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Linux/Documentation/cgroups/cpusets.txt

  1                                 CPUSETS
  2                                 -------
  3 
  4 Copyright (C) 2004 BULL SA.
  5 Written by Simon.Derr@bull.net
  6 
  7 Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
  8 Modified by Paul Jackson <pj@sgi.com>
  9 Modified by Christoph Lameter <clameter@sgi.com>
 10 Modified by Paul Menage <menage@google.com>
 11 Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.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 task's 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/cgroups/cgroups.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 task's 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 task's 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 task's cpuset.
125  - Calls to mbind and set_mempolicy are filtered to just
126    those Memory Nodes allowed in that task's 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 descendants) 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 task's cpuset.
145  - in sched.c migrate_live_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 task's 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 task's 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  - cpuset.cpus: list of CPUs in that cpuset
172  - cpuset.mems: list of Memory Nodes in that cpuset
173  - cpuset.memory_migrate flag: if set, move pages to cpusets nodes
174  - cpuset.cpu_exclusive flag: is cpu placement exclusive?
175  - cpuset.mem_exclusive flag: is memory placement exclusive?
176  - cpuset.mem_hardwall flag:  is memory allocation hardwalled
177  - cpuset.memory_pressure: measure of how much paging pressure in cpuset
178  - cpuset.memory_spread_page flag: if set, spread page cache evenly on allowed nodes
179  - cpuset.memory_spread_slab flag: if set, spread slab cache evenly on allowed nodes
180  - cpuset.sched_load_balance flag: if set, load balance within CPUs on that cpuset
181  - cpuset.sched_relax_domain_level: the searching range when migrating tasks
182 
183 In addition, only the root cpuset has the following file:
184  - cpuset.memory_pressure_enabled flag: compute memory_pressure?
185 
186 New cpusets are created using the mkdir system call or shell
187 command.  The properties of a cpuset, such as its flags, allowed
188 CPUs and Memory Nodes, and attached tasks, are modified by writing
189 to the appropriate file in that cpusets directory, as listed above.
190 
191 The named hierarchical structure of nested cpusets allows partitioning
192 a large system into nested, dynamically changeable, "soft-partitions".
193 
194 The attachment of each task, automatically inherited at fork by any
195 children of that task, to a cpuset allows organizing the work load
196 on a system into related sets of tasks such that each set is constrained
197 to using the CPUs and Memory Nodes of a particular cpuset.  A task
198 may be re-attached to any other cpuset, if allowed by the permissions
199 on the necessary cpuset file system directories.
200 
201 Such management of a system "in the large" integrates smoothly with
202 the detailed placement done on individual tasks and memory regions
203 using the sched_setaffinity, mbind and set_mempolicy system calls.
204 
205 The following rules apply to each cpuset:
206 
207  - Its CPUs and Memory Nodes must be a subset of its parents.
208  - It can't be marked exclusive unless its parent is.
209  - If its cpu or memory is exclusive, they may not overlap any sibling.
210 
211 These rules, and the natural hierarchy of cpusets, enable efficient
212 enforcement of the exclusive guarantee, without having to scan all
213 cpusets every time any of them change to ensure nothing overlaps a
214 exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
215 to represent the cpuset hierarchy provides for a familiar permission
216 and name space for cpusets, with a minimum of additional kernel code.
217 
218 The cpus and mems files in the root (top_cpuset) cpuset are
219 read-only.  The cpus file automatically tracks the value of
220 cpu_online_mask using a CPU hotplug notifier, and the mems file
221 automatically tracks the value of node_states[N_MEMORY]--i.e.,
222 nodes with memory--using the cpuset_track_online_nodes() hook.
223 
224 
225 1.4 What are exclusive cpusets ?
