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NAME

       cpuset - confine processes to processor and memory node subsets

DESCRIPTION

       The  cpuset file system is a pseudo-file-system interface to the kernel
       cpuset mechanism, which is used to control the processor placement  and
       memory  placement of processes.  It is commonly mounted at /dev/cpuset.

       On systems with kernels compiled with built in support for cpusets, all
       processes are attached to a cpuset, and cpusets are always present.  If
       a system supports cpusets, then it will have the entry nodev cpuset  in
       the  file  /proc/filesystems.   By mounting the cpuset file system (see
       the EXAMPLE section below), the administrator can configure the cpusets
       on  a system to control the processor and memory placement of processes
       on that system.  By default, if the cpuset configuration on a system is
       not modified or if the cpuset file system is not even mounted, then the
       cpuset mechanism,  though  present,  has  no  affect  on  the  system's
       behavior.

       A cpuset defines a list of CPUs and memory nodes.

       The  CPUs of a system include all the logical processing units on which
       a process can execute, including, if present, multiple processor  cores
       within  a  package  and  Hyper-Threads within a processor core.  Memory
       nodes include all distinct banks of main memory; small and SMP  systems
       typically have just one memory node that contains all the system's main
       memory, while NUMA (non-uniform memory access)  systems  have  multiple
       memory nodes.

       Cpusets  are  represented  as directories in a hierarchical pseudo-file
       system,  where  the  top  directory  in  the  hierarchy   (/dev/cpuset)
       represents the entire system (all online CPUs and memory nodes) and any
       cpuset that is the child (descendant) of another parent cpuset contains
       a  subset  of that parent's CPUs and memory nodes.  The directories and
       files representing cpusets have normal file-system permissions.

       Every process in the system belongs to exactly one cpuset.   A  process
       is confined to only run on the CPUs in the cpuset it belongs to, and to
       allocate memory only on the  memory  nodes  in  that  cpuset.   When  a
       process fork(2)s, the child process is placed in the same cpuset as its
       parent.  With sufficient privilege, a process may  be  moved  from  one
       cpuset  to another and the allowed CPUs and memory nodes of an existing
       cpuset may be changed.

       When the system  begins  booting,  a  single  cpuset  is  defined  that
       includes all CPUs and memory nodes on the system, and all processes are
       in that cpuset.  During the boot process, or later during normal system
       operation,  other cpusets may be created, as subdirectories of this top
       cpuset, under the control of the system  administrator,  and  processes
       may be placed in these other cpusets.

       Cpusets   are   integrated  with  the  sched_setaffinity(2)  scheduling
       affinity  mechanism  and  the  mbind(2)  and  set_mempolicy(2)  memory-
       placement  mechanisms in the kernel.  Neither of these mechanisms let a
       process make use of a CPU or memory node that is not  allowed  by  that
       process's  cpuset.  If changes to a process's cpuset placement conflict
       with these other mechanisms, then cpuset placement is enforced even  if
       it  means  overriding  these other mechanisms.  The kernel accomplishes
       this overriding by silently  restricting  the  CPUs  and  memory  nodes
       requested  by  these  other mechanisms to those allowed by the invoking
       process's cpuset.  This can result in these other  calls  returning  an
       error,  if  for example, such a call ends up requesting an empty set of
       CPUs or memory nodes, after that request is restricted to the  invoking
       process's cpuset.

       Typically,  a  cpuset  is  used  to  manage  the  CPU  and  memory-node
       confinement for  a  set  of  cooperating  processes  such  as  a  batch
       scheduler  job,  and  these  other  mechanisms  are  used to manage the
       placement of individual processes or memory regions within that set  or
       job.

FILES

       Each  directory  below  /dev/cpuset  represents a cpuset and contains a
       fixed set of pseudo-files describing the state of that cpuset.

       New cpusets are created using the mkdir(2) system call or the  mkdir(1)
       command.   The  properties of a cpuset, such as its flags, allowed CPUs
       and memory nodes, and attached processes, are queried and  modified  by
       reading  or writing to the appropriate file in that cpuset's directory,
       as listed below.

       The pseudo-files in each cpuset  directory  are  automatically  created
       when the cpuset is created, as a result of the mkdir(2) invocation.  It
       is not possible to directly add or remove these pseudo-files.

