGROUP_NAME 180 181 "/proc/$PID/cgroup" lists a process's cgroup membership. If legacy 182 cgroup is in use in the system, this file may contain multiple lines, 183 one for each hierarchy. The entry for cgroup v2 is always in the 184 format "0::$PATH". 185 186 # cat /proc/842/cgroup 187 ... 188 0::/test-cgroup/test-cgroup-nested 189 190 If the process becomes a zombie and the cgroup it was associated with 191 is removed subsequently, " (deleted)" is appended to the path. 192 193 # cat /proc/842/cgroup 194 ... 195 0::/test-cgroup/test-cgroup-nested (deleted) 196 197 198 2-3. [Un]populated Notification 199 200 Each non-root cgroup has a "cgroup.events" file which contains 201 "populated" field indicating whether the cgroup's sub-hierarchy has 202 live processes in it. Its value is 0 if there is no live process in 203 the cgroup and its descendants; otherwise, 1. poll and [id]notify 204 events are triggered when the value changes. This can be used, for 205 example, to start a clean-up operation after all processes of a given 206 sub-hierarchy have exited. The populated state updates and 207 notifications are recursive. Consider the following sub-hierarchy 208 where the numbers in the parentheses represent the numbers of processes 209 in each cgroup. 210 211 A(4) - B(0) - C(1) 212 \ D(0) 213 214 A, B and C's "populated" fields would be 1 while D's 0. After the one 215 process in C exits, B and C's "populated" fields would flip to "0" and 216 file modified events will be generated on the "cgroup.events" files of 217 both cgroups. 218 219 220 2-4. Controlling Controllers 221 222 2-4-1. Enabling and Disabling 223 224 Each cgroup has a "cgroup.controllers" file which lists all 225 controllers available for the cgroup to enable. 226 227 # cat cgroup.controllers 228 cpu io memory 229 230 No controller is enabled by default. Controllers can be enabled and 231 disabled by writing to the "cgroup.subtree_control" file. 232 233 # echo "+cpu +memory -io" > cgroup.subtree_control 234 235 Only controllers which are listed in "cgroup.controllers" can be 236 enabled. When multiple operations are specified as above, either they 237 all succeed or fail. If multiple operations on the same controller 238 are specified, the last one is effective. 239 240 Enabling a controller in a cgroup indicates that the distribution of 241 the target resource across its immediate children will be controlled. 242 Consider the following sub-hierarchy. The enabled controllers are 243 listed in parentheses. 244 245 A(cpu,memory) - B(memory) - C() 246 \ D() 247 248 As A has "cpu" and "memory" enabled, A will control the distribution 249 of CPU cycles and memory to its children, in this case, B. As B has 250 "memory" enabled but not "CPU", C and D will compete freely on CPU 251 cycles but their division of memory available to B will be controlled. 252 253 As a controller regulates the distribution of the target resource to 254 the cgroup's children, enabling it creates the controller's interface 255 files in the child cgroups. In the above example, enabling "cpu" on B 256 would create the "cpu." prefixed controller interface files in C and 257 D. Likewise, disabling "memory" from B would remove the "memory." 258 prefixed controller interface files from C and D. This means that the 259 controller interface files - anything which doesn't start with 260 "cgroup." are owned by the parent rather than the cgroup itself. 261 262 263 2-4-2. Top-down Constraint 264 265 Resources are distributed top-down and a cgroup can further distribute 266 a resource only if the resource has been distributed to it from the 267 parent. This means that all non-root "cgroup.subtree_control" files 268 can only contain controllers which are enabled in the parent's 269 "cgroup.subtree_control" file. A controller can be enabled only if 270 the parent has the controller enabled and a controller can't be 271 disabled if one or more children have it enabled. 272 273 274 2-4-3. No Internal Process Constraint 275 276 Non-root cgroups can only distribute resources to their children when 277 they don't have any processes of their own. In other words, only 278 cgroups which don't contain any processes can have controllers enabled 279 in their "cgroup.subtree_control" files. 280 281 This guarantees that, when a controller is looking at the part of the 282 hierarchy which has it enabled, processes are always only on the 283 leaves. This rules out situations where child cgroups compete against 284 internal processes of the parent. 285 286 The root cgroup is exempt from this restriction. Root contains 287 processes and anonymous resource consumption which can't be associated 288 with any other cgroups and requires special treatment from most 289 controllers. How resource consumption in the root cgroup is governed 290 is up to each controller. 291 292 Note that the restriction doesn't get in the way if there is no 293 enabled controller in the cgroup's "cgroup.subtree_control". This is 294 important as otherwise it wouldn't be possible to create children of a 295 populated cgroup. To control resource distribution of a cgroup, the 296 cgroup must create children and transfer all its processes to the 297 children before enabling controllers in its "cgroup.subtree_control" 298 file. 299 300 301 2-5. Delegation 302 303 2-5-1. Model of Delegation 304 305 A cgroup can be delegated to a less privileged user by granting write 306 access of the directory and its "cgroup.procs" file to the user. Note 307 that resource control interface files in a given directory control the 308 distribution of the parent's resources and thus must not be delegated 309 along with the directory. 310 311 Once delegated, the user can build sub-hierarchy under the directory, 312 organize processes as it sees fit and further distribute the resources 313 it received from the parent. The limits and other settings of all 314 resource controllers are hierarchical and regardless of what happens 315 in the delegated sub-hierarchy, nothing can escape the resource 316 restrictions imposed by the parent. 317 318 Currently, cgroup doesn't impose any restrictions on the number of 319 cgroups in or nesting depth of a delegated sub-hierarchy; however, 320 this may be limited explicitly in the future. 321 322 323 2-5-2. Delegation Containment 324 325 A delegated sub-hierarchy is contained in the sense that processes 326 can't be moved into or out of the sub-hierarchy by the delegatee. For 327 a process with a non-root euid to migrate a target process into a 328 cgroup by writing its PID to the "cgroup.procs" file, the following 329 conditions must be met. 330 331 - The writer's euid must match either uid or suid of the target process. 332 333 - The writer must have write access to the "cgroup.procs" file. 334 335 - The writer must have write access to the "cgroup.procs" file of the 336 common ancestor of the source and destination cgroups. 