Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.
1 CPUSETS 2 ------- 3 4 Copyright (C) 2004 BULL SA. 5 Written by Simon.Derr[AT]bull[DOT]net 6 7 Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. 8 Modified by Paul Jackson <pj[AT]sgi[DOT]com> 9 Modified by Christoph Lameter <clameter[AT]sgi[DOT]com> 10 Modified by Paul Menage <menage[AT]google[DOT]com> 11 Modified by Hidetoshi Seto <seto.hidetoshi[AT]jp.fujitsu[DOT]com> 12 13 CONTENTS: 14 ========= 15 16 1. Cpusets 17 1.1 What are cpusets ? 18 1.2 Why are cpusets needed ? 19 1.3 How are cpusets implemented ? 20 1.4 What are exclusive cpusets ? 21 1.5 What is memory_pressure ? 22 1.6 What is memory spread ? 23 1.7 What is sched_load_balance ? 24 1.8 What is sched_relax_domain_level ? 25 1.9 How do I use cpusets ? 26 2. Usage Examples and Syntax 27 2.1 Basic Usage 28 2.2 Adding/removing cpus 29 2.3 Setting flags 30 2.4 Attaching processes 31 3. Questions 32 4. Contact 33 34 1. Cpusets 35 ========== 36 37 1.1 What are cpusets ? 38 ---------------------- 39 40 Cpusets provide a mechanism for assigning a set of CPUs and Memory 41 Nodes to a set of tasks. In this document "Memory Node" refers to 42 an on-line node that contains memory. 43 44 Cpusets constrain the CPU and Memory placement of tasks to only 45 the resources within a tasks current cpuset. They form a nested 46 hierarchy visible in a virtual file system. These are the essential 47 hooks, beyond what is already present, required to manage dynamic 48 job placement on large systems. 49 50 Cpusets use the generic cgroup subsystem described in 51 Documentation/cgroup.txt. 52 53 Requests by a task, using the sched_setaffinity(2) system call to 54 include CPUs in its CPU affinity mask, and using the mbind(2) and 55 set_mempolicy(2) system calls to include Memory Nodes in its memory 56 policy, are both filtered through that tasks cpuset, filtering out any 57 CPUs or Memory Nodes not in that cpuset. The scheduler will not 58 schedule a task on a CPU that is not allowed in its cpus_allowed 59 vector, and the kernel page allocator will not allocate a page on a 60 node that is not allowed in the requesting tasks mems_allowed vector. 61 62 User level code may create and destroy cpusets by name in the cgroup 63 virtual file system, manage the attributes and permissions of these 64 cpusets and which CPUs and Memory Nodes are assigned to each cpuset, 65 specify and query to which cpuset a task is assigned, and list the 66 task pids assigned to a cpuset. 67 68 69 1.2 Why are cpusets needed ? 70 ---------------------------- 71 72 The management of large computer systems, with many processors (CPUs), 73 complex memory cache hierarchies and multiple Memory Nodes having 74 non-uniform access times (NUMA) presents additional challenges for 75 the efficient scheduling and memory placement of processes. 76 77 Frequently more modest sized systems can be operated with adequate 78 efficiency just by letting the operating system automatically share 79 the available CPU and Memory resources amongst the requesting tasks. 80 81 But larger systems, which benefit more from careful processor and 82 memory placement to reduce memory access times and contention, 83 and which typically represent a larger investment for the customer, 84 can benefit from explicitly placing jobs on properly sized subsets of 85 the system. 86 87 This can be especially valuable on: 88 89 * Web Servers running multiple instances of the same web application, 90 * Servers running different applications (for instance, a web server 91 and a database), or 92 * NUMA systems running large HPC applications with demanding 93 performance characteristics. 94 95 These subsets, or "soft partitions" must be able to be dynamically 96 adjusted, as the job mix changes, without impacting other concurrently 97 executing jobs. The location of the running jobs pages may also be moved 98 when the memory locations are changed. 99 100 The kernel cpuset patch provides the minimum essential kernel 101 mechanisms required to efficiently implement such subsets. It 102 leverages existing CPU and Memory Placement facilities in the Linux 103 kernel to avoid any additional impact on the critical scheduler or 104 memory allocator code. 105 106 107 1.3 How are cpusets implemented ? 108 --------------------------------- 109 110 Cpusets provide a Linux kernel mechanism to constrain which CPUs and 111 Memory Nodes are used by a process or set of processes. 112 113 The Linux kernel already has a pair of mechanisms to specify on which 114 CPUs a task may be scheduled (sched_setaffinity) and on which Memory 115 Nodes it may obtain memory (mbind, set_mempolicy). 116 117 Cpusets extends these two mechanisms as follows: 118 119 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the 120 kernel. 