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


6       cpuset - confine processes to processor and memory node subsets


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.
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       EXAMPLE  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.
24       A cpuset defines a list of CPUs and memory nodes.
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.
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.
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.
49       When  the  system  begins  booting,  a  single  cpuset  is defined that
50       includes 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.
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.
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.


76       Each  directory  below  /dev/cpuset  represents a cpuset and contains a
77       fixed set of pseudo-files describing the state of that cpuset.
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.
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.
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.
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).
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.
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
106              (automatically  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).
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.
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.
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.
126              The  CPUs  allowed  to  a cpuset may be changed by writing a new
127              list to its cpus file.
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
132              default, this is off (0).  Newly created cpusets also  initially
133              default this to off (0).
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
139              another,  and  if both of these cpusets have nonempty cpus, then
140              their cpus must overlap, because the  cpus  of  any  cpuset  are
141              always a subset of the cpus of its parent cpuset.
143       cpuset.mems
144              List  of  memory  nodes  on  which  processes in this cpuset are
145              allowed to  allocate  memory.   See  List  Format  below  for  a
146              description of the format of mems.
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).
155              Regardless  of  the  mem_exclusive setting, if one cpuset is the
156              ancestor of another,  then  their  memory  nodes  must  overlap,
157              because  the  memory  nodes of any cpuset are always a subset of
158              the memory nodes of that cpuset's parent cpuset.
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).
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.
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.
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.
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.
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 defaultBy default, is off (0) in the top cpuset, and
195              inherited  from the parent cpuset in newly created cpusets.  See
196              the Memory Spread section, below.
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.
206       cpuset.sched_relax_domain_level (since Linux 2.6.26)
207              Integer,   between   -1   and   a  small  positive  value.   The
208              sched_relax_domain_level controls the width of the range of CPUs
209              over  which  the kernel scheduler performs immediate rebalancing
210              of runnable tasks across CPUs.  If  sched_load_balance  is  dis‐
211              abled,  then  the  setting  of sched_relax_domain_level does not
212              matter, as no such load balancing is done.   If  sched_load_bal‐
213              ance   is   enabled,   then   the   higher   the  value  of  the
214              sched_relax_domain_level, the wider the range of CPUs over which
215              immediate  load  balancing  is  attempted.   See Scheduler Relax
216              Domain Level, below, for further details.
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.
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:
229           Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
230           Cpus_allowed_list:     0-127
231           Mems_allowed:   ffffffff,ffffffff
232           Mems_allowed_list:     0-63
234       The  "allowed"  fields  were  added in Linux 2.6.24; the "allowed_list"
235       fields were added in Linux 2.6.26.


238       In addition to controlling which cpus and mems a process is allowed  to
239       use, cpusets provide the following extended capabilities.
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.
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.
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.
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.
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.
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.
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.
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].
289       The usual contents of the command /sbin/cpuset_release_agent is  simply
290       the shell script:
292           #!/bin/sh
293           rmdir /dev/cpuset/$1
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.
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.
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.
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.
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.
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,
325       below.
327       A per-cpuset, running average is employed for the following reasons:
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.
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.
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.
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.
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
352       repurpose,  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.
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
360       reclaim  code caused by any process in the cpuset, in units of reclaims
361       attempted per second, times 1000.
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.
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.
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.
381       The  setting  of  these  flags  does  not  affect the data segment (see
382       brk(2)) or stack segment pages of a process.
384       By default, both kinds of memory  spreading  are  off  and  the  kernel
385       prefers  to  allocate  memory  pages  on  the  node  local to where the
386       requesting process is running.  If that node  is  not  allowed  by  the
387       process's  NUMA  memory  policy or cpuset configuration or if there are
388       insufficient free memory pages on that node, then the kernel looks  for
389       the nearest node that is allowed and has sufficient free memory.
391       When  new  cpusets are created, they inherit the memory spread settings
392       of their parent.
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
400       effect, even if it is.  If  cpuset  memory  spreading  is  subsequently
401       turned  off,  the  NUMA  memory policy most recently specified by these
402       calls is automatically reapplied.
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.
409       Cpuset-specified  memory  spreading  behaves similarly to what is known
410       (in other contexts) as round-robin or interleave memory placement.
412       Cpuset-specified memory spreading can provide  substantial  performance
413       improvements for jobs that:
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
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.
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.
426   Memory migration
427       Normally,  under  the  default  setting   (disabled)   of   cpuset.mem‐
428       ory_migrate,  once  a  page is allocated (given a physical page of main
429       memory), then that page stays on whatever node  it  was  allocated,  so
430       long  as  it  remains  allocated, even if the cpuset's memory-placement
431       policy mems subsequently changes.
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.
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.
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.
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).
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
463       details  of  the  kernel  process scheduler, which is subject to change
464       over time, as improved kernel scheduler algorithms are implemented.)
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.
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.)
474       This default load balancing across all CPUs is not well suited  to  the
475       following two situations:
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.
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.
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.
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,
493       except  in  so  far as is necessary because some overlapping cpuset has
494       sched_load_balance enabled.
496       So, for example, if the top  cpuset  has  the  flag  sched_load_balance
497       enabled,  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.
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.
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,
508       depending on  the  particulars  of  this  flag  setting  in  descendant
509       cpusets.   Even  if  such  a process could use spare CPU cycles in some
510       other CPUs, the kernel scheduler might not consider the possibility  of
511       load balancing that process to the underused CPU.
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.
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
523       sched_relax_domain_level applies  only  to  immediate  load  balancing.
524       Regardless  of the sched_relax_domain_level setting, periodic load bal‐
525       ancing is attempted over all  CPUs  (unless  disabled  by  turning  off
526       sched_load_balance.)   In  any case, of course, tasks will be scheduled
527       to  run  only  on  CPUs  allowed  by  their  cpuset,  as  modified   by
528       sched_setaffinity(2) system calls.
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.
537       The   exact    meaning    of    the    small    integer    values    of
538       sched_relax_domain_level will depend on internal implementation details
539       of the kernel scheduler code and on the non-uniform architecture of the
540       hardware.   Both  of  these  will  evolve  over time and vary by system
541       architecture and kernel version.
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.
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].
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.
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.
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.


