1CPUSET(7)                  Linux Programmer's Manual                 CPUSET(7)
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NAME

6       cpuset - confine processes to processor and memory node subsets
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DESCRIPTION

9       The  cpuset  filesystem  is a pseudo-filesystem interface to the kernel
10       cpuset mechanism, which is used to control the processor placement  and
11       memory placement of processes.  It is commonly mounted at /dev/cpuset.
12
13       On systems with kernels compiled with built in support for cpusets, all
14       processes are attached to a cpuset, and cpusets are always present.  If
15       a  system supports cpusets, then it will have the entry nodev cpuset in
16       the file /proc/filesystems.  By mounting the cpuset filesystem (see the
17       EXAMPLES section below), the administrator can configure the cpusets on
18       a system to control the processor and memory placement of processes  on
19       that  system.   By  default, if the cpuset configuration on a system is
20       not modified or if the cpuset filesystem is not even mounted, then  the
21       cpuset  mechanism, though present, has no effect on the system's behav‐
22       ior.
23
24       A cpuset defines a list of CPUs and memory nodes.
25
26       The CPUs of a system include all the logical processing units on  which
27       a  process can execute, including, if present, multiple processor cores
28       within a package and Hyper-Threads within  a  processor  core.   Memory
29       nodes  include all distinct banks of main memory; small and SMP systems
30       typically have just one memory node that contains all the system's main
31       memory,  while  NUMA  (non-uniform memory access) systems have multiple
32       memory nodes.
33
34       Cpusets are  represented  as  directories  in  a  hierarchical  pseudo-
35       filesystem, where the top directory in the hierarchy (/dev/cpuset) rep‐
36       resents the entire system (all online CPUs and memory  nodes)  and  any
37       cpuset that is the child (descendant) of another parent cpuset contains
38       a subset of that parent's CPUs and memory nodes.  The  directories  and
39       files representing cpusets have normal filesystem permissions.
40
41       Every  process  in the system belongs to exactly one cpuset.  A process
42       is confined to run only on the CPUs in the cpuset it belongs to, and to
43       allocate  memory  only  on  the  memory  nodes  in that cpuset.  When a
44       process fork(2)s, the child process is placed in the same cpuset as its
45       parent.   With  sufficient  privilege,  a process may be moved from one
46       cpuset to another and the allowed CPUs and memory nodes of an  existing
47       cpuset may be changed.
48
49       When  the  system  begins  booting, a single cpuset is defined that in‐
50       cludes all CPUs and memory nodes on the system, and all  processes  are
51       in that cpuset.  During the boot process, or later during normal system
52       operation, other cpusets may be created, as subdirectories of this  top
53       cpuset,  under  the  control of the system administrator, and processes
54       may be placed in these other cpusets.
55
56       Cpusets are integrated with the sched_setaffinity(2) scheduling  affin‐
57       ity  mechanism  and  the mbind(2) and set_mempolicy(2) memory-placement
58       mechanisms in the kernel.  Neither of these mechanisms  let  a  process
59       make  use of a CPU or memory node that is not allowed by that process's
60       cpuset.  If changes to a process's cpuset placement conflict with these
61       other  mechanisms,  then  cpuset placement is enforced even if it means
62       overriding these other mechanisms.  The kernel accomplishes this  over‐
63       riding  by  silently restricting the CPUs and memory nodes requested by
64       these other mechanisms to  those  allowed  by  the  invoking  process's
65       cpuset.   This  can  result in these other calls returning an error, if
66       for example, such a call ends up requesting an empty  set  of  CPUs  or
67       memory  nodes,  after  that  request  is  restricted  to  the  invoking
68       process's cpuset.
69
70       Typically, a cpuset is used to manage the CPU and memory-node  confine‐
71       ment  for a set of cooperating processes such as a batch scheduler job,
72       and these other mechanisms are used to manage the placement of individ‐
73       ual processes or memory regions within that set or job.
74