       A cpuset directory that contains no child cpuset directories,  and  has
       no  attached  processes, can be removed using rmdir(2) or rmdir(1).  It
       is not necessary, or possible, to remove the  pseudo-files  inside  the
       directory before removing it.

       The pseudo-files in each cpuset directory are small text files that may
       be read and written using traditional shell utilities such  as  cat(1),
       and  echo(1),  or from a program by using file I/O library functions or
       system calls, such as open(2), read(2), write(2), and close(2).

       The pseudo-files in a cpuset directory represent internal kernel  state
       and do not have any persistent image on disk.  Each of these per-cpuset
       files is listed and described below.

       tasks  List of the process IDs (PIDs) of the processes in that  cpuset.
              The list is formatted as a series of ASCII decimal numbers, each
              followed by a newline.  A process  may  be  added  to  a  cpuset
              (automatically  removing  it  from  the  cpuset  that previously
              contained it) by writing its PID to  that  cpuset's  tasks  file
              (with or without a trailing newline.)

              Warning:  only  one  PID  may  be written to the tasks file at a
              time.  If a string is written that contains more than  one  PID,
              only the first one will be used.

       notify_on_release
              Flag  (0  or  1).   If set (1), that cpuset will receive special
              handling after it is released,  that  is,  after  all  processes
              cease  using  it  (i.e.,  terminate  or are moved to a different
              cpuset) and all child cpuset directories have been removed.  See
              the Notify On Release section, below.

       cpus   List  of  the physical numbers of the CPUs on which processes in
              that cpuset are allowed to execute.  See List Format below for a
              description of the format of cpus.

              The  CPUs  allowed  to  a cpuset may be changed by writing a new
              list to its cpus file.

       cpu_exclusive
              Flag (0 or 1).  If set (1), the cpuset has exclusive use of  its
              CPUs (no sibling or cousin cpuset may overlap CPUs).  By default
              this is off (0).  Newly created cpusets also  initially  default
              this to off (0).

              Two  cpusets  are  sibling cpusets if they share the same parent
              cpuset in the /dev/cpuset hierarchy.   Two  cpusets  are  cousin
              cpusets  if neither is the ancestor of the other.  Regardless of
              the cpu_exclusive setting, if one  cpuset  is  the  ancestor  of
              another,  and  if both of these cpusets have nonempty cpus, then
              their cpus must overlap, because the  cpus  of  any  cpuset  are
              always a subset of the cpus of its parent cpuset.

       mems   List  of  memory  nodes  on  which  processes in this cpuset are
              allowed to  allocate  memory.   See  List  Format  below  for  a
              description of the format of mems.

       mem_exclusive
              Flag  (0 or 1).  If set (1), the cpuset has exclusive use of its
              memory nodes (no sibling or cousin may overlap).   Also  if  set
              (1),  the  cpuset  is a Hardwall cpuset (see below.)  By default
              this is off (0).  Newly created cpusets also  initially  default
              this to off (0).

              Regardless  of  the  mem_exclusive setting, if one cpuset is the
              ancestor of another,  then  their  memory  nodes  must  overlap,
              because  the  memory  nodes of any cpuset are always a subset of
              the memory nodes of that cpuset's parent cpuset.

       mem_hardwall (since Linux 2.6.26)
              Flag (0 or 1).  If set (1), the cpuset is a Hardwall cpuset (see
              below.)  Unlike mem_exclusive, there is no constraint on whether
              cpusets marked mem_hardwall may have  overlapping  memory  nodes
              with  sibling  or  cousin  cpusets.  By default this is off (0).
              Newly created cpusets also initially default this to off (0).

       memory_migrate (since Linux 2.6.16)
              Flag (0 or 1).  If set (1), then memory  migration  is  enabled.
              By  default  this is off (0).  See the Memory Migration section,
              below.

       memory_pressure (since Linux 2.6.16)
              A measure of how much memory  pressure  the  processes  in  this
              cpuset  are  causing.   See  the Memory Pressure section, below.
              Unless memory_pressure_enabled is enabled, always has value zero
              (0).   This file is read-only.  See the WARNINGS section, below.

       memory_pressure_enabled (since Linux 2.6.16)
              Flag (0 or 1).  This file is only present in  the  root  cpuset,
              normally   /dev/cpuset.    If   set   (1),  the  memory_pressure
              calculations are enabled for all  cpusets  in  the  system.   By
              default  this  is  off  (0).   See  the Memory Pressure section,
              below.