337 338 The above three constraints ensure that while a delegatee may migrate 339 processes around freely in the delegated sub-hierarchy it can't pull 340 in from or push out to outside the sub-hierarchy. 341 342 For an example, let's assume cgroups C0 and C1 have been delegated to 343 user U0 who created C00, C01 under C0 and C10 under C1 as follows and 344 all processes under C0 and C1 belong to U0. 345 346 ~~~~~~~~~~~~~ - C0 - C00 347 ~ cgroup ~ \ C01 348 ~ hierarchy ~ 349 ~~~~~~~~~~~~~ - C1 - C10 350 351 Let's also say U0 wants to write the PID of a process which is 352 currently in C10 into "C00/cgroup.procs". U0 has write access to the 353 file and uid match on the process; however, the common ancestor of the 354 source cgroup C10 and the destination cgroup C00 is above the points 355 of delegation and U0 would not have write access to its "cgroup.procs" 356 files and thus the write will be denied with -EACCES. 357 358 359 2-6. Guidelines 360 361 2-6-1. Organize Once and Control 362 363 Migrating a process across cgroups is a relatively expensive operation 364 and stateful resources such as memory are not moved together with the 365 process. This is an explicit design decision as there often exist 366 inherent trade-offs between migration and various hot paths in terms 367 of synchronization cost. 368 369 As such, migrating processes across cgroups frequently as a means to 370 apply different resource restrictions is discouraged. A workload 371 should be assigned to a cgroup according to the system's logical and 372 resource structure once on start-up. Dynamic adjustments to resource 373 distribution can be made by changing controller configuration through 374 the interface files. 375 376 377 2-6-2. Avoid Name Collisions 378 379 Interface files for a cgroup and its children cgroups occupy the same 380 directory and it is possible to create children cgroups which collide 381 with interface files. 382 383 All cgroup core interface files are prefixed with "cgroup." and each 384 controller's interface files are prefixed with the controller name and 385 a dot. A controller's name is composed of lower case alphabets and 386 '_'s but never begins with an '_' so it can be used as the prefix 387 character for collision avoidance. Also, interface file names won't 388 start or end with terms which are often used in categorizing workloads 389 such as job, service, slice, unit or workload. 390 391 cgroup doesn't do anything to prevent name collisions and it's the 392 user's responsibility to avoid them. 393 394 395 3. Resource Distribution Models 396 397 cgroup controllers implement several resource distribution schemes 398 depending on the resource type and expected use cases. This section 399 describes major schemes in use along with their expected behaviors. 400 401 402 3-1. Weights 403 404 A parent's resource is distributed by adding up the weights of all 405 active children and giving each the fraction matching the ratio of its 406 weight against the sum. As only children which can make use of the 407 resource at the moment participate in the distribution, this is 408 work-conserving. Due to the dynamic nature, this model is usually 409 used for stateless resources. 410 411 All weights are in the range [1, 10000] with the default at 100. This 412 allows symmetric multiplicative biases in both directions at fine 413 enough granularity while staying in the intuitive range. 414 415 As long as the weight is in range, all configuration combinations are 416 valid and there is no reason to reject configuration changes or 417 process migrations. 418 419 "cpu.weight" proportionally distributes CPU cycles to active children 420 and is an example of this type. 421 422 423 3-2. Limits 424 425 A child can only consume upto the configured amount of the resource. 426 Limits can be over-committed - the sum of the limits of children can 427 exceed the amount of resource available to the parent. 428 429 Limits are in the range [0, max] and defaults to "max", which is noop. 430 431 As limits can be over-committed, all configuration combinations are 432 valid and there is no reason to reject configuration changes or 433 process migrations. 434 435 "io.max" limits the maximum BPS and/or IOPS that a cgroup can consume 436 on an IO device and is an example of this type. 437 438 439 3-3. Protections 440 441 A cgroup is protected to be allocated upto the configured amount of 442 the resource if the usages of all its ancestors are under their 443 protected levels. Protections can be hard guarantees or best effort 444 soft boundaries. Protections can also be over-committed in which case 445 only upto the amount available to the parent is protected among 446 children. 447 448 Protections are in the range [0, max] and defaults to 0, which is 449 noop. 450 451 As protections can be over-committed, all configuration combinations 452 are valid and there is no reason to reject configuration changes or 453 process migrations. 454 455 "memory.low" implements best-effort memory protection and is an 456 example of this type. 457 458 459 3-4. Allocations 460 461 A cgroup is exclusively allocated a certain amount of a finite 462 resource. Allocations can't be over-committed - the sum of the 463 allocations of children can not exceed the amount of resource 464 available to the parent. 465 466 Allocations are in the range [0, max] and defaults to 0, which is no 467 resource. 468 469 As allocations can't be over-committed, some configuration 470 combinations are invalid and should be rejected. Also, if the 471 resource is mandatory for execution of processes, process migrations 472 may be rejected. 473 474 "cpu.rt.max" hard-allocates realtime slices and is an example of this 475 type. 476 477 478 4. Interface Files 479 480 4-1. Format 481 482 All interface files should be in one of the following formats whenever 483 possible. 484 485 New-line separated values 486 (when only one value can be written at once) 487 488 VAL0\n 489 VAL1\n 490 ... 491 492 Space separated values 493 (when read-only or multiple values can be written at once) 494 495 VAL0 VAL1 ...\n 496 497 Flat keyed 498 499 KEY0 VAL0\n 500 KEY1 VAL1\n 501 ... 502 503 Nested keyed 504 505 KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01... 506 KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11... 507 ... 508 509 For a writable file, the format for writing should generally match 510 reading; however, controllers may allow omitting later fields or 511 implement restricted shortcuts for most common use cases. 512 513 For both flat and nested keyed files, only the values for a single key 514 can be written at a time. For nested keyed files, the sub key pairs 515 may be specified in any order and not all pairs have to be specified. 516 517 518 4-2. Conventions 519 520 - Settings for a single feature should be contained in a single file. 521 522 - The root cgroup should be exempt from resource control and thus 523 shouldn't have resource control interface files. Also, 524 informational files on the root cgroup which end up showing global 525 information available elsewhere shouldn't exist. 