121 - Each task in the system is attached to a cpuset, via a pointer 122 in the task structure to a reference counted cgroup structure. 123 - Calls to sched_setaffinity are filtered to just those CPUs 124 allowed in that tasks cpuset. 125 - Calls to mbind and set_mempolicy are filtered to just 126 those Memory Nodes allowed in that tasks cpuset. 127 - The root cpuset contains all the systems CPUs and Memory 128 Nodes. 129 - For any cpuset, one can define child cpusets containing a subset 130 of the parents CPU and Memory Node resources. 131 - The hierarchy of cpusets can be mounted at /dev/cpuset, for 132 browsing and manipulation from user space. 133 - A cpuset may be marked exclusive, which ensures that no other 134 cpuset (except direct ancestors and descendents) may contain 135 any overlapping CPUs or Memory Nodes. 136 - You can list all the tasks (by pid) attached to any cpuset. 137 138 The implementation of cpusets requires a few, simple hooks 139 into the rest of the kernel, none in performance critical paths: 140 141 - in init/main.c, to initialize the root cpuset at system boot. 142 - in fork and exit, to attach and detach a task from its cpuset. 143 - in sched_setaffinity, to mask the requested CPUs by what's 144 allowed in that tasks cpuset. 145 - in sched.c migrate_all_tasks(), to keep migrating tasks within 146 the CPUs allowed by their cpuset, if possible. 147 - in the mbind and set_mempolicy system calls, to mask the requested 148 Memory Nodes by what's allowed in that tasks cpuset. 149 - in page_alloc.c, to restrict memory to allowed nodes. 150 - in vmscan.c, to restrict page recovery to the current cpuset. 151 152 You should mount the "cgroup" filesystem type in order to enable 153 browsing and modifying the cpusets presently known to the kernel. No 154 new system calls are added for cpusets - all support for querying and 155 modifying cpusets is via this cpuset file system. 156 157 The /proc/<pid>/status file for each task has four added lines, 158 displaying the tasks cpus_allowed (on which CPUs it may be scheduled) 159 and mems_allowed (on which Memory Nodes it may obtain memory), 160 in the two formats seen in the following example: 161 162 Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff 163 Cpus_allowed_list: 0-127 164 Mems_allowed: ffffffff,ffffffff 165 Mems_allowed_list: 0-63 166 167 Each cpuset is represented by a directory in the cgroup file system 168 containing (on top of the standard cgroup files) the following 169 files describing that cpuset: 170 171 - cpus: list of CPUs in that cpuset 172 - mems: list of Memory Nodes in that cpuset 173 - memory_migrate flag: if set, move pages to cpusets nodes 174 - cpu_exclusive flag: is cpu placement exclusive? 175 - mem_exclusive flag: is memory placement exclusive? 176 - mem_hardwall flag: is memory allocation hardwalled 177 - memory_pressure: measure of how much paging pressure in cpuset 178 179 In addition, the root cpuset only has the following file: 180 - memory_pressure_enabled flag: compute memory_pressure? 181 182 New cpusets are created using the mkdir system call or shell 183 command. The properties of a cpuset, such as its flags, allowed 184 CPUs and Memory Nodes, and attached tasks, are modified by writing 185 to the appropriate file in that cpusets directory, as listed above. 186 187 The named hierarchical structure of nested cpusets allows partitioning 188 a large system into nested, dynamically changeable, "soft-partitions". 189 190 The attachment of each task, automatically inherited at fork by any 191 children of that task, to a cpuset allows organizing the work load 192 on a system into related sets of tasks such that each set is constrained 193 to using the CPUs and Memory Nodes of a particular cpuset. A task 194 may be re-attached to any other cpuset, if allowed by the permissions 195 on the necessary cpuset file system directories. 196 197 Such management of a system "in the large" integrates smoothly with 198 the detailed placement done on individual tasks and memory regions 199 using the sched_setaffinity, mbind and set_mempolicy system calls. 200 201 The following rules apply to each cpuset: 202 203 - Its CPUs and Memory Nodes must be a subset of its parents. 204 - It can't be marked exclusive unless its parent is. 205 - If its cpu or memory is exclusive, they may not overlap any sibling. 