       memory_spread_page (since Linux 2.6.17)
              Flag (0 or 1).  If set (1),  pages  in  the  kernel  page  cache
              (file-system  buffers)  are  uniformly spread across the cpuset.
              By default this is off (0) in the top cpuset, and inherited from
              the  parent  cpuset  in  newly  created cpusets.  See the Memory
              Spread section, below.

       memory_spread_slab (since Linux 2.6.17)
              Flag (0 or 1).  If set (1), the kernel slab caches for file  I/O
              (directory and inode structures) are uniformly spread across the
              cpuset.  By default this is off  (0)  in  the  top  cpuset,  and
              inherited  from the parent cpuset in newly created cpusets.  See
              the Memory Spread section, below.

       sched_load_balance (since Linux 2.6.24)
              Flag (0 or  1).   If  set  (1,  the  default)  the  kernel  will
              automatically  load  balance  processes  in that cpuset over the
              allowed CPUs in that cpuset.  If cleared  (0)  the  kernel  will
              avoid load balancing processes in this cpuset, unless some other
              cpuset with overlapping CPUs  has  its  sched_load_balance  flag
              set.   See Scheduler Load Balancing, below, for further details.

       sched_relax_domain_level (since Linux 2.6.26)
              Integer,  between  -1  and  a   small   positive   value.    The
              sched_relax_domain_level controls the width of the range of CPUs
              over which the kernel scheduler performs  immediate  rebalancing
              of   runnable  tasks  across  CPUs.   If  sched_load_balance  is
              disabled, then the setting of sched_relax_domain_level does  not
              matter,    as    no   such   load   balancing   is   done.    If
              sched_load_balance is enabled, then the higher the value of  the
              sched_relax_domain_level, the wider the range of CPUs over which
              immediate load balancing  is  attempted.   See  Scheduler  Relax
              Domain Level, below, for further details.

       In   addition  to  the  above  pseudo-files  in  each  directory  below
       /dev/cpuset, each process has a pseudo-file,  /proc/<pid>/cpuset,  that
       displays  the  path  of  the process's cpuset directory relative to the
       root of the cpuset file system.

       Also the /proc/<pid>/status file for each process has four added lines,
       displaying  the  process's  Cpus_allowed  (on  which  CPUs  it  may  be
       scheduled) and Mems_allowed  (on  which  memory  nodes  it  may  obtain
       memory),  in the two formats Mask Format and List Format (see below) as
       shown in the following example:

              Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
              Cpus_allowed_list:     0-127
              Mems_allowed:   ffffffff,ffffffff
              Mems_allowed_list:     0-63

       The "allowed" fields were added in  Linux  2.6.24;  the  "allowed_list"
       fields were added in Linux 2.6.26.

EXTENDED CAPABILITIES

       In  addition to controlling which cpus and mems a process is allowed to
       use, cpusets provide the following extended capabilities.

   Exclusive Cpusets
       If a cpuset is marked cpu_exclusive or mem_exclusive, no other  cpuset,
       other  than  a direct ancestor or descendant, may share any of the same
       CPUs or memory nodes.

       A cpuset that is mem_exclusive restricts kernel allocations for  buffer
       cache pages and other internal kernel data pages commonly shared by the
       kernel across multiple users.  All cpusets,  whether  mem_exclusive  or
       not,  restrict  allocations  of  memory  for  user space.  This enables
       configuring a system so that several independent jobs can share  common
       kernel  data,  while  isolating  each  job's user allocation in its own
       cpuset.  To do this, construct a large mem_exclusive cpuset to hold all
       the  jobs,  and  construct  child,  non-mem_exclusive  cpusets for each
       individual job.  Only a small amount of kernel memory, such as requests
       from  interrupt  handlers,  is  allowed  to  be  placed on memory nodes
       outside even a mem_exclusive cpuset.

   Hardwall
       A cpuset that has mem_exclusive  or  mem_hardwall  set  is  a  hardwall
       cpuset.   A  hardwall  cpuset  restricts  kernel  allocations for page,
       buffer, and other data commonly shared by the  kernel  across  multiple
       users.   All  cpusets, whether hardwall or not, restrict allocations of
       memory for user space.

       This enables configuring a system so that several independent jobs  can
       share  common  kernel  data, such as file system pages, while isolating
       each job's user allocation in its own cpuset.  To do this, construct  a
       large hardwall cpuset to hold all the jobs, and construct child cpusets
       for each individual job which are not hardwall cpusets.