526 527 - If a controller implements weight based resource distribution, its 528 interface file should be named "weight" and have the range [1, 529 10000] with 100 as the default. The values are chosen to allow 530 enough and symmetric bias in both directions while keeping it 531 intuitive (the default is 100%). 532 533 - If a controller implements an absolute resource guarantee and/or 534 limit, the interface files should be named "min" and "max" 535 respectively. If a controller implements best effort resource 536 guarantee and/or limit, the interface files should be named "low" 537 and "high" respectively. 538 539 In the above four control files, the special token "max" should be 540 used to represent upward infinity for both reading and writing. 541 542 - If a setting has a configurable default value and keyed specific 543 overrides, the default entry should be keyed with "default" and 544 appear as the first entry in the file. 545 546 The default value can be updated by writing either "default $VAL" or 547 "$VAL". 548 549 When writing to update a specific override, "default" can be used as 550 the value to indicate removal of the override. Override entries 551 with "default" as the value must not appear when read. 552 553 For example, a setting which is keyed by major:minor device numbers 554 with integer values may look like the following. 555 556 # cat cgroup-example-interface-file 557 default 150 558 8:0 300 559 560 The default value can be updated by 561 562 # echo 125 > cgroup-example-interface-file 563 564 or 565 566 # echo "default 125" > cgroup-example-interface-file 567 568 An override can be set by 569 570 # echo "8:16 170" > cgroup-example-interface-file 571 572 and cleared by 573 574 # echo "8:0 default" > cgroup-example-interface-file 575 # cat cgroup-example-interface-file 576 default 125 577 8:16 170 578 579 - For events which are not very high frequency, an interface file 580 "events" should be created which lists event key value pairs. 581 Whenever a notifiable event happens, file modified event should be 582 generated on the file. 583 584 585 4-3. Core Interface Files 586 587 All cgroup core files are prefixed with "cgroup." 588 589 cgroup.procs 590 591 A read-write new-line separated values file which exists on 592 all cgroups. 593 594 When read, it lists the PIDs of all processes which belong to 595 the cgroup one-per-line. The PIDs are not ordered and the 596 same PID may show up more than once if the process got moved 597 to another cgroup and then back or the PID got recycled while 598 reading. 599 600 A PID can be written to migrate the process associated with 601 the PID to the cgroup. The writer should match all of the 602 following conditions. 603 604 - Its euid is either root or must match either uid or suid of 605 the target process. 606 607 - It must have write access to the "cgroup.procs" file. 608 609 - It must have write access to the "cgroup.procs" file of the 610 common ancestor of the source and destination cgroups. 611 612 When delegating a sub-hierarchy, write access to this file 613 should be granted along with the containing directory. 614 615 cgroup.controllers 616 617 A read-only space separated values file which exists on all 618 cgroups. 619 620 It shows space separated list of all controllers available to 621 the cgroup. The controllers are not ordered. 622 623 cgroup.subtree_control 624 625 A read-write space separated values file which exists on all 626 cgroups. Starts out empty. 627 628 When read, it shows space separated list of the controllers 629 which are enabled to control resource distribution from the 630 cgroup to its children. 631 632 Space separated list of controllers prefixed with '+' or '-' 633 can be written to enable or disable controllers. A controller 634 name prefixed with '+' enables the controller and '-' 635 disables. If a controller appears more than once on the list, 636 the last one is effective. When multiple enable and disable 637 operations are specified, either all succeed or all fail. 638 639 cgroup.events 640 641 A read-only flat-keyed file which exists on non-root cgroups. 642 The following entries are defined. Unless specified 643 otherwise, a value change in this file generates a file 644 modified event. 645 646 populated 647 648 1 if the cgroup or its descendants contains any live 649 processes; otherwise, 0. 650 651 652 5. Controllers 653 654 5-1. CPU 655 656 [NOTE: The interface for the cpu controller hasn't been merged yet] 657 658 The "cpu" controllers regulates distribution of CPU cycles. This 659 controller implements weight and absolute bandwidth limit models for 660 normal scheduling policy and absolute bandwidth allocation model for 661 realtime scheduling policy. 662 663 664 5-1-1. CPU Interface Files 665 666 All time durations are in microseconds. 667 668 cpu.stat 669 670 A read-only flat-keyed file which exists on non-root cgroups. 671 672 It reports the following six stats. 673 674 usage_usec 675 user_usec 676 system_usec 677 nr_periods 678 nr_throttled 679 throttled_usec 680 681 cpu.weight 682 683 A read-write single value file which exists on non-root 684 cgroups. The default is "100". 685 686 The weight in the range [1, 10000]. 687 688 cpu.max 689 690 A read-write two value file which exists on non-root cgroups. 691 The default is "max 100000". 692 693 The maximum bandwidth limit. It's in the following format. 694 695 $MAX $PERIOD 696 697 which indicates that the group may consume upto $MAX in each 698 $PERIOD duration. "max" for $MAX indicates no limit. If only 699 one number is written, $MAX is updated. 700 701 cpu.rt.max 702 703 [NOTE: The semantics of this file is still under discussion and the 704 interface hasn't been merged yet] 705 706 A read-write two value file which exists on all cgroups. 707 The default is "0 100000". 708 709 The maximum realtime runtime allocation. Over-committing 710 configurations are disallowed and process migrations are 711 rejected if not enough bandwidth is available. It's in the 712 following format. 713 714 $MAX $PERIOD 715 716 which indicates that the group may consume upto $MAX in each 717 $PERIOD duration. If only one number is written, $MAX is 718 updated. 719 720 721 5-2. Memory 722 723 The "memory" controller regulates distribution of memory. Memory is 724 stateful and implements both limit and protection models. Due to the 725 intertwining between memory usage and reclaim pressure and the 726 stateful nature of memory, the distribution model is relatively 727 complex. 728 729 While not completely water-tight, all major memory usages by a given 730 cgroup are tracked so that the total memory consumption can be 731 accounted and controlled to a reasonable extent. Currently, the 732 following types of memory usages are tracked. 733 734 - Userland memory - page cache and anonymous memory. 735 736 - Kernel data structures such as dentries and inodes. 