FILES

76       Each  directory  below  /dev/cpuset  represents a cpuset and contains a
77       fixed set of pseudo-files describing the state of that cpuset.
78
79       New cpusets are created using the mkdir(2) system call or the  mkdir(1)
80       command.   The  properties of a cpuset, such as its flags, allowed CPUs
81       and memory nodes, and attached processes, are queried and  modified  by
82       reading  or writing to the appropriate file in that cpuset's directory,
83       as listed below.
84
85       The pseudo-files in each cpuset  directory  are  automatically  created
86       when the cpuset is created, as a result of the mkdir(2) invocation.  It
87       is not possible to directly add or remove these pseudo-files.
88
89       A cpuset directory that contains no child cpuset directories,  and  has
90       no  attached  processes, can be removed using rmdir(2) or rmdir(1).  It
91       is not necessary, or possible, to remove the  pseudo-files  inside  the
92       directory before removing it.
93
94       The pseudo-files in each cpuset directory are small text files that may
95       be read and written using traditional shell utilities such  as  cat(1),
96       and  echo(1),  or from a program by using file I/O library functions or
97       system calls, such as open(2), read(2), write(2), and close(2).
98
99       The pseudo-files in a cpuset directory represent internal kernel  state
100       and do not have any persistent image on disk.  Each of these per-cpuset
101       files is listed and described below.
102
103       tasks  List of the process IDs (PIDs) of the processes in that  cpuset.
104              The list is formatted as a series of ASCII decimal numbers, each
105              followed by a newline.  A process may be added to a cpuset  (au‐
106              tomatically  removing  it  from  the cpuset that previously con‐
107              tained it) by writing its PID to that cpuset's tasks file  (with
108              or without a trailing newline).
109
110              Warning:  only  one  PID  may  be written to the tasks file at a
111              time.  If a string is written that contains more than  one  PID,
112              only the first one will be used.
113
114       notify_on_release
115              Flag  (0  or  1).   If set (1), that cpuset will receive special
116              handling after it is released,  that  is,  after  all  processes
117              cease  using  it  (i.e.,  terminate  or are moved to a different
118              cpuset) and all child cpuset directories have been removed.  See
119              the Notify On Release section, below.
120
121       cpuset.cpus
122              List  of  the physical numbers of the CPUs on which processes in
123              that cpuset are allowed to execute.  See List Format below for a
124              description of the format of cpus.
125
126              The  CPUs  allowed  to  a cpuset may be changed by writing a new
127              list to its cpus file.
128
129       cpuset.cpu_exclusive
130              Flag (0 or 1).  If set (1), the cpuset has exclusive use of  its
131              CPUs  (no  sibling  or  cousin cpuset may overlap CPUs).  By de‐
132              fault, this is off (0).  Newly created  cpusets  also  initially
133              default this to off (0).
134
135              Two  cpusets  are  sibling cpusets if they share the same parent
136              cpuset in the /dev/cpuset hierarchy.   Two  cpusets  are  cousin
137              cpusets  if neither is the ancestor of the other.  Regardless of
138              the cpu_exclusive setting, if one cpuset is the ancestor of  an‐
139              other,  and  if  both  of these cpusets have nonempty cpus, then
140              their cpus must overlap, because the cpus of any cpuset are  al‐
141              ways a subset of the cpus of its parent cpuset.
142
143       cpuset.mems
144              List  of  memory nodes on which processes in this cpuset are al‐
145              lowed to allocate memory.  See List Format below for a  descrip‐
146              tion of the format of mems.
147
148       cpuset.mem_exclusive
149              Flag  (0 or 1).  If set (1), the cpuset has exclusive use of its
150              memory nodes (no sibling or cousin may overlap).   Also  if  set
151              (1),  the  cpuset is a Hardwall cpuset (see below).  By default,
152              this is off (0).  Newly created cpusets also  initially  default
153              this to off (0).
154
155              Regardless  of  the  mem_exclusive setting, if one cpuset is the
156              ancestor of another, then their memory nodes must  overlap,  be‐
157              cause  the memory nodes of any cpuset are always a subset of the
158              memory nodes of that cpuset's parent cpuset.
159
160       cpuset.mem_hardwall (since Linux 2.6.26)
161              Flag (0 or 1).  If set (1), the cpuset is a Hardwall cpuset (see
162              below).  Unlike mem_exclusive, there is no constraint on whether
163              cpusets marked mem_hardwall may have  overlapping  memory  nodes
164              with  sibling  or  cousin cpusets.  By default, this is off (0).
165              Newly created cpusets also initially default this to off (0).
166
167       cpuset.memory_migrate (since Linux 2.6.16)
168              Flag (0 or 1).  If set (1), then memory  migration  is  enabled.
169              By  default, this is off (0).  See the Memory Migration section,
170              below.
171
172       cpuset.memory_pressure (since Linux 2.6.16)
173              A measure of how much memory  pressure  the  processes  in  this
174              cpuset  are  causing.   See  the Memory Pressure section, below.
175              Unless memory_pressure_enabled is enabled, always has value zero
176              (0).  This file is read-only.  See the WARNINGS section, below.
177
178       cpuset.memory_pressure_enabled (since Linux 2.6.16)
179              Flag  (0  or  1).  This file is present only in the root cpuset,
180              normally /dev/cpuset.  If set (1), the memory_pressure  calcula‐
181              tions  are  enabled  for all cpusets in the system.  By default,
182              this is off (0).  See the Memory Pressure section, below.
183
184       cpuset.memory_spread_page (since Linux 2.6.17)
185              Flag (0 or 1).  If set (1),  pages  in  the  kernel  page  cache
186              (filesystem buffers) are uniformly spread across the cpuset.  By
187              default, this is off (0) in the top cpuset, and  inherited  from
188              the  parent  cpuset  in  newly  created cpusets.  See the Memory
189              Spread section, below.
190
191       cpuset.memory_spread_slab (since Linux 2.6.17)
192              Flag (0 or 1).  If set (1), the kernel slab caches for file  I/O
193              (directory and inode structures) are uniformly spread across the
194              cpuset.  By default, is off (0) in the top cpuset, and inherited
195              from the parent cpuset in newly created cpusets.  See the Memory
196              Spread section, below.
197
198       cpuset.sched_load_balance (since Linux 2.6.24)
199              Flag (0 or 1).  If set (1, the default) the kernel will automat‐
200              ically  load  balance  processes in that cpuset over the allowed
201              CPUs in that cpuset.  If cleared (0) the kernel will avoid  load
202              balancing  processes  in  this  cpuset, unless some other cpuset
203              with overlapping CPUs has its sched_load_balance flag set.   See
204              Scheduler Load Balancing, below, for further details.
205
206       cpuset.sched_relax_domain_level (since Linux 2.6.26)
207              Integer,  between  -1 and a small positive value.  The sched_re‐
208              lax_domain_level controls the width of the range  of  CPUs  over
209              which  the  kernel  scheduler  performs immediate rebalancing of
210              runnable tasks across CPUs.  If sched_load_balance is  disabled,
211              then the setting of sched_relax_domain_level does not matter, as
212              no such load balancing is done.  If  sched_load_balance  is  en‐
213              abled,   then  the  higher  the  value  of  the  sched_relax_do‐
214              main_level, the wider the range of  CPUs  over  which  immediate
215              load  balancing is attempted.  See Scheduler Relax Domain Level,
216              below, for further details.
217
218       In  addition  to  the  above  pseudo-files  in  each  directory   below
219       /dev/cpuset,  each  process has a pseudo-file, /proc/<pid>/cpuset, that
220       displays the path of the process's cpuset  directory  relative  to  the
221       root of the cpuset filesystem.
222
223       Also the /proc/<pid>/status file for each process has four added lines,
224       displaying the process's Cpus_allowed (on which CPUs it may  be  sched‐
225       uled) and Mems_allowed (on which memory nodes it may obtain memory), in
226       the two formats Mask Format and List Format (see below) as shown in the
227       following example:
228
229           Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
230           Cpus_allowed_list:     0-127
231           Mems_allowed:   ffffffff,ffffffff
232           Mems_allowed_list:     0-63
233
234       The  "allowed"  fields  were  added in Linux 2.6.24; the "allowed_list"
235       fields were added in Linux 2.6.26.
236