       Only a small amount of kernel memory, such as requests  from  interrupt
       handlers, is allowed to be taken outside even a hardwall cpuset.

   Notify On Release
       If the notify_on_release flag is enabled (1) in a cpuset, then whenever
       the last process in the cpuset leaves (exits or attaches to some  other
       cpuset) and the last child cpuset of that cpuset is removed, the kernel
       will run the command /sbin/cpuset_release_agent, supplying the pathname
       (relative  to  the  mount  point  of  the  cpuset  file  system) of the
       abandoned cpuset.  This enables automatic removal of abandoned cpusets.

       The  default  value  of  notify_on_release in the root cpuset at system
       boot is disabled (0).  The default value of other cpusets  at  creation
       is the current value of their parent's notify_on_release setting.

       The  command  /sbin/cpuset_release_agent  is  invoked,  with  the  name
       (/dev/cpuset relative path) of the to-be-released cpuset in argv[1].

       The usual contents of the command /sbin/cpuset_release_agent is  simply
       the shell script:

           #!/bin/sh
           rmdir /dev/cpuset/$1

       As with other flag values below, this flag can be changed by writing an
       ASCII number 0 or 1 (with optional trailing newline) into the file,  to
       clear or set the flag, respectively.

   Memory Pressure
       The  memory_pressure  of  a cpuset provides a simple per-cpuset running
       average of the rate that the processes in a cpuset  are  attempting  to
       free  up in-use memory on the nodes of the cpuset to satisfy additional
       memory requests.

       This enables  batch  managers  that  are  monitoring  jobs  running  in
       dedicated  cpusets  to efficiently detect what level of memory pressure
       that job is causing.

       This is useful both on tightly managed systems running a  wide  mix  of
       submitted jobs, which may choose to terminate or reprioritize jobs that
       are trying to use more memory than allowed on the nodes assigned  them,
       and  with  tightly coupled, long-running, massively parallel scientific
       computing jobs that will dramatically fail to meet required performance
       goals if they start to use more memory than allowed to them.

       This  mechanism provides a very economical way for the batch manager to
       monitor a cpuset for signs of memory pressure.  It's up  to  the  batch
       manager  or other user code to decide what action to take if it detects
       signs of memory pressure.

       Unless memory pressure calculation is enabled by  setting  the  pseudo-
       file  /dev/cpuset/memory_pressure_enabled,  it  is not computed for any
       cpuset, and reads from  any  memory_pressure  always  return  zero,  as
       represented  by  the  ASCII  string  "0\n".   See the WARNINGS section,
       below.

       A per-cpuset, running average is employed for the following reasons:

       *  Because this meter is per-cpuset  rather  than  per-process  or  per
          virtual  memory region, the system load imposed by a batch scheduler
          monitoring this metric is sharply reduced on large systems,  because
          a scan of the tasklist can be avoided on each set of queries.

       *  Because  this meter is a running average rather than an accumulating
          counter, a batch scheduler can detect memory pressure with a  single
          read,  instead of having to read and accumulate results for a period
          of time.

       *  Because this meter is per-cpuset rather than per-process, the  batch
          scheduler  can  obtain  the  key information -- memory pressure in a
          cpuset -- with a single  read,  rather  than  having  to  query  and
          accumulate  results  over  all  the  (dynamically  changing)  set of
          processes in the cpuset.

       The memory_pressure of a cpuset is calculated using a per-cpuset simple
       digital  filter  that is kept within the kernel.  For each cpuset, this
       filter tracks the recent rate  at  which  processes  attached  to  that
       cpuset enter the kernel direct reclaim code.

       The  kernel  direct  reclaim  code is entered whenever a process has to
       satisfy a memory page request by  first  finding  some  other  page  to
       repurpose,  due  to  lack  of any readily available already free pages.
       Dirty file system pages are repurposed by first writing them  to  disk.
       Unmodified  file  system buffer pages are repurposed by simply dropping
       them, though if that page is needed again, it will have  to  be  reread
       from disk.

       The  memory_pressure  file  provides an integer number representing the
       recent (half-life of 10 seconds) rate of entries to the direct  reclaim
       code  caused  by  any  process  in  the  cpuset,  in  units of reclaims
       attempted per second, times 1000.