737 738 - TCP socket buffers. 739 740 The above list may expand in the future for better coverage. 741 742 743 5-2-1. Memory Interface Files 744 745 All memory amounts are in bytes. If a value which is not aligned to 746 PAGE_SIZE is written, the value may be rounded up to the closest 747 PAGE_SIZE multiple when read back. 748 749 memory.current 750 751 A read-only single value file which exists on non-root 752 cgroups. 753 754 The total amount of memory currently being used by the cgroup 755 and its descendants. 756 757 memory.low 758 759 A read-write single value file which exists on non-root 760 cgroups. The default is "0". 761 762 Best-effort memory protection. If the memory usages of a 763 cgroup and all its ancestors are below their low boundaries, 764 the cgroup's memory won't be reclaimed unless memory can be 765 reclaimed from unprotected cgroups. 766 767 Putting more memory than generally available under this 768 protection is discouraged. 769 770 memory.high 771 772 A read-write single value file which exists on non-root 773 cgroups. The default is "max". 774 775 Memory usage throttle limit. This is the main mechanism to 776 control memory usage of a cgroup. If a cgroup's usage goes 777 over the high boundary, the processes of the cgroup are 778 throttled and put under heavy reclaim pressure. 779 780 Going over the high limit never invokes the OOM killer and 781 under extreme conditions the limit may be breached. 782 783 memory.max 784 785 A read-write single value file which exists on non-root 786 cgroups. The default is "max". 787 788 Memory usage hard limit. This is the final protection 789 mechanism. If a cgroup's memory usage reaches this limit and 790 can't be reduced, the OOM killer is invoked in the cgroup. 791 Under certain circumstances, the usage may go over the limit 792 temporarily. 793 794 This is the ultimate protection mechanism. As long as the 795 high limit is used and monitored properly, this limit's 796 utility is limited to providing the final safety net. 797 798 memory.events 799 800 A read-only flat-keyed file which exists on non-root cgroups. 801 The following entries are defined. Unless specified 802 otherwise, a value change in this file generates a file 803 modified event. 804 805 low 806 807 The number of times the cgroup is reclaimed due to 808 high memory pressure even though its usage is under 809 the low boundary. This usually indicates that the low 810 boundary is over-committed. 811 812 high 813 814 The number of times processes of the cgroup are 815 throttled and routed to perform direct memory reclaim 816 because the high memory boundary was exceeded. For a 817 cgroup whose memory usage is capped by the high limit 818 rather than global memory pressure, this event's 819 occurrences are expected. 820 821 max 822 823 The number of times the cgroup's memory usage was 824 about to go over the max boundary. If direct reclaim 825 fails to bring it down, the OOM killer is invoked. 826 827 oom 828 829 The number of times the OOM killer has been invoked in 830 the cgroup. This may not exactly match the number of 831 processes killed but should generally be close. 832 833 memory.stat 834 835 A read-only flat-keyed file which exists on non-root cgroups. 836 837 This breaks down the cgroup's memory footprint into different 838 types of memory, type-specific details, and other information 839 on the state and past events of the memory management system. 840 841 All memory amounts are in bytes. 842 843 The entries are ordered to be human readable, and new entries 844 can show up in the middle. Don't rely on items remaining in a 845 fixed position; use the keys to look up specific values! 846 847 anon 848 849 Amount of memory used in anonymous mappings such as 850 brk(), sbrk(), and mmap(MAP_ANONYMOUS) 851 852 file 853 854 Amount of memory used to cache filesystem data, 855 including tmpfs and shared memory. 856 857 kernel_stack 858 859 Amount of memory allocated to kernel stacks. 860 861 slab 862 863 Amount of memory used for storing in-kernel data 864 structures. 865 866 sock 867 868 Amount of memory used in network transmission buffers 869 870 file_mapped 871 872 Amount of cached filesystem data mapped with mmap() 873 874 file_dirty 875 876 Amount of cached filesystem data that was modified but 877 not yet written back to disk 878 879 file_writeback 880 881 Amount of cached filesystem data that was modified and 882 is currently being written back to disk 883 884 inactive_anon 885 active_anon 886 inactive_file 887 active_file 888 unevictable 889 890 Amount of memory, swap-backed and filesystem-backed, 891 on the internal memory management lists used by the 892 page reclaim algorithm 893 894 slab_reclaimable 895 896 Part of "slab" that might be reclaimed, such as 897 dentries and inodes. 898 899 slab_unreclaimable 900 901 Part of "slab" that cannot be reclaimed on memory 902 pressure. 903 904 pgfault 905 906 Total number of page faults incurred 907 908 pgmajfault 909 910 Number of major page faults incurred 911 912 memory.swap.current 913 914 A read-only single value file which exists on non-root 915 cgroups. 916 917 The total amount of swap currently being used by the cgroup 918 and its descendants. 919 920 memory.swap.max 921 922 A read-write single value file which exists on non-root 923 cgroups. The default is "max". 924 925 Swap usage hard limit. If a cgroup's swap usage reaches this 926 limit, anonymous meomry of the cgroup will not be swapped out. 927 928 929 5-2-2. Usage Guidelines 930 931 "memory.high" is the main mechanism to control memory usage. 932 Over-committing on high limit (sum of high limits > available memory) 933 and letting global memory pressure to distribute memory according to 934 usage is a viable strategy. 935 936 Because breach of the high limit doesn't trigger the OOM killer but 937 throttles the offending cgroup, a management agent has ample 938 opportunities to monitor and take appropriate actions such as granting 939 more memory or terminating the workload. 940 941 Determining whether a cgroup has enough memory is not trivial as 942 memory usage doesn't indicate whether the workload can benefit from 943 more memory. For example, a workload which writes data received from 944 network to a file can use all available memory but can also operate as 945 performant with a small amount of memory. A measure of memory 946 pressure - how much the workload is being impacted due to lack of 947 memory - is necessary to determine whether a workload needs more 948 memory; unfortunately, memory pressure monitoring mechanism isn't 949 implemented yet. 950 951 952 5-2-3. Memory Ownership 953 954 A memory area is charged to the cgroup which instantiated it and stays 955 charged to the cgroup until the area is released. Migrating a process 956 to a different cgroup doesn't move the memory usages that it 957 instantiated while in the previous cgroup to the new cgroup. 958 959 A memory area may be used by processes belonging to different cgroups. 