226 --------------------------------
227 
228 If a cpuset is cpu or mem exclusive, no other cpuset, other than
229 a direct ancestor or descendant, may share any of the same CPUs or
230 Memory Nodes.
231 
232 A cpuset that is cpuset.mem_exclusive *or* cpuset.mem_hardwall is "hardwalled",
233 i.e. it restricts kernel allocations for page, buffer and other data
234 commonly shared by the kernel across multiple users.  All cpusets,
235 whether hardwalled or not, restrict allocations of memory for user
236 space.  This enables configuring a system so that several independent
237 jobs can share common kernel data, such as file system pages, while
238 isolating each job's user allocation in its own cpuset.  To do this,
239 construct a large mem_exclusive cpuset to hold all the jobs, and
240 construct child, non-mem_exclusive cpusets for each individual job.
241 Only a small amount of typical kernel memory, such as requests from
242 interrupt handlers, is allowed to be taken outside even a
243 mem_exclusive cpuset.
244 
245 
246 1.5 What is memory_pressure ?
247 -----------------------------
248 The memory_pressure of a cpuset provides a simple per-cpuset metric
249 of the rate that the tasks in a cpuset are attempting to free up in
250 use memory on the nodes of the cpuset to satisfy additional memory
251 requests.
252 
253 This enables batch managers monitoring jobs running in dedicated
254 cpusets to efficiently detect what level of memory pressure that job
255 is causing.
256 
257 This is useful both on tightly managed systems running a wide mix of
258 submitted jobs, which may choose to terminate or re-prioritize jobs that
259 are trying to use more memory than allowed on the nodes assigned to them,
260 and with tightly coupled, long running, massively parallel scientific
261 computing jobs that will dramatically fail to meet required performance
262 goals if they start to use more memory than allowed to them.
263 
264 This mechanism provides a very economical way for the batch manager
265 to monitor a cpuset for signs of memory pressure.  It's up to the
266 batch manager or other user code to decide what to do about it and
267 take action.
268 
269 ==> Unless this feature is enabled by writing "1" to the special file
270     /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
271     code of __alloc_pages() for this metric reduces to simply noticing
272     that the cpuset_memory_pressure_enabled flag is zero.  So only
273     systems that enable this feature will compute the metric.
274 
275 Why a per-cpuset, running average:
276 
277     Because this meter is per-cpuset, rather than per-task or mm,
278     the system load imposed by a batch scheduler monitoring this
279     metric is sharply reduced on large systems, because a scan of
280     the tasklist can be avoided on each set of queries.
281 
282     Because this meter is a running average, instead of an accumulating
283     counter, a batch scheduler can detect memory pressure with a
284     single read, instead of having to read and accumulate results
285     for a period of time.
286 
287     Because this meter is per-cpuset rather than per-task or mm,
288     the batch scheduler can obtain the key information, memory
289     pressure in a cpuset, with a single read, rather than having to
290     query and accumulate results over all the (dynamically changing)
291     set of tasks in the cpuset.
292 
293 A per-cpuset simple digital filter (requires a spinlock and 3 words
294 of data per-cpuset) is kept, and updated by any task attached to that
295 cpuset, if it enters the synchronous (direct) page reclaim code.
296 
297 A per-cpuset file provides an integer number representing the recent
298 (half-life of 10 seconds) rate of direct page reclaims caused by
299 the tasks in the cpuset, in units of reclaims attempted per second,
300 times 1000.
301 
302 
303 1.6 What is memory spread ?
304 ---------------------------
305 There are two boolean flag files per cpuset that control where the
306 kernel allocates pages for the file system buffers and related in
307 kernel data structures.  They are called 'cpuset.memory_spread_page' and
308 'cpuset.memory_spread_slab'.
309 
310 If the per-cpuset boolean flag file 'cpuset.memory_spread_page' is set, then
311 the kernel will spread the file system buffers (page cache) evenly
312 over all the nodes that the faulting task is allowed to use, instead
313 of preferring to put those pages on the node where the task is running.