   Memory Spread
       There are two Boolean flag files per  cpuset  that  control  where  the
       kernel  allocates  pages  for  the  file-system buffers and related in-
       kernel  data  structures.   They  are  called  memory_spread_page   and
       memory_spread_slab.

       If the per-cpuset Boolean flag file memory_spread_page is set, then the
       kernel will spread the file-system buffers (page cache) evenly over all
       the  nodes  that  the  faulting  process  is allowed to use, instead of
       preferring to put those pages on the node where the process is running.

       If the per-cpuset Boolean flag file memory_spread_slab is set, then the
       kernel will spread some file-system-related slab caches, such as  those
       for  inodes  and  directory entries, evenly over all the nodes that the
       faulting process is allowed to use, instead of preferring to put  those
       pages on the node where the process is running.

       The  setting  of  these  flags  does  not  affect the data segment (see
       brk(2)) or stack segment pages of a process.

       By default, both kinds of memory  spreading  are  off  and  the  kernel
       prefers  to  allocate  memory  pages  on  the  node  local to where the
       requesting process is running.  If that node  is  not  allowed  by  the
       process's  NUMA  memory  policy or cpuset configuration or if there are
       insufficient free memory pages on that node, then the kernel looks  for
       the nearest node that is allowed and has sufficient free memory.

       When  new  cpusets are created, they inherit the memory spread settings
       of their parent.

       Setting memory spreading causes allocations for the  affected  page  or
       slab  caches  to  ignore the process's NUMA memory policy and be spread
       instead.  However, the effect of  these  changes  in  memory  placement
       caused by cpuset-specified memory spreading is hidden from the mbind(2)
       or set_mempolicy(2) calls.  These two NUMA memory policy  calls  always
       appear  to  behave  as  if  no  cpuset-specified memory spreading is in
       effect, even if it is.  If  cpuset  memory  spreading  is  subsequently
       turned  off,  the  NUMA  memory policy most recently specified by these
       calls is automatically reapplied.

       Both memory_spread_page and memory_spread_slab are Boolean flag  files.
       By  default  they contain "0", meaning that the feature is off for that
       cpuset.  If a "1" is written to that file, that turns the named feature
       on.

       Cpuset-specified  memory  spreading  behaves similarly to what is known
       (in other contexts) as round-robin or interleave memory placement.

       Cpuset-specified memory spreading can provide  substantial  performance
       improvements for jobs that:

       a) need  to  place  thread-local data on memory nodes close to the CPUs
          which are running the threads that most frequently access that data;
          but also

       b) need  to  access  large file-system data sets that must to be spread
          across the several nodes in the job's cpuset in order to fit.

       Without this policy, the memory allocation  across  the  nodes  in  the
       job's  cpuset  can  become  very uneven, especially for jobs that might
       have just a single thread initializing or reading in the data set.

   Memory Migration
       Normally, under the default setting (disabled) of memory_migrate,  once
       a  page  is  allocated (given a physical page of main memory) then that
       page stays on whatever node it was allocated, so  long  as  it  remains
       allocated,   even   if   the   cpuset's  memory-placement  policy  mems
       subsequently changes.

       When memory migration is enabled in a cpuset, if the  mems  setting  of
       the  cpuset  is  changed, then any memory page in use by any process in
       the cpuset that is on a memory node that is no longer allowed  will  be
       migrated to a memory node that is allowed.

       Furthermore,  if  a  process is moved into a cpuset with memory_migrate
       enabled, any memory pages it uses that were on memory nodes allowed  in
       its  previous cpuset, but which are not allowed in its new cpuset, will
       be migrated to a memory node allowed in the new cpuset.

       The relative  placement  of  a  migrated  page  within  the  cpuset  is
       preserved  during these migration operations if possible.  For example,
       if the page was on the second valid node of the prior cpuset, then  the
       page  will  be  placed  on  the second valid node of the new cpuset, if
       possible.

   Scheduler Load Balancing
       The kernel scheduler automatically load balances processes.  If one CPU
       is  underutilized,  the  kernel  will  look for processes on other more
       overloaded CPUs and move those  processes  to  the  underutilized  CPU,
       within  the  constraints  of  such  placement mechanisms as cpusets and
       sched_setaffinity(2).