960 To which cgroup the area will be charged is in-deterministic; however, 961 over time, the memory area is likely to end up in a cgroup which has 962 enough memory allowance to avoid high reclaim pressure. 963 964 If a cgroup sweeps a considerable amount of memory which is expected 965 to be accessed repeatedly by other cgroups, it may make sense to use 966 POSIX_FADV_DONTNEED to relinquish the ownership of memory areas 967 belonging to the affected files to ensure correct memory ownership. 968 969 970 5-3. IO 971 972 The "io" controller regulates the distribution of IO resources. This 973 controller implements both weight based and absolute bandwidth or IOPS 974 limit distribution; however, weight based distribution is available 975 only if cfq-iosched is in use and neither scheme is available for 976 blk-mq devices. 977 978 979 5-3-1. IO Interface Files 980 981 io.stat 982 983 A read-only nested-keyed file which exists on non-root 984 cgroups. 985 986 Lines are keyed by $MAJ:$MIN device numbers and not ordered. 987 The following nested keys are defined. 988 989 rbytes Bytes read 990 wbytes Bytes written 991 rios Number of read IOs 992 wios Number of write IOs 993 994 An example read output follows. 995 996 8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 997 8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 998 999 io.weight 1000 1001 A read-write flat-keyed file which exists on non-root cgroups. 1002 The default is "default 100". 1003 1004 The first line is the default weight applied to devices 1005 without specific override. The rest are overrides keyed by 1006 $MAJ:$MIN device numbers and not ordered. The weights are in 1007 the range [1, 10000] and specifies the relative amount IO time 1008 the cgroup can use in relation to its siblings. 1009 1010 The default weight can be updated by writing either "default 1011 $WEIGHT" or simply "$WEIGHT". Overrides can be set by writing 1012 "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default". 1013 1014 An example read output follows. 1015 1016 default 100 1017 8:16 200 1018 8:0 50 1019 1020 io.max 1021 1022 A read-write nested-keyed file which exists on non-root 1023 cgroups. 1024 1025 BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN 1026 device numbers and not ordered. The following nested keys are 1027 defined. 1028 1029 rbps Max read bytes per second 1030 wbps Max write bytes per second 1031 riops Max read IO operations per second 1032 wiops Max write IO operations per second 1033 1034 When writing, any number of nested key-value pairs can be 1035 specified in any order. "max" can be specified as the value 1036 to remove a specific limit. If the same key is specified 1037 multiple times, the outcome is undefined. 1038 1039 BPS and IOPS are measured in each IO direction and IOs are 1040 delayed if limit is reached. Temporary bursts are allowed. 1041 1042 Setting read limit at 2M BPS and write at 120 IOPS for 8:16. 1043 1044 echo "8:16 rbps=2097152 wiops=120" > io.max 1045 1046 Reading returns the following. 1047 1048 8:16 rbps=2097152 wbps=max riops=max wiops=120 1049 1050 Write IOPS limit can be removed by writing the following. 1051 1052 echo "8:16 wiops=max" > io.max 1053 1054 Reading now returns the following. 1055 1056 8:16 rbps=2097152 wbps=max riops=max wiops=max 1057 1058 1059 5-3-2. Writeback 1060 1061 Page cache is dirtied through buffered writes and shared mmaps and 1062 written asynchronously to the backing filesystem by the writeback 1063 mechanism. Writeback sits between the memory and IO domains and 1064 regulates the proportion of dirty memory by balancing dirtying and 1065 write IOs. 1066 1067 The io controller, in conjunction with the memory controller, 1068 implements control of page cache writeback IOs. The memory controller 1069 defines the memory domain that dirty memory ratio is calculated and 1070 maintained for and the io controller defines the io domain which 1071 writes out dirty pages for the memory domain. Both system-wide and 1072 per-cgroup dirty memory states are examined and the more restrictive 1073 of the two is enforced. 1074 1075 cgroup writeback requires explicit support from the underlying 1076 filesystem. Currently, cgroup writeback is implemented on ext2, ext4 1077 and btrfs. On other filesystems, all writeback IOs are attributed to 1078 the root cgroup. 1079 1080 There are inherent differences in memory and writeback management 1081 which affects how cgroup ownership is tracked. Memory is tracked per 1082 page while writeback per inode. For the purpose of writeback, an 1083 inode is assigned to a cgroup and all IO requests to write dirty pages 1084 from the inode are attributed to that cgroup. 1085 1086 As cgroup ownership for memory is tracked per page, there can be pages 1087 which are associated with different cgroups than the one the inode is 1088 associated with. These are called foreign pages. The writeback 1089 constantly keeps track of foreign pages and, if a particular foreign 1090 cgroup becomes the majority over a certain period of time, switches 1091 the ownership of the inode to that cgroup. 1092 1093 While this model is enough for most use cases where a given inode is 1094 mostly dirtied by a single cgroup even when the main writing cgroup 1095 changes over time, use cases where multiple cgroups write to a single 1096 inode simultaneously are not supported well. In such circumstances, a 1097 significant portion of IOs are likely to be attributed incorrectly. 1098 As memory controller assigns page ownership on the first use and 1099 doesn't update it until the page is released, even if writeback 1100 strictly follows page ownership, multiple cgroups dirtying overlapping 1101 areas wouldn't work as expected. It's recommended to avoid such usage 1102 patterns. 1103 1104 The sysctl knobs which affect writeback behavior are applied to cgroup 1105 writeback as follows. 1106 1107 vm.dirty_background_ratio 1108 vm.dirty_ratio 1109 1110 These ratios apply the same to cgroup writeback with the 1111 amount of available memory capped by limits imposed by the 1112 memory controller and system-wide clean memory. 1113 1114 vm.dirty_background_bytes 1115 vm.dirty_bytes 1116 1117 For cgroup writeback, this is calculated into ratio against 1118 total available memory and applied the same way as 1119 vm.dirty[_background]_ratio. 1120 1121 1122 6. Namespace 1123 1124 6-1. Basics 1125 1126 cgroup namespace provides a mechanism to virtualize the view of the 1127 "/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone 1128 flag can be used with clone(2) and unshare(2) to create a new cgroup 1129 namespace. The process running inside the cgroup namespace will have 1130 its "/proc/$PID/cgroup" output restricted to cgroupns root. The 1131 cgroupns root is the cgroup of the process at the time of creation of 1132 the cgroup namespace. 