206 207 These rules, and the natural hierarchy of cpusets, enable efficient 208 enforcement of the exclusive guarantee, without having to scan all 209 cpusets every time any of them change to ensure nothing overlaps a 210 exclusive cpuset. Also, the use of a Linux virtual file system (vfs) 211 to represent the cpuset hierarchy provides for a familiar permission 212 and name space for cpusets, with a minimum of additional kernel code. 213 214 The cpus and mems files in the root (top_cpuset) cpuset are 215 read-only. The cpus file automatically tracks the value of 216 cpu_online_map using a CPU hotplug notifier, and the mems file 217 automatically tracks the value of node_states[N_HIGH_MEMORY]--i.e., 218 nodes with memory--using the cpuset_track_online_nodes() hook. 219 220 221 1.4 What are exclusive cpusets ? 222 -------------------------------- 223 224 If a cpuset is cpu or mem exclusive, no other cpuset, other than 225 a direct ancestor or descendent, may share any of the same CPUs or 226 Memory Nodes. 227 228 A cpuset that is mem_exclusive *or* mem_hardwall is "hardwalled", 229 i.e. it restricts kernel allocations for page, buffer and other data 230 commonly shared by the kernel across multiple users. All cpusets, 231 whether hardwalled or not, restrict allocations of memory for user 232 space. This enables configuring a system so that several independent 233 jobs can share common kernel data, such as file system pages, while 234 isolating each job's user allocation in its own cpuset. To do this, 235 construct a large mem_exclusive cpuset to hold all the jobs, and 236 construct child, non-mem_exclusive cpusets for each individual job. 237 Only a small amount of typical kernel memory, such as requests from 238 interrupt handlers, is allowed to be taken outside even a 239 mem_exclusive cpuset. 240 241 242 1.5 What is memory_pressure ? 243 ----------------------------- 244 The memory_pressure of a cpuset provides a simple per-cpuset metric 245 of the rate that the tasks in a cpuset are attempting to free up in 246 use memory on the nodes of the cpuset to satisfy additional memory 247 requests. 248 249 This enables batch managers monitoring jobs running in dedicated 250 cpusets to efficiently detect what level of memory pressure that job 251 is causing. 252 253 This is useful both on tightly managed systems running a wide mix of 254 submitted jobs, which may choose to terminate or re-prioritize jobs that 255 are trying to use more memory than allowed on the nodes assigned them, 256 and with tightly coupled, long running, massively parallel scientific 257 computing jobs that will dramatically fail to meet required performance 258 goals if they start to use more memory than allowed to them. 259 260 This mechanism provides a very economical way for the batch manager 261 to monitor a cpuset for signs of memory pressure. It's up to the 262 batch manager or other user code to decide what to do about it and 263 take action. 264 265 ==> Unless this feature is enabled by writing "1" to the special file 266 /dev/cpuset/memory_pressure_enabled, the hook in the rebalance 267 code of __alloc_pages() for this metric reduces to simply noticing 268 that the cpuset_memory_pressure_enabled flag is zero. So only 269 systems that enable this feature will compute the metric. 270 271 Why a per-cpuset, running average: 272 273 Because this meter is per-cpuset, rather than per-task or mm, 274 the system load imposed by a batch scheduler monitoring this 275 metric is sharply reduced on large systems, because a scan of 276 the tasklist can be avoided on each set of queries. 277 278 Because this meter is a running average, instead of an accumulating 279 counter, a batch scheduler can detect memory pressure with a 280 single read, instead of having to read and accumulate results 281 for a period of time. 282 283 Because this meter is per-cpuset rather than per-task or mm, 284 the batch scheduler can obtain the key information, memory 285 pressure in a cpuset, with a single read, rather than having to 286 query and accumulate results over all the (dynamically changing) 287 set of tasks in the cpuset. 288 289 A per-cpuset simple digital filter (requires a spinlock and 3 words 290 of data per-cpuset) is kept, and updated by any task attached to that 291 cpuset, if it enters the synchronous (direct) page reclaim code. 292 293 A per-cpuset file provides an integer number representing the recent 294 (half-life of 10 seconds) rate of direct page reclaims caused by 295 the tasks in the cpuset, in units of reclaims attempted per second, 296 times 1000. 297 298 299 1.6 What is memory spread ? 300 --------------------------- 301 There are two boolean flag files per cpuset that control where the 302 kernel allocates pages for the file system buffers and related in 303 kernel data structures. They are called 'memory_spread_page' and 304 'memory_spread_slab'. 305 306 If the per-cpuset boolean flag file 'memory_spread_page' is set, then 307 the kernel will spread the file system buffers (page cache) evenly 308 over all the nodes that the faulting task is allowed to use, instead 309 of preferring to put those pages on the node where the task is running. 