314 
315 If the per-cpuset boolean flag file 'cpuset.memory_spread_slab' is set,
316 then the kernel will spread some file system related slab caches,
317 such as for inodes and dentries evenly over all the nodes that the
318 faulting task is allowed to use, instead of preferring to put those
319 pages on the node where the task is running.
320 
321 The setting of these flags does not affect anonymous data segment or
322 stack segment pages of a task.
323 
324 By default, both kinds of memory spreading are off, and memory
325 pages are allocated on the node local to where the task is running,
326 except perhaps as modified by the task's NUMA mempolicy or cpuset
327 configuration, so long as sufficient free memory pages are available.
328 
329 When new cpusets are created, they inherit the memory spread settings
330 of their parent.
331 
332 Setting memory spreading causes allocations for the affected page
333 or slab caches to ignore the task's NUMA mempolicy and be spread
334 instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
335 mempolicies will not notice any change in these calls as a result of
336 their containing task's memory spread settings.  If memory spreading
337 is turned off, then the currently specified NUMA mempolicy once again
338 applies to memory page allocations.
339 
340 Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boolean flag
341 files.  By default they contain "0", meaning that the feature is off
342 for that cpuset.  If a "1" is written to that file, then that turns
343 the named feature on.
344 
345 The implementation is simple.
346 
347 Setting the flag 'cpuset.memory_spread_page' turns on a per-process flag
348 PFA_SPREAD_PAGE for each task that is in that cpuset or subsequently
349 joins that cpuset.  The page allocation calls for the page cache
350 is modified to perform an inline check for this PFA_SPREAD_PAGE task
351 flag, and if set, a call to a new routine cpuset_mem_spread_node()
352 returns the node to prefer for the allocation.
353 
354 Similarly, setting 'cpuset.memory_spread_slab' turns on the flag
355 PFA_SPREAD_SLAB, and appropriately marked slab caches will allocate
356 pages from the node returned by cpuset_mem_spread_node().
357 
358 The cpuset_mem_spread_node() routine is also simple.  It uses the
359 value of a per-task rotor cpuset_mem_spread_rotor to select the next
360 node in the current task's mems_allowed to prefer for the allocation.
361 
362 This memory placement policy is also known (in other contexts) as
363 round-robin or interleave.
364 
365 This policy can provide substantial improvements for jobs that need
366 to place thread local data on the corresponding node, but that need
367 to access large file system data sets that need to be spread across
368 the several nodes in the jobs cpuset in order to fit.  Without this
369 policy, especially for jobs that might have one thread reading in the
370 data set, the memory allocation across the nodes in the jobs cpuset
371 can become very uneven.
372 
373 1.7 What is sched_load_balance ?
374 --------------------------------
375 
376 The kernel scheduler (kernel/sched/core.c) automatically load balances
377 tasks.  If one CPU is underutilized, kernel code running on that
378 CPU will look for tasks on other more overloaded CPUs and move those
379 tasks to itself, within the constraints of such placement mechanisms
380 as cpusets and sched_setaffinity.
381 
382 The algorithmic cost of load balancing and its impact on key shared
383 kernel data structures such as the task list increases more than
384 linearly with the number of CPUs being balanced.  So the scheduler
385 has support to partition the systems CPUs into a number of sched
386 domains such that it only load balances within each sched domain.
387 Each sched domain covers some subset of the CPUs in the system;
388 no two sched domains overlap; some CPUs might not be in any sched
389 domain and hence won't be load balanced.
390 
391 Put simply, it costs less to balance between two smaller sched domains
392 than one big one, but doing so means that overloads in one of the
393 two domains won't be load balanced to the other one.
394 
395 By default, there is one sched domain covering all CPUs, except those
396 marked isolated using the kernel boot time "isolcpus=" argument.
397 
398 This default load balancing across all CPUs is not well suited for
399 the following two situations:
400  1) On large systems, load balancing across many CPUs is expensive.
401     If the system is managed using cpusets to place independent jobs
402     on separate sets of CPUs, full load balancing is unnecessary.