       The algorithmic cost of load balancing and its  impact  on  key  shared
       kernel  data  structures  such  as the process list increases more than
       linearly with the number of CPUs being balanced.  For example, it costs
       more  to  load  balance  across  one  large set of CPUs than it does to
       balance across two smaller sets of CPUs, each of half the size  of  the
       larger set.  (The precise relationship between the number of CPUs being
       balanced and the cost  of  load  balancing  depends  on  implementation
       details  of  the  kernel  process scheduler, which is subject to change
       over time, as improved kernel scheduler algorithms are implemented.)

       The per-cpuset flag sched_load_balance provides a mechanism to suppress
       this automatic scheduler load balancing in cases where it is not needed
       and suppressing it would have worthwhile performance benefits.

       By default, load balancing is done across all CPUs, except those marked
       isolated  using  the  kernel  boot  time  "isolcpus="  argument.   (See
       Scheduler Relax Domain Level, below, to change this default.)

       This default load balancing across all CPUs is not well suited  to  the
       following two situations:

       *  On  large systems, load balancing across many CPUs is expensive.  If
          the system is managed using cpusets to  place  independent  jobs  on
          separate sets of CPUs, full load balancing is unnecessary.

       *  Systems  supporting  real-time  on some CPUs need to minimize system
          overhead on those CPUs, including avoiding process load balancing if
          that is not needed.

       When  the  per-cpuset  flag  sched_load_balance is enabled (the default
       setting), it requests load  balancing  across  all  the  CPUs  in  that
       cpuset's  allowed CPUs, ensuring that load balancing can move a process
       (not otherwise pinned, as by sched_setaffinity(2)) from any CPU in that
       cpuset to any other.

       When  the  per-cpuset  flag  sched_load_balance  is  disabled, then the
       scheduler will avoid load balancing across the  CPUs  in  that  cpuset,
       except  in  so  far as is necessary because some overlapping cpuset has
       sched_load_balance enabled.

       So, for example, if the top  cpuset  has  the  flag  sched_load_balance
       enabled,  then the scheduler will load balance across all CPUs, and the
       setting of the sched_load_balance flag in other cpusets has no  effect,
       as we're already fully load balancing.

       Therefore  in  the  above  two  situations, the flag sched_load_balance
       should be disabled in the top cpuset, and only  some  of  the  smaller,
       child cpusets would have this flag enabled.

       When doing this, you don't usually want to leave any unpinned processes
       in the top cpuset that might use nontrivial amounts  of  CPU,  as  such
       processes  may  be  artificially  constrained  to  some subset of CPUs,
       depending on  the  particulars  of  this  flag  setting  in  descendant
       cpusets.   Even  if  such  a process could use spare CPU cycles in some
       other CPUs, the kernel scheduler might not consider the possibility  of
       load balancing that process to the underused CPU.

       Of course, processes pinned to a particular CPU can be left in a cpuset
       that  disables  sched_load_balance  as  those  processes  aren't  going
       anywhere else anyway.

   Scheduler Relax Domain Level
       The  kernel  scheduler performs immediate load balancing whenever a CPU
       becomes free or another task becomes  runnable.   This  load  balancing
       works  to  ensure  that  as many CPUs as possible are usefully employed
       running tasks.  The kernel also performs periodic  load  balancing  off
       the   software   clock   described   in   time(7).    The   setting  of
       sched_relax_domain_level only  applies  to  immediate  load  balancing.
       Regardless  of  the  sched_relax_domain_level  setting,  periodic  load
       balancing is attempted over all CPUs (unless disabled  by  turning  off
       sched_load_balance.)   In  any  case,  of  course,  tasks  will only be
       scheduled to run on CPUs  allowed  by  their  cpuset,  as  modified  by
       sched_setaffinity(2) system calls.

       On  small  systems,  such as those with just a few CPUs, immediate load
       balancing is useful to improve system  interactivity  and  to  minimize
       wasteful  idle  CPU cycles.  But on large systems, attempting immediate
       load balancing across a large number of CPUs can be more costly than it
       is  worth,  depending  on the particular performance characteristics of
       the job mix and the hardware.

       The   exact    meaning    of    the    small    integer    values    of
       sched_relax_domain_level will depend on internal implementation details
       of the kernel scheduler code and on the non-uniform architecture of the
       hardware.   Both  of  these  will  evolve  over time and vary by system
       architecture and kernel version.

       As of this writing,  when  this  capability  was  introduced  in  Linux
       2.6.26,  on  certain  popular  architectures,  the  positive  values of
       sched_relax_domain_level have the following meanings.