EXTENDED CAPABILITIES

238       In addition to controlling which cpus and mems a process is allowed  to
239       use, cpusets provide the following extended capabilities.
240
241   Exclusive cpusets
242       If  a cpuset is marked cpu_exclusive or mem_exclusive, no other cpuset,
243       other than a direct ancestor or descendant, may share any of  the  same
244       CPUs or memory nodes.
245
246       A  cpuset that is mem_exclusive restricts kernel allocations for buffer
247       cache pages and other internal kernel data pages commonly shared by the
248       kernel  across  multiple  users.  All cpusets, whether mem_exclusive or
249       not, restrict allocations of memory for user space.  This enables  con‐
250       figuring  a  system  so  that several independent jobs can share common
251       kernel data, while isolating each job's  user  allocation  in  its  own
252       cpuset.  To do this, construct a large mem_exclusive cpuset to hold all
253       the jobs, and construct child, non-mem_exclusive cpusets for each indi‐
254       vidual  job.   Only  a  small amount of kernel memory, such as requests
255       from interrupt handlers, is allowed to be placed on memory  nodes  out‐
256       side even a mem_exclusive cpuset.
257
258   Hardwall
259       A  cpuset  that  has  mem_exclusive  or  mem_hardwall set is a hardwall
260       cpuset.  A hardwall cpuset restricts kernel allocations for page,  buf‐
261       fer,  and  other  data  commonly  shared  by the kernel across multiple
262       users.  All cpusets, whether hardwall or not, restrict  allocations  of
263       memory for user space.
264
265       This  enables configuring a system so that several independent jobs can
266       share common kernel data, such as  filesystem  pages,  while  isolating
267       each  job's user allocation in its own cpuset.  To do this, construct a
268       large hardwall cpuset to hold all the jobs, and construct child cpusets
269       for each individual job which are not hardwall cpusets.
270
271       Only  a  small amount of kernel memory, such as requests from interrupt
272       handlers, is allowed to be taken outside even a hardwall cpuset.
273
274   Notify on release
275       If the notify_on_release flag is enabled (1) in a cpuset, then whenever
276       the  last process in the cpuset leaves (exits or attaches to some other
277       cpuset) and the last child cpuset of that cpuset is removed, the kernel
278       will run the command /sbin/cpuset_release_agent, supplying the pathname
279       (relative to the mount point of the cpuset filesystem) of the abandoned
280       cpuset.  This enables automatic removal of abandoned cpusets.
281
282       The  default  value  of  notify_on_release in the root cpuset at system
283       boot is disabled (0).  The default value of other cpusets  at  creation
284       is the current value of their parent's notify_on_release setting.
285
286       The  command  /sbin/cpuset_release_agent  is  invoked,  with  the  name
287       (/dev/cpuset relative path) of the to-be-released cpuset in argv[1].
288
289       The usual contents of the command /sbin/cpuset_release_agent is  simply
290       the shell script:
291
292           #!/bin/sh
293           rmdir /dev/cpuset/$1
294
295       As with other flag values below, this flag can be changed by writing an
296       ASCII number 0 or 1 (with optional trailing newline) into the file,  to
297       clear or set the flag, respectively.
298
299   Memory pressure
300       The  memory_pressure  of  a cpuset provides a simple per-cpuset running
301       average of the rate that the processes in a cpuset  are  attempting  to
302       free  up in-use memory on the nodes of the cpuset to satisfy additional
303       memory requests.
304
305       This enables batch managers that are monitoring jobs running  in  dedi‐
306       cated  cpusets to efficiently detect what level of memory pressure that
307       job is causing.
308
309       This is useful both on tightly managed systems running a  wide  mix  of
310       submitted jobs, which may choose to terminate or reprioritize jobs that
311       are trying to use more memory than allowed on the nodes assigned  them,
312       and  with  tightly coupled, long-running, massively parallel scientific
313       computing jobs that will dramatically fail to meet required performance
314       goals if they start to use more memory than allowed to them.
315
316       This  mechanism provides a very economical way for the batch manager to
317       monitor a cpuset for signs of memory pressure.  It's up  to  the  batch
318       manager  or other user code to decide what action to take if it detects
319       signs of memory pressure.
320
321       Unless memory pressure calculation is enabled by  setting  the  pseudo-
322       file /dev/cpuset/cpuset.memory_pressure_enabled, it is not computed for
323       any cpuset, and reads from any memory_pressure always return  zero,  as
324       represented  by  the ASCII string "0\n".  See the WARNINGS section, be‐
325       low.
326
327       A per-cpuset, running average is employed for the following reasons:
328
329       *  Because this meter is per-cpuset rather than per-process or per vir‐
330          tual  memory  region,  the  system load imposed by a batch scheduler
331          monitoring this metric is sharply reduced on large systems,  because
332          a scan of the tasklist can be avoided on each set of queries.
333
334       *  Because  this meter is a running average rather than an accumulating
335          counter, a batch scheduler can detect memory pressure with a  single
336          read,  instead of having to read and accumulate results for a period
337          of time.
338
339       *  Because this meter is per-cpuset rather than per-process, the  batch
340          scheduler  can  obtain  the  key  information—memory  pressure  in a
341          cpuset—with a single read, rather than having to query  and  accumu‐
342          late results over all the (dynamically changing) set of processes in
343          the cpuset.
344
345       The memory_pressure of a cpuset is calculated using a per-cpuset simple
346       digital  filter  that is kept within the kernel.  For each cpuset, this
347       filter tracks the recent rate  at  which  processes  attached  to  that
348       cpuset enter the kernel direct reclaim code.
349
350       The  kernel  direct  reclaim  code is entered whenever a process has to
351       satisfy a memory page request by first finding some other page  to  re‐