1133 1134 Without cgroup namespace, the "/proc/$PID/cgroup" file shows the 1135 complete path of the cgroup of a process. In a container setup where 1136 a set of cgroups and namespaces are intended to isolate processes the 1137 "/proc/$PID/cgroup" file may leak potential system level information 1138 to the isolated processes. For Example: 1139 1140 # cat /proc/self/cgroup 1141 0::/batchjobs/container_id1 1142 1143 The path '/batchjobs/container_id1' can be considered as system-data 1144 and undesirable to expose to the isolated processes. cgroup namespace 1145 can be used to restrict visibility of this path. For example, before 1146 creating a cgroup namespace, one would see: 1147 1148 # ls -l /proc/self/ns/cgroup 1149 lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup: 1150 # cat /proc/self/cgroup 1151 0::/batchjobs/container_id1 1152 1153 After unsharing a new namespace, the view changes. 1154 1155 # ls -l /proc/self/ns/cgroup 1156 lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup: 1157 # cat /proc/self/cgroup 1158 0::/ 1159 1160 When some thread from a multi-threaded process unshares its cgroup 1161 namespace, the new cgroupns gets applied to the entire process (all 1162 the threads). This is natural for the v2 hierarchy; however, for the 1163 legacy hierarchies, this may be unexpected. 1164 1165 A cgroup namespace is alive as long as there are processes inside or 1166 mounts pinning it. When the last usage goes away, the cgroup 1167 namespace is destroyed. The cgroupns root and the actual cgroups 1168 remain. 1169 1170 1171 6-2. The Root and Views 1172 1173 The 'cgroupns root' for a cgroup namespace is the cgroup in which the 1174 process calling unshare(2) is running. For example, if a process in 1175 /batchjobs/container_id1 cgroup calls unshare, cgroup 1176 /batchjobs/container_id1 becomes the cgroupns root. For the 1177 init_cgroup_ns, this is the real root ('/') cgroup. 1178 1179 The cgroupns root cgroup does not change even if the namespace creator 1180 process later moves to a different cgroup. 1181 1182 # ~/unshare -c # unshare cgroupns in some cgroup 1183 # cat /proc/self/cgroup 1184 0::/ 1185 # mkdir sub_cgrp_1 1186 # echo 0 > sub_cgrp_1/cgroup.procs 1187 # cat /proc/self/cgroup 1188 0::/sub_cgrp_1 1189 1190 Each process gets its namespace-specific view of "/proc/$PID/cgroup" 1191 1192 Processes running inside the cgroup namespace will be able to see 1193 cgroup paths (in /proc/self/cgroup) only inside their root cgroup. 1194 From within an unshared cgroupns: 1195 1196 # sleep 100000 & 1197  7353 1198 # echo 7353 > sub_cgrp_1/cgroup.procs 1199 # cat /proc/7353/cgroup 1200 0::/sub_cgrp_1 1201 1202 From the initial cgroup namespace, the real cgroup path will be 1203 visible: 1204 1205 $ cat /proc/7353/cgroup 1206 0::/batchjobs/container_id1/sub_cgrp_1 1207 1208 From a sibling cgroup namespace (that is, a namespace rooted at a 1209 different cgroup), the cgroup path relative to its own cgroup 1210 namespace root will be shown. For instance, if PID 7353's cgroup 1211 namespace root is at '/batchjobs/container_id2', then it will see 1212 1213 # cat /proc/7353/cgroup 1214 0::/../container_id2/sub_cgrp_1 1215 1216 Note that the relative path always starts with '/' to indicate that 1217 its relative to the cgroup namespace root of the caller. 1218 1219 1220 6-3. Migration and setns(2) 1221 1222 Processes inside a cgroup namespace can move into and out of the 1223 namespace root if they have proper access to external cgroups. For 1224 example, from inside a namespace with cgroupns root at 1225 /batchjobs/container_id1, and assuming that the global hierarchy is 1226 still accessible inside cgroupns: 1227 1228 # cat /proc/7353/cgroup 1229 0::/sub_cgrp_1 1230 # echo 7353 > batchjobs/container_id2/cgroup.procs 1231 # cat /proc/7353/cgroup 1232 0::/../container_id2 1233 1234 Note that this kind of setup is not encouraged. A task inside cgroup 1235 namespace should only be exposed to its own cgroupns hierarchy. 1236 1237 setns(2) to another cgroup namespace is allowed when: 1238 1239 (a) the process has CAP_SYS_ADMIN against its current user namespace 1240 (b) the process has CAP_SYS_ADMIN against the target cgroup 1241 namespace's userns 1242 1243 No implicit cgroup changes happen with attaching to another cgroup 1244 namespace. It is expected that the someone moves the attaching 1245 process under the target cgroup namespace root. 1246 1247 1248 6-4. Interaction with Other Namespaces 1249 1250 Namespace specific cgroup hierarchy can be mounted by a process 1251 running inside a non-init cgroup namespace. 1252 1253 # mount -t cgroup2 none $MOUNT_POINT 1254 1255 This will mount the unified cgroup hierarchy with cgroupns root as the 1256 filesystem root. The process needs CAP_SYS_ADMIN against its user and 1257 mount namespaces. 1258 1259 The virtualization of /proc/self/cgroup file combined with restricting 1260 the view of cgroup hierarchy by namespace-private cgroupfs mount 1261 provides a properly isolated cgroup view inside the container. 1262 1263 1264 P. Information on Kernel Programming 1265 1266 This section contains kernel programming information in the areas 1267 where interacting with cgroup is necessary. cgroup core and 1268 controllers are not covered. 1269 1270 1271 P-1. Filesystem Support for Writeback 1272 1273 A filesystem can support cgroup writeback by updating 1274 address_space_operations->writepage[s]() to annotate bio's using the 1275 following two functions. 1276 1277 wbc_init_bio(@wbc, @bio) 1278 1279 Should be called for each bio carrying writeback data and 1280 associates the bio with the inode's owner cgroup. Can be 1281 called anytime between bio allocation and submission. 1282 1283 wbc_account_io(@wbc, @page, @bytes) 1284 1285 Should be called for each data segment being written out. 1286 While this function doesn't care exactly when it's called 1287 during the writeback session, it's the easiest and most 1288 natural to call it as data segments are added to a bio. 1289 1290 With writeback bio's annotated, cgroup support can be enabled per 1291 super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for 1292 selective disabling of cgroup writeback support which is helpful when 1293 certain filesystem features, e.g. journaled data mode, are 1294 incompatible. 1295 1296 wbc_init_bio() binds the specified bio to its cgroup. Depending on 1297 the configuration, the bio may be executed at a lower priority and if 1298 the writeback session is holding shared resources, e.g. a journal 1299 entry, may lead to priority inversion. There is no one easy solution 1300 for the problem. Filesystems can try to work around specific problem 1301 cases by skipping wbc_init_bio() or using bio_associate_blkcg() 1302 directly. 1303 1304 1305 D. Deprecated v1 Core Features 1306 1307 - Multiple hierarchies including named ones are not supported. 1308 1309 - All mount options and remounting are not supported. 1310 1311 - The "tasks" file is removed and "cgroup.procs" is not sorted. 1312 1313 - "cgroup.clone_children" is removed. 