310 311 If the per-cpuset boolean flag file 'memory_spread_slab' is set, 312 then the kernel will spread some file system related slab caches, 313 such as for inodes and dentries evenly over all the nodes that the 314 faulting task is allowed to use, instead of preferring to put those 315 pages on the node where the task is running. 316 317 The setting of these flags does not affect anonymous data segment or 318 stack segment pages of a task. 319 320 By default, both kinds of memory spreading are off, and memory 321 pages are allocated on the node local to where the task is running, 322 except perhaps as modified by the tasks NUMA mempolicy or cpuset 323 configuration, so long as sufficient free memory pages are available. 324 325 When new cpusets are created, they inherit the memory spread settings 326 of their parent. 327 328 Setting memory spreading causes allocations for the affected page 329 or slab caches to ignore the tasks NUMA mempolicy and be spread 330 instead. Tasks using mbind() or set_mempolicy() calls to set NUMA 331 mempolicies will not notice any change in these calls as a result of 332 their containing tasks memory spread settings. If memory spreading 333 is turned off, then the currently specified NUMA mempolicy once again 334 applies to memory page allocations. 335 336 Both 'memory_spread_page' and 'memory_spread_slab' are boolean flag 337 files. By default they contain "0", meaning that the feature is off 338 for that cpuset. If a "1" is written to that file, then that turns 339 the named feature on. 340 341 The implementation is simple. 342 343 Setting the flag 'memory_spread_page' turns on a per-process flag 344 PF_SPREAD_PAGE for each task that is in that cpuset or subsequently 345 joins that cpuset. The page allocation calls for the page cache 346 is modified to perform an inline check for this PF_SPREAD_PAGE task 347 flag, and if set, a call to a new routine cpuset_mem_spread_node() 348 returns the node to prefer for the allocation. 349 350 Similarly, setting 'memory_spread_slab' turns on the flag 351 PF_SPREAD_SLAB, and appropriately marked slab caches will allocate 352 pages from the node returned by cpuset_mem_spread_node(). 353 354 The cpuset_mem_spread_node() routine is also simple. It uses the 355 value of a per-task rotor cpuset_mem_spread_rotor to select the next 356 node in the current tasks mems_allowed to prefer for the allocation. 357 358 This memory placement policy is also known (in other contexts) as 359 round-robin or interleave. 360 361 This policy can provide substantial improvements for jobs that need 362 to place thread local data on the corresponding node, but that need 363 to access large file system data sets that need to be spread across 364 the several nodes in the jobs cpuset in order to fit. Without this 365 policy, especially for jobs that might have one thread reading in the 366 data set, the memory allocation across the nodes in the jobs cpuset 367 can become very uneven. 368 369 1.7 What is sched_load_balance ? 370 -------------------------------- 371 372 The kernel scheduler (kernel/sched.c) automatically load balances 373 tasks. If one CPU is underutilized, kernel code running on that 374 CPU will look for tasks on other more overloaded CPUs and move those 375 tasks to itself, within the constraints of such placement mechanisms 376 as cpusets and sched_setaffinity. 377 378 The algorithmic cost of load balancing and its impact on key shared 379 kernel data structures such as the task list increases more than 380 linearly with the number of CPUs being balanced. So the scheduler 381 has support to partition the systems CPUs into a number of sched 382 domains such that it only load balances within each sched domain. 383 Each sched domain covers some subset of the CPUs in the system; 384 no two sched domains overlap; some CPUs might not be in any sched 385 domain and hence won't be load balanced. 386 387 Put simply, it costs less to balance between two smaller sched domains 388 than one big one, but doing so means that overloads in one of the 389 two domains won't be load balanced to the other one. 390 391 By default, there is one sched domain covering all CPUs, except those 392 marked isolated using the kernel boot time "isolcpus=" argument. 393 394 This default load balancing across all CPUs is not well suited for 395 the following two situations: 396 1) On large systems, load balancing across many CPUs is expensive. 397 If the system is managed using cpusets to place independent jobs 398 on separate sets of CPUs, full load balancing is unnecessary. 