352       purpose,  due  to  lack  of  any  readily available already free pages.
353       Dirty filesystem pages are repurposed by first writing  them  to  disk.
354       Unmodified  filesystem  buffer  pages are repurposed by simply dropping
355       them, though if that page is needed again, it will have  to  be  reread
356       from disk.
357
358       The cpuset.memory_pressure file provides an integer number representing
359       the recent (half-life of 10 seconds) rate of entries to the direct  re‐
360       claim  code  caused  by any process in the cpuset, in units of reclaims
361       attempted per second, times 1000.
362
363   Memory spread
364       There are two Boolean flag files per cpuset that control where the ker‐
365       nel  allocates  pages  for the filesystem buffers and related in-kernel
366       data  structures.   They  are  called   cpuset.memory_spread_page   and
367       cpuset.memory_spread_slab.
368
369       If  the  per-cpuset Boolean flag file cpuset.memory_spread_page is set,
370       then the kernel will spread the filesystem buffers (page cache)  evenly
371       over all the nodes that the faulting process is allowed to use, instead
372       of preferring to put those pages on the node where the process is  run‐
373       ning.
374
375       If  the  per-cpuset Boolean flag file cpuset.memory_spread_slab is set,
376       then the kernel will spread some filesystem-related slab  caches,  such
377       as  those  for  inodes and directory entries, evenly over all the nodes
378       that the faulting process is allowed to use, instead of  preferring  to
379       put those pages on the node where the process is running.
380
381       The  setting  of  these  flags  does  not  affect the data segment (see
382       brk(2)) or stack segment pages of a process.
383
384       By default, both kinds of memory spreading are off and the kernel  pre‐
385       fers to allocate memory pages on the node local to where the requesting
386       process is running.  If that node is not allowed by the process's  NUMA
387       memory policy or cpuset configuration or if there are insufficient free
388       memory pages on that node, then the kernel looks for the  nearest  node
389       that is allowed and has sufficient free memory.
390
391       When  new  cpusets are created, they inherit the memory spread settings
392       of their parent.
393
394       Setting memory spreading causes allocations for the  affected  page  or
395       slab  caches  to  ignore the process's NUMA memory policy and be spread
396       instead.  However, the effect of  these  changes  in  memory  placement
397       caused by cpuset-specified memory spreading is hidden from the mbind(2)
398       or set_mempolicy(2) calls.  These two NUMA memory policy  calls  always
399       appear  to  behave as if no cpuset-specified memory spreading is in ef‐
400       fect, even if it is.  If cpuset memory spreading is subsequently turned
401       off,  the  NUMA memory policy most recently specified by these calls is
402       automatically reapplied.
403
404       Both cpuset.memory_spread_page and cpuset.memory_spread_slab are  Bool‐
405       ean flag files.  By default, they contain "0", meaning that the feature
406       is off for that cpuset.  If a "1" is written to that file,  that  turns
407       the named feature on.
408
409       Cpuset-specified  memory  spreading  behaves similarly to what is known
410       (in other contexts) as round-robin or interleave memory placement.
411
412       Cpuset-specified memory spreading can provide  substantial  performance
413       improvements for jobs that:
414
415       a) need  to  place  thread-local data on memory nodes close to the CPUs
416          which are running the threads that most frequently access that data;
417          but also
418
419       b) need  to  access  large  filesystem data sets that must to be spread
420          across the several nodes in the job's cpuset in order to fit.
421
422       Without this policy, the memory allocation  across  the  nodes  in  the
423       job's  cpuset  can  become  very uneven, especially for jobs that might
424       have just a single thread initializing or reading in the data set.
425
426   Memory migration
427       Normally, under the default  setting  (disabled)  of  cpuset.memory_mi‐
428       grate, once a page is allocated (given a physical page of main memory),
429       then that page stays on whatever node it was allocated, so long  as  it
430       remains  allocated,  even  if the cpuset's memory-placement policy mems
431       subsequently changes.
432
433       When memory migration is enabled in a cpuset, if the  mems  setting  of
434       the  cpuset  is  changed, then any memory page in use by any process in
435       the cpuset that is on a memory node that is no longer allowed  will  be
436       migrated to a memory node that is allowed.
437
438       Furthermore,  if  a  process is moved into a cpuset with memory_migrate
439       enabled, any memory pages it uses that were on memory nodes allowed  in
440       its  previous cpuset, but which are not allowed in its new cpuset, will
441       be migrated to a memory node allowed in the new cpuset.
442
443       The relative placement of a migrated page within  the  cpuset  is  pre‐
444       served  during these migration operations if possible.  For example, if
445       the page was on the second valid node of the  prior  cpuset,  then  the
446       page will be placed on the second valid node of the new cpuset, if pos‐
447       sible.
448
449   Scheduler load balancing
450       The kernel scheduler automatically load balances processes.  If one CPU
451       is  underutilized,  the  kernel  will  look for processes on other more
452       overloaded CPUs and move those  processes  to  the  underutilized  CPU,
453       within  the  constraints  of  such  placement mechanisms as cpusets and
454       sched_setaffinity(2).
455
456       The algorithmic cost of load balancing and its  impact  on  key  shared
457       kernel  data  structures  such  as the process list increases more than
458       linearly with the number of CPUs being balanced.  For example, it costs
459       more  to load balance across one large set of CPUs than it does to bal‐
460       ance across two smaller sets of CPUs, each of  half  the  size  of  the
461       larger set.  (The precise relationship between the number of CPUs being
462       balanced and the cost of load balancing depends on  implementation  de‐