403  2) Systems supporting realtime on some CPUs need to minimize
404     system overhead on those CPUs, including avoiding task load
405     balancing if that is not needed.
406 
407 When the per-cpuset flag "cpuset.sched_load_balance" is enabled (the default
408 setting), it requests that all the CPUs in that cpusets allowed 'cpuset.cpus'
409 be contained in a single sched domain, ensuring that load balancing
410 can move a task (not otherwised pinned, as by sched_setaffinity)
411 from any CPU in that cpuset to any other.
412 
413 When the per-cpuset flag "cpuset.sched_load_balance" is disabled, then the
414 scheduler will avoid load balancing across the CPUs in that cpuset,
415 --except-- in so far as is necessary because some overlapping cpuset
416 has "sched_load_balance" enabled.
417 
418 So, for example, if the top cpuset has the flag "cpuset.sched_load_balance"
419 enabled, then the scheduler will have one sched domain covering all
420 CPUs, and the setting of the "cpuset.sched_load_balance" flag in any other
421 cpusets won't matter, as we're already fully load balancing.
422 
423 Therefore in the above two situations, the top cpuset flag
424 "cpuset.sched_load_balance" should be disabled, and only some of the smaller,
425 child cpusets have this flag enabled.
426 
427 When doing this, you don't usually want to leave any unpinned tasks in
428 the top cpuset that might use non-trivial amounts of CPU, as such tasks
429 may be artificially constrained to some subset of CPUs, depending on
430 the particulars of this flag setting in descendant cpusets.  Even if
431 such a task could use spare CPU cycles in some other CPUs, the kernel
432 scheduler might not consider the possibility of load balancing that
433 task to that underused CPU.
434 
435 Of course, tasks pinned to a particular CPU can be left in a cpuset
436 that disables "cpuset.sched_load_balance" as those tasks aren't going anywhere
437 else anyway.
438 
439 There is an impedance mismatch here, between cpusets and sched domains.
440 Cpusets are hierarchical and nest.  Sched domains are flat; they don't
441 overlap and each CPU is in at most one sched domain.
442 
443 It is necessary for sched domains to be flat because load balancing
444 across partially overlapping sets of CPUs would risk unstable dynamics
445 that would be beyond our understanding.  So if each of two partially
446 overlapping cpusets enables the flag 'cpuset.sched_load_balance', then we
447 form a single sched domain that is a superset of both.  We won't move
448 a task to a CPU outside its cpuset, but the scheduler load balancing
449 code might waste some compute cycles considering that possibility.
450 
451 This mismatch is why there is not a simple one-to-one relation
452 between which cpusets have the flag "cpuset.sched_load_balance" enabled,
453 and the sched domain configuration.  If a cpuset enables the flag, it
454 will get balancing across all its CPUs, but if it disables the flag,
455 it will only be assured of no load balancing if no other overlapping
456 cpuset enables the flag.
457 
458 If two cpusets have partially overlapping 'cpuset.cpus' allowed, and only
459 one of them has this flag enabled, then the other may find its
460 tasks only partially load balanced, just on the overlapping CPUs.
461 This is just the general case of the top_cpuset example given a few
462 paragraphs above.  In the general case, as in the top cpuset case,
463 don't leave tasks that might use non-trivial amounts of CPU in
464 such partially load balanced cpusets, as they may be artificially
465 constrained to some subset of the CPUs allowed to them, for lack of
466 load balancing to the other CPUs.
467 
468 1.7.1 sched_load_balance implementation details.
469 ------------------------------------------------
470 
471 The per-cpuset flag 'cpuset.sched_load_balance' defaults to enabled (contrary
472 to most cpuset flags.)  When enabled for a cpuset, the kernel will
473 ensure that it can load balance across all the CPUs in that cpuset
474 (makes sure that all the CPUs in the cpus_allowed of that cpuset are
475 in the same sched domain.)