399 2) Systems supporting realtime on some CPUs need to minimize 400 system overhead on those CPUs, including avoiding task load 401 balancing if that is not needed. 402 403 When the per-cpuset flag "sched_load_balance" is enabled (the default 404 setting), it requests that all the CPUs in that cpusets allowed 'cpus' 405 be contained in a single sched domain, ensuring that load balancing 406 can move a task (not otherwised pinned, as by sched_setaffinity) 407 from any CPU in that cpuset to any other. 408 409 When the per-cpuset flag "sched_load_balance" is disabled, then the 410 scheduler will avoid load balancing across the CPUs in that cpuset, 411 --except-- in so far as is necessary because some overlapping cpuset 412 has "sched_load_balance" enabled. 413 414 So, for example, if the top cpuset has the flag "sched_load_balance" 415 enabled, then the scheduler will have one sched domain covering all 416 CPUs, and the setting of the "sched_load_balance" flag in any other 417 cpusets won't matter, as we're already fully load balancing. 418 419 Therefore in the above two situations, the top cpuset flag 420 "sched_load_balance" should be disabled, and only some of the smaller, 421 child cpusets have this flag enabled. 422 423 When doing this, you don't usually want to leave any unpinned tasks in 424 the top cpuset that might use non-trivial amounts of CPU, as such tasks 425 may be artificially constrained to some subset of CPUs, depending on 426 the particulars of this flag setting in descendent cpusets. Even if 427 such a task could use spare CPU cycles in some other CPUs, the kernel 428 scheduler might not consider the possibility of load balancing that 429 task to that underused CPU. 430 431 Of course, tasks pinned to a particular CPU can be left in a cpuset 432 that disables "sched_load_balance" as those tasks aren't going anywhere 433 else anyway. 434 435 There is an impedance mismatch here, between cpusets and sched domains. 436 Cpusets are hierarchical and nest. Sched domains are flat; they don't 437 overlap and each CPU is in at most one sched domain. 438 439 It is necessary for sched domains to be flat because load balancing 440 across partially overlapping sets of CPUs would risk unstable dynamics 441 that would be beyond our understanding. So if each of two partially 442 overlapping cpusets enables the flag 'sched_load_balance', then we 443 form a single sched domain that is a superset of both. We won't move 444 a task to a CPU outside it cpuset, but the scheduler load balancing 445 code might waste some compute cycles considering that possibility. 446 447 This mismatch is why there is not a simple one-to-one relation 448 between which cpusets have the flag "sched_load_balance" enabled, 449 and the sched domain configuration. If a cpuset enables the flag, it 450 will get balancing across all its CPUs, but if it disables the flag, 451 it will only be assured of no load balancing if no other overlapping 452 cpuset enables the flag. 453 454 If two cpusets have partially overlapping 'cpus' allowed, and only 455 one of them has this flag enabled, then the other may find its 456 tasks only partially load balanced, just on the overlapping CPUs. 457 This is just the general case of the top_cpuset example given a few 458 paragraphs above. In the general case, as in the top cpuset case, 459 don't leave tasks that might use non-trivial amounts of CPU in 460 such partially load balanced cpusets, as they may be artificially 461 constrained to some subset of the CPUs allowed to them, for lack of 462 load balancing to the other CPUs. 463 464 1.7.1 sched_load_balance implementation details. 465 ------------------------------------------------ 466 467 The per-cpuset flag 'sched_load_balance' defaults to enabled (contrary 468 to most cpuset flags.) When enabled for a cpuset, the kernel will 469 ensure that it can load balance across all the CPUs in that cpuset 470 (makes sure that all the CPUs in the cpus_allowed of that cpuset are 471 in the same sched domain.) 472 473 If two overlapping cpusets both have 'sched_load_balance' enabled, 474 then they will be (must be) both in the same sched domain. 475 476 If, as is the default, the top cpuset has 'sched_load_balance' enabled, 477 then by the above that means there is a single sched domain covering 478 the whole system, regardless of any other cpuset settings. 479 480 The kernel commits to user space that it will avoid load balancing 481 where it can. It will pick as fine a granularity partition of sched 482 domains as it can while still providing load balancing for any set 483 of CPUs allowed to a cpuset having 'sched_load_balance' enabled. 