463       tails  of the kernel process scheduler, which is subject to change over
464       time, as improved kernel scheduler algorithms are implemented.)
465
466       The per-cpuset flag sched_load_balance provides a mechanism to suppress
467       this automatic scheduler load balancing in cases where it is not needed
468       and suppressing it would have worthwhile performance benefits.
469
470       By default, load balancing is done across all CPUs, except those marked
471       isolated  using the kernel boot time "isolcpus=" argument.  (See Sched‐
472       uler Relax Domain Level, below, to change this default.)
473
474       This default load balancing across all CPUs is not well suited  to  the
475       following two situations:
476
477       *  On  large systems, load balancing across many CPUs is expensive.  If
478          the system is managed using cpusets to  place  independent  jobs  on
479          separate sets of CPUs, full load balancing is unnecessary.
480
481       *  Systems  supporting  real-time  on some CPUs need to minimize system
482          overhead on those CPUs, including avoiding process load balancing if
483          that is not needed.
484
485       When  the  per-cpuset  flag  sched_load_balance is enabled (the default
486       setting), it requests load  balancing  across  all  the  CPUs  in  that
487       cpuset's  allowed CPUs, ensuring that load balancing can move a process
488       (not otherwise pinned, as by sched_setaffinity(2)) from any CPU in that
489       cpuset to any other.
490
491       When  the  per-cpuset  flag  sched_load_balance  is  disabled, then the
492       scheduler will avoid load balancing across the CPUs in that cpuset, ex‐
493       cept  in  so  far  as  is necessary because some overlapping cpuset has
494       sched_load_balance enabled.
495
496       So, for example, if the top cpuset has the flag sched_load_balance  en‐
497       abled,  then  the  scheduler will load balance across all CPUs, and the
498       setting of the sched_load_balance flag in other cpusets has no  effect,
499       as we're already fully load balancing.
500
501       Therefore  in  the  above  two  situations, the flag sched_load_balance
502       should be disabled in the top cpuset, and only  some  of  the  smaller,
503       child cpusets would have this flag enabled.
504
505       When doing this, you don't usually want to leave any unpinned processes
506       in the top cpuset that might use nontrivial amounts  of  CPU,  as  such
507       processes  may  be artificially constrained to some subset of CPUs, de‐
508       pending on the particulars of this flag setting in descendant  cpusets.
509       Even  if  such a process could use spare CPU cycles in some other CPUs,
510       the kernel scheduler might not consider the possibility of load balanc‐
511       ing that process to the underused CPU.
512
513       Of course, processes pinned to a particular CPU can be left in a cpuset
514       that disables sched_load_balance as those processes aren't  going  any‐
515       where else anyway.
516
517   Scheduler relax domain level
518       The  kernel  scheduler performs immediate load balancing whenever a CPU
519       becomes free or another task becomes  runnable.   This  load  balancing
520       works  to  ensure  that  as many CPUs as possible are usefully employed
521       running tasks.  The kernel also performs periodic  load  balancing  off
522       the  software  clock  described  in  time(7).  The setting of sched_re‐
523       lax_domain_level applies only to immediate load balancing.   Regardless
524       of the sched_relax_domain_level setting, periodic load balancing is at‐
525       tempted over all CPUs (unless disabled by turning  off  sched_load_bal‐
526       ance.)   In any case, of course, tasks will be scheduled to run only on
527       CPUs allowed by their cpuset, as modified by sched_setaffinity(2)  sys‐
528       tem calls.
529
530       On  small  systems,  such as those with just a few CPUs, immediate load
531       balancing is useful to improve system  interactivity  and  to  minimize
532       wasteful  idle  CPU cycles.  But on large systems, attempting immediate
533       load balancing across a large number of CPUs can be more costly than it
534       is  worth,  depending  on the particular performance characteristics of
535       the job mix and the hardware.
536
537       The exact meaning  of  the  small  integer  values  of  sched_relax_do‐
538       main_level will depend on internal implementation details of the kernel
539       scheduler code and on the non-uniform  architecture  of  the  hardware.
540       Both of these will evolve over time and vary by system architecture and
541       kernel version.
542
543       As of this writing,  when  this  capability  was  introduced  in  Linux
544       2.6.26,  on  certain  popular  architectures,  the  positive  values of
545       sched_relax_domain_level have the following meanings.
546
547       (1) Perform immediate load balancing across  Hyper-Thread  siblings  on
548           the same core.
549       (2) Perform  immediate  load  balancing  across other cores in the same
550           package.
551       (3) Perform immediate load balancing across other CPUs on the same node
552           or blade.
553       (4) Perform  immediate  load balancing across over several (implementa‐
554           tion detail) nodes [On NUMA systems].
555       (5) Perform immediate load balancing across over all CPUs in system [On
556           NUMA systems].
557
558       The  sched_relax_domain_level value of zero (0) always means don't per‐
559       form immediate load balancing, hence that load balancing is  done  only
560       periodically,  not  immediately when a CPU becomes available or another
561       task becomes runnable.
562
563       The sched_relax_domain_level value of minus one (-1) always  means  use
564       the  system default value.  The system default value can vary by archi‐
565       tecture and kernel version.  This system default value can  be  changed
566       by kernel boot-time "relax_domain_level=" argument.
567
568       In  the  case  of  multiple  overlapping cpusets which have conflicting
569       sched_relax_domain_level values, then the highest such value applies to
570       all  CPUs  in any of the overlapping cpusets.  In such cases, the value
571       minus one (-1) is the lowest value, overridden by any other value,  and
572       the value zero (0) is the next lowest value.
573