1314 1315 - /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file 1316 at the root instead. 1317 1318 1319 R. Issues with v1 and Rationales for v2 1320 1321 R-1. Multiple Hierarchies 1322 1323 cgroup v1 allowed an arbitrary number of hierarchies and each 1324 hierarchy could host any number of controllers. While this seemed to 1325 provide a high level of flexibility, it wasn't useful in practice. 1326 1327 For example, as there is only one instance of each controller, utility 1328 type controllers such as freezer which can be useful in all 1329 hierarchies could only be used in one. The issue is exacerbated by 1330 the fact that controllers couldn't be moved to another hierarchy once 1331 hierarchies were populated. Another issue was that all controllers 1332 bound to a hierarchy were forced to have exactly the same view of the 1333 hierarchy. It wasn't possible to vary the granularity depending on 1334 the specific controller. 1335 1336 In practice, these issues heavily limited which controllers could be 1337 put on the same hierarchy and most configurations resorted to putting 1338 each controller on its own hierarchy. Only closely related ones, such 1339 as the cpu and cpuacct controllers, made sense to be put on the same 1340 hierarchy. This often meant that userland ended up managing multiple 1341 similar hierarchies repeating the same steps on each hierarchy 1342 whenever a hierarchy management operation was necessary. 1343 1344 Furthermore, support for multiple hierarchies came at a steep cost. 1345 It greatly complicated cgroup core implementation but more importantly 1346 the support for multiple hierarchies restricted how cgroup could be 1347 used in general and what controllers was able to do. 1348 1349 There was no limit on how many hierarchies there might be, which meant 1350 that a thread's cgroup membership couldn't be described in finite 1351 length. The key might contain any number of entries and was unlimited 1352 in length, which made it highly awkward to manipulate and led to 1353 addition of controllers which existed only to identify membership, 1354 which in turn exacerbated the original problem of proliferating number 1355 of hierarchies. 1356 1357 Also, as a controller couldn't have any expectation regarding the 1358 topologies of hierarchies other controllers might be on, each 1359 controller had to assume that all other controllers were attached to 1360 completely orthogonal hierarchies. This made it impossible, or at 1361 least very cumbersome, for controllers to cooperate with each other. 1362 1363 In most use cases, putting controllers on hierarchies which are 1364 completely orthogonal to each other isn't necessary. What usually is 1365 called for is the ability to have differing levels of granularity 1366 depending on the specific controller. In other words, hierarchy may 1367 be collapsed from leaf towards root when viewed from specific 1368 controllers. For example, a given configuration might not care about 1369 how memory is distributed beyond a certain level while still wanting 1370 to control how CPU cycles are distributed. 1371 1372 1373 R-2. Thread Granularity 1374 1375 cgroup v1 allowed threads of a process to belong to different cgroups. 1376 This didn't make sense for some controllers and those controllers 1377 ended up implementing different ways to ignore such situations but 1378 much more importantly it blurred the line between API exposed to 1379 individual applications and system management interface. 1380 1381 Generally, in-process knowledge is available only to the process 1382 itself; thus, unlike service-level organization of processes, 1383 categorizing threads of a process requires active participation from 1384 the application which owns the target process. 1385 1386 cgroup v1 had an ambiguously defined delegation model which got abused 1387 in combination with thread granularity. cgroups were delegated to 1388 individual applications so that they can create and manage their own 1389 sub-hierarchies and control resource distributions along them. This 1390 effectively raised cgroup to the status of a syscall-like API exposed 1391 to lay programs. 1392 1393 First of all, cgroup has a fundamentally inadequate interface to be 1394 exposed this way. For a process to access its own knobs, it has to 1395 extract the path on the target hierarchy from /proc/self/cgroup, 1396 construct the path by appending the name of the knob to the path, open 1397 and then read and/or write to it. This is not only extremely clunky 1398 and unusual but also inherently racy. There is no conventional way to 1399 define transaction across the required steps and nothing can guarantee 1400 that the process would actually be operating on its own sub-hierarchy. 1401 1402 cgroup controllers implemented a number of knobs which would never be 1403 accepted as public APIs because they were just adding control knobs to 1404 system-management pseudo filesystem. cgroup ended up with interface 1405 knobs which were not properly abstracted or refined and directly 1406 revealed kernel internal details. These knobs got exposed to 1407 individual applications through the ill-defined delegation mechanism 1408 effectively abusing cgroup as a shortcut to implementing public APIs 1409 without going through the required scrutiny. 1410 1411 This was painful for both userland and kernel. Userland ended up with 1412 misbehaving and poorly abstracted interfaces and kernel exposing and 1413 locked into constructs inadvertently. 1414 1415 1416 R-3. Competition Between Inner Nodes and Threads 1417 1418 cgroup v1 allowed threads to be in any cgroups which created an 1419 interesting problem where threads belonging to a parent cgroup and its 1420 children cgroups competed for resources. This was nasty as two 1421 different types of entities competed and there was no obvious way to 1422 settle it. Different controllers did different things. 1423 1424 The cpu controller considered threads and cgroups as equivalents and 1425 mapped nice levels to cgroup weights. This worked for some cases but 1426 fell flat when children wanted to be allocated specific ratios of CPU 1427 cycles and the number of internal threads fluctuated - the ratios 1428 constantly changed as the number of competing entities fluctuated. 1429 There also were other issues. The mapping from nice level to weight 1430 wasn't obvious or universal, and there were various other knobs which 1431 simply weren't available for threads. 1432 1433 The io controller implicitly created a hidden leaf node for each 1434 cgroup to host the threads. The hidden leaf had its own copies of all 1435 the knobs with "leaf_" prefixed. While this allowed equivalent 1436 control over internal threads, it was with serious drawbacks. It 1437 always added an extra layer of nesting which wouldn't be necessary 1438 otherwise, made the interface messy and significantly complicated the 1439 implementation. 