484 485 The internal kernel cpuset to scheduler interface passes from the 486 cpuset code to the scheduler code a partition of the load balanced 487 CPUs in the system. This partition is a set of subsets (represented 488 as an array of cpumask_t) of CPUs, pairwise disjoint, that cover all 489 the CPUs that must be load balanced. 490 491 Whenever the 'sched_load_balance' flag changes, or CPUs come or go 492 from a cpuset with this flag enabled, or a cpuset with this flag 493 enabled is removed, the cpuset code builds a new such partition and 494 passes it to the scheduler sched domain setup code, to have the sched 495 domains rebuilt as necessary. 496 497 This partition exactly defines what sched domains the scheduler should 498 setup - one sched domain for each element (cpumask_t) in the partition. 499 500 The scheduler remembers the currently active sched domain partitions. 501 When the scheduler routine partition_sched_domains() is invoked from 502 the cpuset code to update these sched domains, it compares the new 503 partition requested with the current, and updates its sched domains, 504 removing the old and adding the new, for each change. 505 506 507 1.8 What is sched_relax_domain_level ? 508 -------------------------------------- 509 510 In sched domain, the scheduler migrates tasks in 2 ways; periodic load 511 balance on tick, and at time of some schedule events. 512 513 When a task is woken up, scheduler try to move the task on idle CPU. 514 For example, if a task A running on CPU X activates another task B 515 on the same CPU X, and if CPU Y is X's sibling and performing idle, 516 then scheduler migrate task B to CPU Y so that task B can start on 517 CPU Y without waiting task A on CPU X. 518 519 And if a CPU run out of tasks in its runqueue, the CPU try to pull 520 extra tasks from other busy CPUs to help them before it is going to 521 be idle. 522 523 Of course it takes some searching cost to find movable tasks and/or 524 idle CPUs, the scheduler might not search all CPUs in the domain 525 everytime. In fact, in some architectures, the searching ranges on 526 events are limited in the same socket or node where the CPU locates, 527 while the load balance on tick searchs all. 528 529 For example, assume CPU Z is relatively far from CPU X. Even if CPU Z 530 is idle while CPU X and the siblings are busy, scheduler can't migrate 531 woken task B from X to Z since it is out of its searching range. 532 As the result, task B on CPU X need to wait task A or wait load balance 533 on the next tick. For some applications in special situation, waiting 534 1 tick may be too long. 535 536 The 'sched_relax_domain_level' file allows you to request changing 537 this searching range as you like. This file takes int value which 538 indicates size of searching range in levels ideally as follows, 539 otherwise initial value -1 that indicates the cpuset has no request. 540 541 -1 : no request. use system default or follow request of others. 542 0 : no search. 543 1 : search siblings (hyperthreads in a core). 544 2 : search cores in a package. 545 3 : search cpus in a node [= system wide on non-NUMA system] 546 ( 4 : search nodes in a chunk of node [on NUMA system] ) 547 ( 5 : search system wide [on NUMA system] ) 548 549 The system default is architecture dependent. The system default 550 can be changed using the relax_domain_level= boot parameter. 551 552 This file is per-cpuset and affect the sched domain where the cpuset 553 belongs to. Therefore if the flag 'sched_load_balance' of a cpuset 554 is disabled, then 'sched_relax_domain_level' have no effect since 555 there is no sched domain belonging the cpuset. 556 557 If multiple cpusets are overlapping and hence they form a single sched 558 domain, the largest value among those is used. Be careful, if one 559 requests 0 and others are -1 then 0 is used. 560 561 Note that modifying this file will have both good and bad effects, 562 and whether it is acceptable or not will be depend on your situation. 563 Don't modify this file if you are not sure. 564 565 If your situation is: 566 - The migration costs between each cpu can be assumed considerably 567 small(for you) due to your special application's behavior or 568 special hardware support for CPU cache etc. 569 - The searching cost doesn't have impact(for you) or you can make 570 the searching cost enough small by managing cpuset to compact etc. 571 - The latency is required even it sacrifices cache hit rate etc. 572 then increasing 'sched_relax_domain_level' would benefit you. 573 574 575 1.9 How do I use cpusets ? 576 -------------------------- 577 578 In order to minimize the impact of cpusets on critical kernel 579 code, such as the scheduler, and due to the fact that the kernel 580 does not support one task updating the memory placement of another 581 task directly, the impact on a task of changing its cpuset CPU 582 or Memory Node placement, or of changing to which cpuset a task 583 is attached, is subtle. 