FORMATS

575       The  following  formats  are  used to represent sets of CPUs and memory
576       nodes.
577
578   Mask format
579       The Mask Format is used to represent CPU and memory-node bit  masks  in
580       the /proc/<pid>/status file.
581
582       This format displays each 32-bit word in hexadecimal (using ASCII char‐
583       acters "0" - "9" and "a" - "f"); words are filled with  leading  zeros,
584       if required.  For masks longer than one word, a comma separator is used
585       between words.  Words are displayed in big-endian order, which has  the
586       most  significant  bit first.  The hex digits within a word are also in
587       big-endian order.
588
589       The number of 32-bit words displayed is the minimum  number  needed  to
590       display all bits of the bit mask, based on the size of the bit mask.
591
592       Examples of the Mask Format:
593
594           00000001                        # just bit 0 set
595           40000000,00000000,00000000      # just bit 94 set
596           00000001,00000000,00000000      # just bit 64 set
597           000000ff,00000000               # bits 32-39 set
598           00000000,000e3862               # 1,5,6,11-13,17-19 set
599
600       A mask with bits 0, 1, 2, 4, 8, 16, 32, and 64 set displays as:
601
602           00000001,00000001,00010117
603
604       The  first  "1" is for bit 64, the second for bit 32, the third for bit
605       16, the fourth for bit 8, the fifth for bit 4, and the "7" is for  bits
606       2, 1, and 0.
607
608   List format
609       The  List  Format for cpus and mems is a comma-separated list of CPU or
610       memory-node numbers and ranges of numbers, in ASCII decimal.
611
612       Examples of the List Format:
613
614           0-4,9           # bits 0, 1, 2, 3, 4, and 9 set
615           0-2,7,12-14     # bits 0, 1, 2, 7, 12, 13, and 14 set
616

RULES

618       The following rules apply to each cpuset:
619
620       *  Its CPUs and memory nodes must be a (possibly equal) subset  of  its
621          parent's.
622
623       *  It can be marked cpu_exclusive only if its parent is.
624
625       *  It can be marked mem_exclusive only if its parent is.
626
627       *  If it is cpu_exclusive, its CPUs may not overlap any sibling.
628
629       *  If it is memory_exclusive, its memory nodes may not overlap any sib‐
630          ling.
631

PERMISSIONS

633       The permissions of a cpuset are determined by the  permissions  of  the
634       directories and pseudo-files in the cpuset filesystem, normally mounted
635       at /dev/cpuset.
636
637       For instance, a process can put itself in some other cpuset  (than  its
638       current  one) if it can write the tasks file for that cpuset.  This re‐
639       quires execute permission on the  encompassing  directories  and  write
640       permission on the tasks file.
641
642       An  additional  constraint  is  applied to requests to place some other
643       process in a cpuset.  One process may not attach another  to  a  cpuset
644       unless  it  would  have  permission  to send that process a signal (see
645       kill(2)).
646
647       A process may create a child cpuset if it can access and write the par‐
648       ent  cpuset  directory.   It  can  modify the CPUs or memory nodes in a
649       cpuset if it can access that cpuset's directory (execute permissions on
650       the each of the parent directories) and write the corresponding cpus or
651       mems file.
652
653       There is one minor difference between the manner in which these permis‐
654       sions are evaluated and the manner in which normal filesystem operation
655       permissions are evaluated.  The kernel  interprets  relative  pathnames
656       starting  at a process's current working directory.  Even if one is op‐
657       erating on a cpuset file, relative pathnames are  interpreted  relative
658       to  the  process's  current  working  directory,  not  relative  to the
659       process's current cpuset.  The only ways that cpuset paths relative  to
660       a process's current cpuset can be used are if either the process's cur‐
661       rent working directory is its cpuset (it first did a cd or chdir(2)  to
662       its cpuset directory beneath /dev/cpuset, which is a bit unusual) or if
663       some user code converts the relative cpuset path to a  full  filesystem
664       path.
665
666       In theory, this means that user code should specify cpusets using abso‐
667       lute pathnames, which requires knowing the mount point  of  the  cpuset
668       filesystem  (usually,  but not necessarily, /dev/cpuset).  In practice,
669       all user level code that this author is aware of simply assumes that if
670       the  cpuset  filesystem  is mounted, then it is mounted at /dev/cpuset.
671       Furthermore, it is common practice for carefully written user  code  to
672       verify  the  presence  of the pseudo-file /dev/cpuset/tasks in order to
673       verify that the cpuset pseudo-filesystem is currently mounted.
674

WARNINGS

676   Enabling memory_pressure
677       By default, the per-cpuset file cpuset.memory_pressure always  contains
678       zero (0).  Unless this feature is enabled by writing "1" to the pseudo-
679       file /dev/cpuset/cpuset.memory_pressure_enabled, the  kernel  does  not
680       compute per-cpuset memory_pressure.
681
682   Using the echo command
683       When using the echo command at the shell prompt to change the values of
684       cpuset files, beware that the built-in echo command in some shells does
685       not  display  an  error message if the write(2) system call fails.  For
686       example, if the command:
687
688           echo 19 > cpuset.mems
689
690       failed because memory node 19 was not allowed (perhaps the current sys‐
691       tem  does  not  have a memory node 19), then the echo command might not
692       display any error.  It is better to use the /bin/echo external  command
693       to  change  cpuset file settings, as this command will display write(2)
694       errors, as in the example:
695
696           /bin/echo 19 > cpuset.mems
697           /bin/echo: write error: Invalid argument
698