476 
477 If two overlapping cpusets both have 'cpuset.sched_load_balance' enabled,
478 then they will be (must be) both in the same sched domain.
479 
480 If, as is the default, the top cpuset has 'cpuset.sched_load_balance' enabled,
481 then by the above that means there is a single sched domain covering
482 the whole system, regardless of any other cpuset settings.
483 
484 The kernel commits to user space that it will avoid load balancing
485 where it can.  It will pick as fine a granularity partition of sched
486 domains as it can while still providing load balancing for any set
487 of CPUs allowed to a cpuset having 'cpuset.sched_load_balance' enabled.
488 
489 The internal kernel cpuset to scheduler interface passes from the
490 cpuset code to the scheduler code a partition of the load balanced
491 CPUs in the system. This partition is a set of subsets (represented
492 as an array of struct cpumask) of CPUs, pairwise disjoint, that cover
493 all the CPUs that must be load balanced.
494 
495 The cpuset code builds a new such partition and passes it to the
496 scheduler sched domain setup code, to have the sched domains rebuilt
497 as necessary, whenever:
498  - the 'cpuset.sched_load_balance' flag of a cpuset with non-empty CPUs changes,
499  - or CPUs come or go from a cpuset with this flag enabled,
500  - or 'cpuset.sched_relax_domain_level' value of a cpuset with non-empty CPUs
501    and with this flag enabled changes,
502  - or a cpuset with non-empty CPUs and with this flag enabled is removed,
503  - or a cpu is offlined/onlined.
504 
505 This partition exactly defines what sched domains the scheduler should
506 setup - one sched domain for each element (struct cpumask) in the
507 partition.
508 
509 The scheduler remembers the currently active sched domain partitions.
510 When the scheduler routine partition_sched_domains() is invoked from
511 the cpuset code to update these sched domains, it compares the new
512 partition requested with the current, and updates its sched domains,
513 removing the old and adding the new, for each change.
514 
515 
516 1.8 What is sched_relax_domain_level ?
517 --------------------------------------
518 
519 In sched domain, the scheduler migrates tasks in 2 ways; periodic load
520 balance on tick, and at time of some schedule events.
521 
522 When a task is woken up, scheduler try to move the task on idle CPU.
523 For example, if a task A running on CPU X activates another task B
524 on the same CPU X, and if CPU Y is X's sibling and performing idle,
525 then scheduler migrate task B to CPU Y so that task B can start on
526 CPU Y without waiting task A on CPU X.
527 
528 And if a CPU run out of tasks in its runqueue, the CPU try to pull
529 extra tasks from other busy CPUs to help them before it is going to
530 be idle.
531 
532 Of course it takes some searching cost to find movable tasks and/or
533 idle CPUs, the scheduler might not search all CPUs in the domain
534 every time.  In fact, in some architectures, the searching ranges on
535 events are limited in the same socket or node where the CPU locates,
536 while the load balance on tick searches all.
537 
538 For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
539 is idle while CPU X and the siblings are busy, scheduler can't migrate
540 woken task B from X to Z since it is out of its searching range.
541 As the result, task B on CPU X need to wait task A or wait load balance
542 on the next tick.  For some applications in special situation, waiting
543 1 tick may be too long.
544 
545 The 'cpuset.sched_relax_domain_level' file allows you to request changing
546 this searching range as you like.  This file takes int value which
547 indicates size of searching range in levels ideally as follows,
548 otherwise initial value -1 that indicates the cpuset has no request.
549 
550   -1  : no request. use system default or follow request of others.
551    0  : no search.
552    1  : search siblings (hyperthreads in a core).
553    2  : search cores in a package.
554    3  : search cpus in a node [= system wide on non-NUMA system]
555    4  : search nodes in a chunk of node [on NUMA system]
556    5  : search system wide [on NUMA system]
557 
558 The system default is architecture dependent.  The system default
559 can be changed using the relax_domain_level= boot parameter.