584 585 If a cpuset has its Memory Nodes modified, then for each task attached 586 to that cpuset, the next time that the kernel attempts to allocate 587 a page of memory for that task, the kernel will notice the change 588 in the tasks cpuset, and update its per-task memory placement to 589 remain within the new cpusets memory placement. If the task was using 590 mempolicy MPOL_BIND, and the nodes to which it was bound overlap with 591 its new cpuset, then the task will continue to use whatever subset 592 of MPOL_BIND nodes are still allowed in the new cpuset. If the task 593 was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed 594 in the new cpuset, then the task will be essentially treated as if it 595 was MPOL_BIND bound to the new cpuset (even though its numa placement, 596 as queried by get_mempolicy(), doesn't change). If a task is moved 597 from one cpuset to another, then the kernel will adjust the tasks 598 memory placement, as above, the next time that the kernel attempts 599 to allocate a page of memory for that task. 600 601 If a cpuset has its 'cpus' modified, then each task in that cpuset 602 will have its allowed CPU placement changed immediately. Similarly, 603 if a tasks pid is written to a cpusets 'tasks' file, in either its 604 current cpuset or another cpuset, then its allowed CPU placement is 605 changed immediately. If such a task had been bound to some subset 606 of its cpuset using the sched_setaffinity() call, the task will be 607 allowed to run on any CPU allowed in its new cpuset, negating the 608 affect of the prior sched_setaffinity() call. 609 610 In summary, the memory placement of a task whose cpuset is changed is 611 updated by the kernel, on the next allocation of a page for that task, 612 but the processor placement is not updated, until that tasks pid is 613 rewritten to the 'tasks' file of its cpuset. This is done to avoid 614 impacting the scheduler code in the kernel with a check for changes 615 in a tasks processor placement. 616 617 Normally, once a page is allocated (given a physical page 618 of main memory) then that page stays on whatever node it 619 was allocated, so long as it remains allocated, even if the 620 cpusets memory placement policy 'mems' subsequently changes. 621 If the cpuset flag file 'memory_migrate' is set true, then when 622 tasks are attached to that cpuset, any pages that task had 623 allocated to it on nodes in its previous cpuset are migrated 624 to the tasks new cpuset. The relative placement of the page within 625 the cpuset is preserved during these migration operations if possible. 626 For example if the page was on the second valid node of the prior cpuset 627 then the page will be placed on the second valid node of the new cpuset. 628 629 Also if 'memory_migrate' is set true, then if that cpusets 630 'mems' file is modified, pages allocated to tasks in that 631 cpuset, that were on nodes in the previous setting of 'mems', 632 will be moved to nodes in the new setting of 'mems.' 633 Pages that were not in the tasks prior cpuset, or in the cpusets 634 prior 'mems' setting, will not be moved. 635 636 There is an exception to the above. If hotplug functionality is used 637 to remove all the CPUs that are currently assigned to a cpuset, 638 then all the tasks in that cpuset will be moved to the nearest ancestor 639 with non-empty cpus. But the moving of some (or all) tasks might fail if 640 cpuset is bound with another cgroup subsystem which has some restrictions 641 on task attaching. In this failing case, those tasks will stay 642 in the original cpuset, and the kernel will automatically update 643 their cpus_allowed to allow all online CPUs. When memory hotplug 644 functionality for removing Memory Nodes is available, a similar exception 645 is expected to apply there as well. In general, the kernel prefers to 646 violate cpuset placement, over starving a task that has had all 647 its allowed CPUs or Memory Nodes taken offline. 648 649 There is a second exception to the above. GFP_ATOMIC requests are 650 kernel internal allocations that must be satisfied, immediately. 651 The kernel may drop some request, in rare cases even panic, if a 652 GFP_ATOMIC alloc fails. If the request cannot be satisfied within 653 the current tasks cpuset, then we relax the cpuset, and look for 654 memory anywhere we can find it. It's better to violate the cpuset 655 than stress the kernel. 656 657 To start a new job that is to be contained within a cpuset, the steps are: 658 659 1) mkdir /dev/cpuset 660 2) mount -t cgroup -ocpuset cpuset /dev/cpuset 661 3) Create the new cpuset by doing mkdir's and write's (or echo's) in 662 the /dev/cpuset virtual file system. 