575       The  following  formats  are  used to represent sets of CPUs and memory
576       nodes.
578   Mask format
579       The Mask Format is used to represent CPU and memory-node bit  masks  in
580       the /proc/<pid>/status file.
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.
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.
592       Examples of the Mask Format:
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
600       A mask with bits 0, 1, 2, 4, 8, 16, 32, and 64 set displays as:
602           00000001,00000001,00010117
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.
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.
612       Examples of the List Format:
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


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


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.
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
639       requires execute permission on the encompassing directories  and  write
640       permission on the tasks file.
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)).
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.
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
657       operating 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.
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.


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.
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:
688           echo 19 > cpuset.mems
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:
696           /bin/echo 19 > cpuset.mems
697           /bin/echo: write error: Invalid argument


700   Memory placement
701       Not all allocations of system memory are constrained  by  cpusets,  for
702       the following reasons.
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.
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
718       request  cannot  be satisfied within the current process's cpuset, then
719       we relax the cpuset, and look for memory anywhere we can find it.  It's
720       better to violate the cpuset than stress the kernel.
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.
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.


733       The Linux kernel implementation of cpusets sets errno  to  specify  the
734       reason for a failed system call affecting cpusets.
736       The  possible  errno  settings  and  their meaning when set on a failed
737       cpuset call are as listed below.
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.
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.
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.
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.
755       EACCES Attempted to write(2) a cpuset.memory_pressure file.
757       EACCES Attempted to create a file in a cpuset directory.
759       EBUSY  Attempted to remove, using rmdir(2), a cpuset with attached pro‐
760              cesses.
762       EBUSY  Attempted  to  remove,  using  rmdir(2),  a  cpuset  with  child
763              cpusets.
765       EBUSY  Attempted to remove a CPU or memory node from a cpuset  that  is
766              also in a child of that cpuset.
768       EEXIST Attempted  to  create,  using  mkdir(2),  a  cpuset that already
769              exists.
771       EEXIST Attempted to rename(2) a cpuset to a name that already exists.
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.
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.
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.
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.
787       EINVAL Attempted to write(2) a cpuset.cpus or  cpuset.mems  list  which
788              included an invalid character in the string.
790       EINVAL Attempted  to write(2) a list to a cpuset.cpus file that did not
791              include any online CPUs.
793       EINVAL Attempted to write(2) a list to a cpuset.mems file that did  not
794              include any online memory nodes.
796       EINVAL Attempted to write(2) a list to a cpuset.mems file that included
797              a node that held no memory.
799       EIO    Attempted to write(2) a string to a cpuset tasks file that  does
800              not begin with an ASCII decimal integer.
802       EIO    Attempted to rename(2) a cpuset into a different directory.
805              Attempted to read(2) a /proc/<pid>/cpuset file for a cpuset path
806              that is longer than the kernel page size.
809              Attempted to create, using mkdir(2), a cpuset whose base  direc‐
810              tory name is longer than 255 characters.
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.
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.
821       ENOENT Attempted to create, using mkdir(2), a cpuset in a parent cpuset
822              that doesn't exist.
824       ENOENT Attempted to access(2) or open(2) a nonexistent file in a cpuset
825              directory.
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.
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.
835       ENOSPC Attempted  to  write(2) an empty cpuset.cpus or cpuset.mems set‐
836              ting to a cpuset that has tasks attached.
838       ENOTDIR
839              Attempted to rename(2) a nonexistent cpuset.
841       EPERM  Attempted to remove a file from a cpuset directory.
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.
847       ESRCH  Attempted to write(2) the process  ID  (PID)  of  a  nonexistent
848              process to a cpuset tasks file.


851       Cpusets appeared in version 2.6.12 of the Linux kernel.


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.


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
863       effect.


866       The  following examples demonstrate querying and setting cpuset options
867       using shell commands.
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:
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.
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.
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
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.
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.
908       The following sequence of commands accomplishes this.
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
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,
920       respectively.
922       Notice that the last step of the above sequence did not do:
924           $ cp ../alpha/tasks tasks
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.
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
933       (unbuffered) option of sed(1):
935           $ sed -un p < ../alpha/tasks > tasks


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)
943       Documentation/cgroup-v1/cpusets.txt in the Linux kernel source tree (or
944       Documentation/cpusets.txt before Linux 2.6.29)


947       This page is part of release 4.15 of the Linux  man-pages  project.   A
948       description  of  the project, information about reporting bugs, and the
949       latest    version    of    this    page,    can     be     found     at
950       https://www.kernel.org/doc/man-pages/.
954Linux                             2017-09-15                         CPUSET(7)