560 
561 This file is per-cpuset and affect the sched domain where the cpuset
562 belongs to.  Therefore if the flag 'cpuset.sched_load_balance' of a cpuset
563 is disabled, then 'cpuset.sched_relax_domain_level' have no effect since
564 there is no sched domain belonging the cpuset.
565 
566 If multiple cpusets are overlapping and hence they form a single sched
567 domain, the largest value among those is used.  Be careful, if one
568 requests 0 and others are -1 then 0 is used.
569 
570 Note that modifying this file will have both good and bad effects,
571 and whether it is acceptable or not depends on your situation.
572 Don't modify this file if you are not sure.
573 
574 If your situation is:
575  - The migration costs between each cpu can be assumed considerably
576    small(for you) due to your special application's behavior or
577    special hardware support for CPU cache etc.
578  - The searching cost doesn't have impact(for you) or you can make
579    the searching cost enough small by managing cpuset to compact etc.
580  - The latency is required even it sacrifices cache hit rate etc.
581 then increasing 'sched_relax_domain_level' would benefit you.
582 
583 
584 1.9 How do I use cpusets ?
585 --------------------------
586 
587 In order to minimize the impact of cpusets on critical kernel
588 code, such as the scheduler, and due to the fact that the kernel
589 does not support one task updating the memory placement of another
590 task directly, the impact on a task of changing its cpuset CPU
591 or Memory Node placement, or of changing to which cpuset a task
592 is attached, is subtle.
593 
594 If a cpuset has its Memory Nodes modified, then for each task attached
595 to that cpuset, the next time that the kernel attempts to allocate
596 a page of memory for that task, the kernel will notice the change
597 in the task's cpuset, and update its per-task memory placement to
598 remain within the new cpusets memory placement.  If the task was using
599 mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
600 its new cpuset, then the task will continue to use whatever subset
601 of MPOL_BIND nodes are still allowed in the new cpuset.  If the task
602 was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
603 in the new cpuset, then the task will be essentially treated as if it
604 was MPOL_BIND bound to the new cpuset (even though its NUMA placement,
605 as queried by get_mempolicy(), doesn't change).  If a task is moved
606 from one cpuset to another, then the kernel will adjust the task's
607 memory placement, as above, the next time that the kernel attempts
608 to allocate a page of memory for that task.
609 
610 If a cpuset has its 'cpuset.cpus' modified, then each task in that cpuset
611 will have its allowed CPU placement changed immediately.  Similarly,
612 if a task's pid is written to another cpusets 'cpuset.tasks' file, then its
613 allowed CPU placement is changed immediately.  If such a task had been
614 bound to some subset of its cpuset using the sched_setaffinity() call,
615 the task will be allowed to run on any CPU allowed in its new cpuset,
616 negating the effect of the prior sched_setaffinity() call.
617 
618 In summary, the memory placement of a task whose cpuset is changed is
619 updated by the kernel, on the next allocation of a page for that task,
620 and the processor placement is updated immediately.
621 
622 Normally, once a page is allocated (given a physical page
623 of main memory) then that page stays on whatever node it
624 was allocated, so long as it remains allocated, even if the
625 cpusets memory placement policy 'cpuset.mems' subsequently changes.
626 If the cpuset flag file 'cpuset.memory_migrate' is set true, then when
627 tasks are attached to that cpuset, any pages that task had
628 allocated to it on nodes in its previous cpuset are migrated
629 to the task's new cpuset. The relative placement of the page within
630 the cpuset is preserved during these migration operations if possible.
631 For example if the page was on the second valid node of the prior cpuset
632 then the page will be placed on the second valid node of the new cpuset.
633 
634 Also if 'cpuset.memory_migrate' is set true, then if that cpuset's
635 'cpuset.mems' file is modified, pages allocated to tasks in that
636 cpuset, that were on nodes in the previous setting of 'cpuset.mems',
637 will be moved to nodes in the new setting of 'mems.'