663 4) Start a task that will be the "founding father" of the new job. 664 5) Attach that task to the new cpuset by writing its pid to the 665 /dev/cpuset tasks file for that cpuset. 666 6) fork, exec or clone the job tasks from this founding father task. 667 668 For example, the following sequence of commands will setup a cpuset 669 named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, 670 and then start a subshell 'sh' in that cpuset: 671 672 mount -t cgroup -ocpuset cpuset /dev/cpuset 673 cd /dev/cpuset 674 mkdir Charlie 675 cd Charlie 676 /bin/echo 2-3 > cpus 677 /bin/echo 1 > mems 678 /bin/echo $$ > tasks 679 sh 680 # The subshell 'sh' is now running in cpuset Charlie 681 # The next line should display '/Charlie' 682 cat /proc/self/cpuset 683 684 In the future, a C library interface to cpusets will likely be 685 available. For now, the only way to query or modify cpusets is 686 via the cpuset file system, using the various cd, mkdir, echo, cat, 687 rmdir commands from the shell, or their equivalent from C. 688 689 The sched_setaffinity calls can also be done at the shell prompt using 690 SGI's runon or Robert Love's taskset. The mbind and set_mempolicy 691 calls can be done at the shell prompt using the numactl command 692 (part of Andi Kleen's numa package). 693 694 2. Usage Examples and Syntax 695 ============================ 696 697 2.1 Basic Usage 698 --------------- 699 700 Creating, modifying, using the cpusets can be done through the cpuset 701 virtual filesystem. 702 703 To mount it, type: 704 # mount -t cgroup -o cpuset cpuset /dev/cpuset 705 706 Then under /dev/cpuset you can find a tree that corresponds to the 707 tree of the cpusets in the system. For instance, /dev/cpuset 708 is the cpuset that holds the whole system. 709 710 If you want to create a new cpuset under /dev/cpuset: 711 # cd /dev/cpuset 712 # mkdir my_cpuset 713 714 Now you want to do something with this cpuset. 715 # cd my_cpuset 716 717 In this directory you can find several files: 718 # ls 719 cpu_exclusive memory_migrate mems tasks 720 cpus memory_pressure notify_on_release 721 mem_exclusive memory_spread_page sched_load_balance 722 mem_hardwall memory_spread_slab sched_relax_domain_level 723 724 Reading them will give you information about the state of this cpuset: 725 the CPUs and Memory Nodes it can use, the processes that are using 726 it, its properties. By writing to these files you can manipulate 727 the cpuset. 728 729 Set some flags: 730 # /bin/echo 1 > cpu_exclusive 731 732 Add some cpus: 733 # /bin/echo 0-7 > cpus 734 735 Add some mems: 736 # /bin/echo 0-7 > mems 737 738 Now attach your shell to this cpuset: 739 # /bin/echo $$ > tasks 740 741 You can also create cpusets inside your cpuset by using mkdir in this 742 directory. 743 # mkdir my_sub_cs 744 745 To remove a cpuset, just use rmdir: 746 # rmdir my_sub_cs 747 This will fail if the cpuset is in use (has cpusets inside, or has 748 processes attached). 749 750 Note that for legacy reasons, the "cpuset" filesystem exists as a 751 wrapper around the cgroup filesystem. 752 753 The command 754 755 mount -t cpuset X /dev/cpuset 756 757 is equivalent to 758 759 mount -t cgroup -ocpuset X /dev/cpuset 760 echo "/sbin/cpuset_release_agent" > /dev/cpuset/release_agent 761 762 2.2 Adding/removing cpus 763 ------------------------ 764 765 This is the syntax to use when writing in the cpus or mems files 766 in cpuset directories: 767 768 # /bin/echo 1-4 > cpus -> set cpus list to cpus 1,2,3,4 769 # /bin/echo 1,2,3,4 > cpus -> set cpus list to cpus 1,2,3,4 770 771 2.3 Setting flags 772 ----------------- 773 774 The syntax is very simple: 775 776 # /bin/echo 1 > cpu_exclusive -> set flag 'cpu_exclusive' 777 # /bin/echo 0 > cpu_exclusive -> unset flag 'cpu_exclusive' 778 779 2.4 Attaching processes 780 ----------------------- 781 782 # /bin/echo PID > tasks 783 784 Note that it is PID, not PIDs. You can only attach ONE task at a time. 785 If you have several tasks to attach, you have to do it one after another: 786 787 # /bin/echo PID1 > tasks 788 # /bin/echo PID2 > tasks 789 ... 790 # /bin/echo PIDn > tasks 791 792 793 3. Questions 794 ============ 795 796 Q: what's up with this '/bin/echo' ? 797 A: bash's builtin 'echo' command does not check calls to write() against 798 errors. If you use it in the cpuset file system, you won't be 799 able to tell whether a command succeeded or failed. 800 801 Q: When I attach processes, only the first of the line gets really attached ! 802 A: We can only return one error code per call to write(). So you should also 803 put only ONE pid. 804 805 4. Contact 806 ========== 807 808 Web: http://www.bullopensource.org/cpuset
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