EXCEPTIONS

700   Memory placement
701       Not all allocations of system memory are constrained  by  cpusets,  for
702       the following reasons.
703
704       If  hot-plug functionality is used to remove all the CPUs that are cur‐
705       rently assigned to a cpuset, then the kernel will automatically  update
706       the  cpus_allowed  of  all processes attached to CPUs in that cpuset to
707       allow all CPUs.  When memory hot-plug functionality for removing memory
708       nodes  is  available, a similar exception is expected to apply there as
709       well.  In general, the kernel  prefers  to  violate  cpuset  placement,
710       rather  than  starving  a  process that has had all its allowed CPUs or
711       memory nodes taken offline.  User code should  reconfigure  cpusets  to
712       refer  only  to online CPUs and memory nodes when using hot-plug to add
713       or remove such resources.
714
715       A few  kernel-critical,  internal  memory-allocation  requests,  marked
716       GFP_ATOMIC,  must  be  satisfied immediately.  The kernel may drop some
717       request or malfunction if one of these allocations fail.  If such a re‐
718       quest  cannot be satisfied within the current process's cpuset, then we
719       relax the cpuset, and look for memory anywhere we can  find  it.   It's
720       better to violate the cpuset than stress the kernel.
721
722       Allocations  of  memory requested by kernel drivers while processing an
723       interrupt lack any relevant process context, and are  not  confined  by
724       cpusets.
725
726   Renaming cpusets
727       You  can  use the rename(2) system call to rename cpusets.  Only simple
728       renaming is supported; that is, changing the name of a cpuset directory
729       is  permitted, but moving a directory into a different directory is not
730       permitted.
731

ERRORS

733       The Linux kernel implementation of cpusets sets errno  to  specify  the
734       reason for a failed system call affecting cpusets.
735
736       The  possible  errno  settings  and  their meaning when set on a failed
737       cpuset call are as listed below.
738
739       E2BIG  Attempted a write(2) on a special  cpuset  file  with  a  length
740              larger  than some kernel-determined upper limit on the length of
741              such writes.
742
743       EACCES Attempted to write(2) the process ID (PID) of  a  process  to  a
744              cpuset  tasks  file  when  one  lacks  permission  to  move that
745              process.
746
747       EACCES Attempted to add, using write(2), a CPU  or  memory  node  to  a
748              cpuset, when that CPU or memory node was not already in its par‐
749              ent.
750
751       EACCES Attempted  to  set,  using  write(2),  cpuset.cpu_exclusive   or
752              cpuset.mem_exclusive  on  a  cpuset  whose parent lacks the same
753              setting.
754
755       EACCES Attempted to write(2) a cpuset.memory_pressure file.
756
757       EACCES Attempted to create a file in a cpuset directory.
758
759       EBUSY  Attempted to remove, using rmdir(2), a cpuset with attached pro‐
760              cesses.
761
762       EBUSY  Attempted  to  remove,  using  rmdir(2),  a  cpuset  with  child
763              cpusets.
764
765       EBUSY  Attempted to remove a CPU or memory node from a cpuset  that  is
766              also in a child of that cpuset.
767
768       EEXIST Attempted  to  create, using mkdir(2), a cpuset that already ex‐
769              ists.
770
771       EEXIST Attempted to rename(2) a cpuset to a name that already exists.
772
773       EFAULT Attempted to read(2) or write(2) a cpuset file  using  a  buffer
774              that is outside the writing processes accessible address space.
775
776       EINVAL Attempted  to  change  a  cpuset,  using write(2), in a way that
777              would violate a cpu_exclusive or mem_exclusive attribute of that
778              cpuset or any of its siblings.
779
780       EINVAL Attempted  to  write(2) an empty cpuset.cpus or cpuset.mems list
781              to a cpuset which has attached processes or child cpusets.
782
783       EINVAL Attempted to write(2) a cpuset.cpus or  cpuset.mems  list  which
784              included  a  range with the second number smaller than the first
785              number.
786
787       EINVAL Attempted to write(2) a cpuset.cpus or  cpuset.mems  list  which
788              included an invalid character in the string.
789
790       EINVAL Attempted  to write(2) a list to a cpuset.cpus file that did not
791              include any online CPUs.
792
793       EINVAL Attempted to write(2) a list to a cpuset.mems file that did  not
794              include any online memory nodes.
795
796       EINVAL Attempted to write(2) a list to a cpuset.mems file that included
797              a node that held no memory.
798
799       EIO    Attempted to write(2) a string to a cpuset tasks file that  does
800              not begin with an ASCII decimal integer.
801
802       EIO    Attempted to rename(2) a cpuset into a different directory.
803
804       ENAMETOOLONG
805              Attempted to read(2) a /proc/<pid>/cpuset file for a cpuset path
806              that is longer than the kernel page size.
807
808       ENAMETOOLONG
809              Attempted to create, using mkdir(2), a cpuset whose base  direc‐
810              tory name is longer than 255 characters.
811
812       ENAMETOOLONG
813              Attempted  to  create, using mkdir(2), a cpuset whose full path‐
814              name, including the mount point (typically "/dev/cpuset/")  pre‐
815              fix, is longer than 4095 characters.
816
817       ENODEV The  cpuset was removed by another process at the same time as a
818              write(2) was attempted on one of the pseudo-files in the  cpuset
819              directory.
820
821       ENOENT Attempted to create, using mkdir(2), a cpuset in a parent cpuset
822              that doesn't exist.
823
824       ENOENT Attempted to access(2) or open(2) a nonexistent file in a cpuset
825              directory.
826
827       ENOMEM Insufficient memory is available within the kernel; can occur on
828              a variety of system calls affecting cpusets,  but  only  if  the
829              system is extremely short of memory.
830
831       ENOSPC Attempted  to  write(2)  the  process ID (PID) of a process to a
832              cpuset tasks file when the cpuset had an  empty  cpuset.cpus  or
833              empty cpuset.mems setting.
834
835       ENOSPC Attempted  to  write(2) an empty cpuset.cpus or cpuset.mems set‐
836              ting to a cpuset that has tasks attached.
837
838       ENOTDIR
839              Attempted to rename(2) a nonexistent cpuset.
840
841       EPERM  Attempted to remove a file from a cpuset directory.
842
843       ERANGE Specified a cpuset.cpus or cpuset.mems list to the kernel  which
844              included  a  number  too  large for the kernel to set in its bit
845              masks.
846
847       ESRCH  Attempted to write(2) the process  ID  (PID)  of  a  nonexistent
848              process to a cpuset tasks file.
849