638 Pages that were not in the task's prior cpuset, or in the cpuset's
639 prior 'cpuset.mems' setting, will not be moved.
640 
641 There is an exception to the above.  If hotplug functionality is used
642 to remove all the CPUs that are currently assigned to a cpuset,
643 then all the tasks in that cpuset will be moved to the nearest ancestor
644 with non-empty cpus.  But the moving of some (or all) tasks might fail if
645 cpuset is bound with another cgroup subsystem which has some restrictions
646 on task attaching.  In this failing case, those tasks will stay
647 in the original cpuset, and the kernel will automatically update
648 their cpus_allowed to allow all online CPUs.  When memory hotplug
649 functionality for removing Memory Nodes is available, a similar exception
650 is expected to apply there as well.  In general, the kernel prefers to
651 violate cpuset placement, over starving a task that has had all
652 its allowed CPUs or Memory Nodes taken offline.
653 
654 There is a second exception to the above.  GFP_ATOMIC requests are
655 kernel internal allocations that must be satisfied, immediately.
656 The kernel may drop some request, in rare cases even panic, if a
657 GFP_ATOMIC alloc fails.  If the request cannot be satisfied within
658 the current task's cpuset, then we relax the cpuset, and look for
659 memory anywhere we can find it.  It's better to violate the cpuset
660 than stress the kernel.
661 
662 To start a new job that is to be contained within a cpuset, the steps are:
663 
664  1) mkdir /sys/fs/cgroup/cpuset
665  2) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
666  3) Create the new cpuset by doing mkdir's and write's (or echo's) in
667     the /sys/fs/cgroup/cpuset virtual file system.
668  4) Start a task that will be the "founding father" of the new job.
669  5) Attach that task to the new cpuset by writing its pid to the
670     /sys/fs/cgroup/cpuset tasks file for that cpuset.
671  6) fork, exec or clone the job tasks from this founding father task.
672 
673 For example, the following sequence of commands will setup a cpuset
674 named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
675 and then start a subshell 'sh' in that cpuset:
676 
677   mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
678   cd /sys/fs/cgroup/cpuset
679   mkdir Charlie
680   cd Charlie
681   /bin/echo 2-3 > cpuset.cpus
682   /bin/echo 1 > cpuset.mems
683   /bin/echo $$ > tasks
684   sh
685   # The subshell 'sh' is now running in cpuset Charlie
686   # The next line should display '/Charlie'
687   cat /proc/self/cpuset
688 
689 There are ways to query or modify cpusets:
690  - via the cpuset file system directly, using the various cd, mkdir, echo,
691    cat, rmdir commands from the shell, or their equivalent from C.
692  - via the C library libcpuset.
693  - via the C library libcgroup.
694    (http://sourceforge.net/projects/libcg/)
695  - via the python application cset.
696    (http://code.google.com/p/cpuset/)
697 
698 The sched_setaffinity calls can also be done at the shell prompt using
699 SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy
700 calls can be done at the shell prompt using the numactl command
701 (part of Andi Kleen's numa package).
702 
703 2. Usage Examples and Syntax
704 ============================
705 
706 2.1 Basic Usage
707 ---------------
708 
709 Creating, modifying, using the cpusets can be done through the cpuset
710 virtual filesystem.
711 
712 To mount it, type:
713 # mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset
714 
715 Then under /sys/fs/cgroup/cpuset you can find a tree that corresponds to the
716 tree of the cpusets in the system. For instance, /sys/fs/cgroup/cpuset
717 is the cpuset that holds the whole system.