1440 1441 The memory controller didn't have a way to control what happened 1442 between internal tasks and child cgroups and the behavior was not 1443 clearly defined. There were attempts to add ad-hoc behaviors and 1444 knobs to tailor the behavior to specific workloads which would have 1445 led to problems extremely difficult to resolve in the long term. 1446 1447 Multiple controllers struggled with internal tasks and came up with 1448 different ways to deal with it; unfortunately, all the approaches were 1449 severely flawed and, furthermore, the widely different behaviors 1450 made cgroup as a whole highly inconsistent. 1451 1452 This clearly is a problem which needs to be addressed from cgroup core 1453 in a uniform way. 1454 1455 1456 R-4. Other Interface Issues 1457 1458 cgroup v1 grew without oversight and developed a large number of 1459 idiosyncrasies and inconsistencies. One issue on the cgroup core side 1460 was how an empty cgroup was notified - a userland helper binary was 1461 forked and executed for each event. The event delivery wasn't 1462 recursive or delegatable. The limitations of the mechanism also led 1463 to in-kernel event delivery filtering mechanism further complicating 1464 the interface. 1465 1466 Controller interfaces were problematic too. An extreme example is 1467 controllers completely ignoring hierarchical organization and treating 1468 all cgroups as if they were all located directly under the root 1469 cgroup. Some controllers exposed a large amount of inconsistent 1470 implementation details to userland. 1471 1472 There also was no consistency across controllers. When a new cgroup 1473 was created, some controllers defaulted to not imposing extra 1474 restrictions while others disallowed any resource usage until 1475 explicitly configured. Configuration knobs for the same type of 1476 control used widely differing naming schemes and formats. Statistics 1477 and information knobs were named arbitrarily and used different 1478 formats and units even in the same controller. 1479 1480 cgroup v2 establishes common conventions where appropriate and updates 1481 controllers so that they expose minimal and consistent interfaces. 1482 1483 1484 R-5. Controller Issues and Remedies 1485 1486 R-5-1. Memory 1487 1488 The original lower boundary, the soft limit, is defined as a limit 1489 that is per default unset. As a result, the set of cgroups that 1490 global reclaim prefers is opt-in, rather than opt-out. The costs for 1491 optimizing these mostly negative lookups are so high that the 1492 implementation, despite its enormous size, does not even provide the 1493 basic desirable behavior. First off, the soft limit has no 1494 hierarchical meaning. All configured groups are organized in a global 1495 rbtree and treated like equal peers, regardless where they are located 1496 in the hierarchy. This makes subtree delegation impossible. Second, 1497 the soft limit reclaim pass is so aggressive that it not just 1498 introduces high allocation latencies into the system, but also impacts 1499 system performance due to overreclaim, to the point where the feature 1500 becomes self-defeating. 1501 1502 The memory.low boundary on the other hand is a top-down allocated 1503 reserve. A cgroup enjoys reclaim protection when it and all its 1504 ancestors are below their low boundaries, which makes delegation of 1505 subtrees possible. Secondly, new cgroups have no reserve per default 1506 and in the common case most cgroups are eligible for the preferred 1507 reclaim pass. This allows the new low boundary to be efficiently 1508 implemented with just a minor addition to the generic reclaim code, 1509 without the need for out-of-band data structures and reclaim passes. 1510 Because the generic reclaim code considers all cgroups except for the 1511 ones running low in the preferred first reclaim pass, overreclaim of 1512 individual groups is eliminated as well, resulting in much better 1513 overall workload performance. 1514 1515 The original high boundary, the hard limit, is defined as a strict 1516 limit that can not budge, even if the OOM killer has to be called. 1517 But this generally goes against the goal of making the most out of the 1518 available memory. The memory consumption of workloads varies during 1519 runtime, and that requires users to overcommit. But doing that with a 1520 strict upper limit requires either a fairly accurate prediction of the 1521 working set size or adding slack to the limit. Since working set size 1522 estimation is hard and error prone, and getting it wrong results in 1523 OOM kills, most users tend to err on the side of a looser limit and 1524 end up wasting precious resources. 1525 1526 The memory.high boundary on the other hand can be set much more 1527 conservatively. When hit, it throttles allocations by forcing them 1528 into direct reclaim to work off the excess, but it never invokes the 1529 OOM killer. As a result, a high boundary that is chosen too 1530 aggressively will not terminate the processes, but instead it will 1531 lead to gradual performance degradation. The user can monitor this 1532 and make corrections until the minimal memory footprint that still 1533 gives acceptable performance is found. 1534 1535 In extreme cases, with many concurrent allocations and a complete 1536 breakdown of reclaim progress within the group, the high boundary can 1537 be exceeded. But even then it's mostly better to satisfy the 1538 allocation from the slack available in other groups or the rest of the 1539 system than killing the group. Otherwise, memory.max is there to 1540 limit this type of spillover and ultimately contain buggy or even 1541 malicious applications. 1542 1543 Setting the original memory.limit_in_bytes below the current usage was 1544 subject to a race condition, where concurrent charges could cause the 1545 limit setting to fail. memory.max on the other hand will first set the 1546 limit to prevent new charges, and then reclaim and OOM kill until the 1547 new limit is met - or the task writing to memory.max is killed. 1548 1549 The combined memory+swap accounting and limiting is replaced by real 1550 control over swap space. 1551 1552 The main argument for a combined memory+swap facility in the original 1553 cgroup design was that global or parental pressure would always be 1554 able to swap all anonymous memory of a child group, regardless of the 1555 child's own (possibly untrusted) configuration. However, untrusted 1556 groups can sabotage swapping by other means - such as referencing its 1557 anonymous memory in a tight loop - and an admin can not assume full 1558 swappability when overcommitting untrusted jobs. 1559 1560 For trusted jobs, on the other hand, a combined counter is not an 1561 intuitive userspace interface, and it flies in the face of the idea 1562 that cgroup controllers should account and limit specific physical 1563 resources. Swap space is a resource like all others in the system, 1564 and that's why unified hierarchy allows distributing it separately.