VERSIONS

851       Cpusets appeared in version 2.6.12 of the Linux kernel.
852

NOTES

854       Despite  its  name, the pid parameter is actually a thread ID, and each
855       thread in a threaded group can be attached to a different cpuset.   The
856       value  returned  from a call to gettid(2) can be passed in the argument
857       pid.
858

BUGS

860       cpuset.memory_pressure cpuset files can be  opened  for  writing,  cre‐
861       ation,  or  truncation,  but  then the write(2) fails with errno set to
862       EACCES, and the creation and truncation options on open(2) have no  ef‐
863       fect.
864

EXAMPLES

866       The  following examples demonstrate querying and setting cpuset options
867       using shell commands.
868
869   Creating and attaching to a cpuset.
870       To create a new cpuset and attach the current command shell to it,  the
871       steps are:
872
873       1)  mkdir /dev/cpuset (if not already done)
874       2)  mount -t cpuset none /dev/cpuset (if not already done)
875       3)  Create the new cpuset using mkdir(1).
876       4)  Assign CPUs and memory nodes to the new cpuset.
877       5)  Attach the shell to the new cpuset.
878
879       For  example,  the  following sequence of commands will set up a cpuset
880       named "Charlie", containing just CPUs 2 and 3, and memory node  1,  and
881       then attach the current shell to that cpuset.
882
883           $ mkdir /dev/cpuset
884           $ mount -t cpuset cpuset /dev/cpuset
885           $ cd /dev/cpuset
886           $ mkdir Charlie
887           $ cd Charlie
888           $ /bin/echo 2-3 > cpuset.cpus
889           $ /bin/echo 1 > cpuset.mems
890           $ /bin/echo $$ > tasks
891           # The current shell is now running in cpuset Charlie
892           # The next line should display '/Charlie'
893           $ cat /proc/self/cpuset
894
895   Migrating a job to different memory nodes.
896       To migrate a job (the set of processes attached to a cpuset) to differ‐
897       ent CPUs and memory nodes in the system, including  moving  the  memory
898       pages currently allocated to that job, perform the following steps.
899
900       1)  Let's  say  we  want  to move the job in cpuset alpha (CPUs 4–7 and
901           memory nodes 2–3) to a new cpuset beta (CPUs 16–19 and memory nodes
902           8–9).
903       2)  First create the new cpuset beta.
904       3)  Then allow CPUs 16–19 and memory nodes 8–9 in beta.
905       4)  Then enable memory_migration in beta.
906       5)  Then move each process from alpha to beta.
907
908       The following sequence of commands accomplishes this.
909
910           $ cd /dev/cpuset
911           $ mkdir beta
912           $ cd beta
913           $ /bin/echo 16-19 > cpuset.cpus
914           $ /bin/echo 8-9 > cpuset.mems
915           $ /bin/echo 1 > cpuset.memory_migrate
916           $ while read i; do /bin/echo $i; done < ../alpha/tasks > tasks
917
918       The  above  should  move any processes in alpha to beta, and any memory
919       held by these processes on memory nodes 2–3 to memory  nodes  8–9,  re‐
920       spectively.
921
922       Notice that the last step of the above sequence did not do:
923
924           $ cp ../alpha/tasks tasks
925
926       The  while loop, rather than the seemingly easier use of the cp(1) com‐
927       mand, was necessary because only one process PID at a time may be writ‐
928       ten to the tasks file.
929
930       The  same  effect  (writing one PID at a time) as the while loop can be
931       accomplished more efficiently, in fewer keystrokes and in  syntax  that
932       works  on  any  shell,  but  alas  more obscurely, by using the -u (un‐
933       buffered) option of sed(1):
934
935           $ sed -un p < ../alpha/tasks > tasks
936

SEE ALSO

938       taskset(1),  get_mempolicy(2),  getcpu(2),  mbind(2),   sched_getaffin‐
939       ity(2),  sched_setaffinity(2), sched_setscheduler(2), set_mempolicy(2),
940       CPU_SET(3), proc(5), cgroups(7),  numa(7),  sched(7),  migratepages(8),
941       numactl(8)
942
943       Documentation/admin-guide/cgroup-v1/cpusets.rst  in  the  Linux  kernel
944       source tree (or Documentation/cgroup-v1/cpusets.txt before Linux  4.18,
945       and Documentation/cpusets.txt before Linux 2.6.29)
946

COLOPHON

948       This  page  is  part of release 5.13 of the Linux man-pages project.  A
949       description of the project, information about reporting bugs,  and  the
950       latest     version     of     this    page,    can    be    found    at
951       https://www.kernel.org/doc/man-pages/.
952
953
954
955Linux                             2020-11-01                         CPUSET(7)
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