718 
719 If you want to create a new cpuset under /sys/fs/cgroup/cpuset:
720 # cd /sys/fs/cgroup/cpuset
721 # mkdir my_cpuset
722 
723 Now you want to do something with this cpuset.
724 # cd my_cpuset
725 
726 In this directory you can find several files:
727 # ls
728 cgroup.clone_children  cpuset.memory_pressure
729 cgroup.event_control   cpuset.memory_spread_page
730 cgroup.procs           cpuset.memory_spread_slab
731 cpuset.cpu_exclusive   cpuset.mems
732 cpuset.cpus            cpuset.sched_load_balance
733 cpuset.mem_exclusive   cpuset.sched_relax_domain_level
734 cpuset.mem_hardwall    notify_on_release
735 cpuset.memory_migrate  tasks
736 
737 Reading them will give you information about the state of this cpuset:
738 the CPUs and Memory Nodes it can use, the processes that are using
739 it, its properties.  By writing to these files you can manipulate
740 the cpuset.
741 
742 Set some flags:
743 # /bin/echo 1 > cpuset.cpu_exclusive
744 
745 Add some cpus:
746 # /bin/echo 0-7 > cpuset.cpus
747 
748 Add some mems:
749 # /bin/echo 0-7 > cpuset.mems
750 
751 Now attach your shell to this cpuset:
752 # /bin/echo $$ > tasks
753 
754 You can also create cpusets inside your cpuset by using mkdir in this
755 directory.
756 # mkdir my_sub_cs
757 
758 To remove a cpuset, just use rmdir:
759 # rmdir my_sub_cs
760 This will fail if the cpuset is in use (has cpusets inside, or has
761 processes attached).
762 
763 Note that for legacy reasons, the "cpuset" filesystem exists as a
764 wrapper around the cgroup filesystem.
765 
766 The command
767 
768 mount -t cpuset X /sys/fs/cgroup/cpuset
769 
770 is equivalent to
771 
772 mount -t cgroup -ocpuset,noprefix X /sys/fs/cgroup/cpuset
773 echo "/sbin/cpuset_release_agent" > /sys/fs/cgroup/cpuset/release_agent
774 
775 2.2 Adding/removing cpus
776 ------------------------
777 
778 This is the syntax to use when writing in the cpus or mems files
779 in cpuset directories:
780 
781 # /bin/echo 1-4 > cpuset.cpus           -> set cpus list to cpus 1,2,3,4
782 # /bin/echo 1,2,3,4 > cpuset.cpus       -> set cpus list to cpus 1,2,3,4
783 
784 To add a CPU to a cpuset, write the new list of CPUs including the
785 CPU to be added. To add 6 to the above cpuset:
786 
787 # /bin/echo 1-4,6 > cpuset.cpus -> set cpus list to cpus 1,2,3,4,6
788 
789 Similarly to remove a CPU from a cpuset, write the new list of CPUs
790 without the CPU to be removed.
791 
792 To remove all the CPUs:
793 
794 # /bin/echo "" > cpuset.cpus            -> clear cpus list
795 
796 2.3 Setting flags
797 -----------------
798 
799 The syntax is very simple:
800 
801 # /bin/echo 1 > cpuset.cpu_exclusive    -> set flag 'cpuset.cpu_exclusive'
802 # /bin/echo 0 > cpuset.cpu_exclusive    -> unset flag 'cpuset.cpu_exclusive'
803 
804 2.4 Attaching processes
805 -----------------------
806 
807 # /bin/echo PID > tasks
808 
809 Note that it is PID, not PIDs. You can only attach ONE task at a time.
810 If you have several tasks to attach, you have to do it one after another:
811 
812 # /bin/echo PID1 > tasks
813 # /bin/echo PID2 > tasks
814         ...
815 # /bin/echo PIDn > tasks
816 
817 
818 3. Questions
819 ============
820 
821 Q: what's up with this '/bin/echo' ?
822 A: bash's builtin 'echo' command does not check calls to write() against
823    errors. If you use it in the cpuset file system, you won't be
824    able to tell whether a command succeeded or failed.
825 
826 Q: When I attach processes, only the first of the line gets really attached !
827 A: We can only return one error code per call to write(). So you should also
828    put only ONE pid.
829 
830 4. Contact
831 ==========
832 
833 Web: http://www.bullopensource.org/cpuset

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