       (1) Perform immediate load balancing across  Hyper-Thread  siblings  on
           the same core.
       (2) Perform  immediate  load  balancing  across other cores in the same
           package.
       (3) Perform immediate load balancing across other CPUs on the same node
           or blade.
       (4) Perform    immediate    load    balancing   across   over   several
           (implementation detail) nodes [On NUMA systems].
       (5) Perform immediate load balancing across over all CPUs in system [On
           NUMA systems].

       The  sched_relax_domain_level  value  of  zero  (0)  always means don't
       perform immediate load balancing, hence that  load  balancing  is  only
       done  periodically,  not  immediately  when  a CPU becomes available or
       another task becomes runnable.

       The sched_relax_domain_level value of minus one (-1) always  means  use
       the  system  default  value.   The  system  default  value  can vary by
       architecture and kernel version.  This  system  default  value  can  be
       changed by kernel boot-time "relax_domain_level=" argument.

       In  the  case  of  multiple  overlapping cpusets which have conflicting
       sched_relax_domain_level values, then the highest such value applies to
       all  CPUs  in any of the overlapping cpusets.  In such cases, the value
       minus one (-1) is the lowest value, overridden by any other value,  and
       the value zero (0) is the next lowest value.

FORMATS

       The  following  formats  are  used to represent sets of CPUs and memory
       nodes.

   Mask Format
       The Mask Format is used to represent CPU and  memory-node  bitmasks  in
       the /proc/<pid>/status file.

       This  format  displays  each  32-bit  word  in hexadecimal (using ASCII
       characters "0" - "9" and "a" - "f");  words  are  filled  with  leading
       zeros,  if required.  For masks longer than one word, a comma separator
       is used between words.  Words are displayed in big-endian order,  which
       has  the  most significant bit first.  The hex digits within a word are
       also in big-endian order.

       The number of 32-bit words displayed is the minimum  number  needed  to
       display all bits of the bitmask, based on the size of the bitmask.

       Examples of the Mask Format:

              00000001                        # just bit 0 set
              40000000,00000000,00000000      # just bit 94 set
              00000001,00000000,00000000      # just bit 64 set
              000000ff,00000000               # bits 32-39 set
              00000000,000E3862               # 1,5,6,11-13,17-19 set

       A mask with bits 0, 1, 2, 4, 8, 16, 32, and 64 set displays as:

              00000001,00000001,00010117

       The  first  "1" is for bit 64, the second for bit 32, the third for bit
       16, the fourth for bit 8, the fifth for bit 4, and the "7" is for  bits
       2, 1, and 0.

   List Format
       The  List  Format for cpus and mems is a comma-separated list of CPU or
       memory-node numbers and ranges of numbers, in ASCII decimal.

       Examples of the List Format:

              0-4,9           # bits 0, 1, 2, 3, 4, and 9 set
              0-2,7,12-14     # bits 0, 1, 2, 7, 12, 13, and 14 set

RULES

       The following rules apply to each cpuset:

       *  Its CPUs and memory nodes must be a (possibly equal) subset  of  its
          parent's.

       *  It can only be marked cpu_exclusive if its parent is.

       *  It can only be marked mem_exclusive if its parent is.

       *  If it is cpu_exclusive, its CPUs may not overlap any sibling.

       *  If  it  is  memory_exclusive,  its  memory nodes may not overlap any
          sibling.

PERMISSIONS

       The permissions of a cpuset are determined by the  permissions  of  the
       directories  and  pseudo-files  in  the  cpuset  file  system, normally
       mounted at /dev/cpuset.

       For instance, a process can put itself in some other cpuset  (than  its
       current  one)  if  it  can  write the tasks file for that cpuset.  This
       requires execute permission on the encompassing directories  and  write
       permission on the tasks file.

       An  additional  constraint  is  applied to requests to place some other
       process in a cpuset.  One process may not attach another  to  a  cpuset
       unless  it  would  have  permission  to send that process a signal (see
       kill(2)).

       A process may create a child cpuset if it  can  access  and  write  the
       parent  cpuset  directory.  It can modify the CPUs or memory nodes in a
       cpuset if it can access that cpuset's directory (execute permissions on
       the each of the parent directories) and write the corresponding cpus or
       mems file.

       There is one  minor  difference  between  the  manner  in  which  these
       permissions  are  evaluated  and the manner in which normal file-system
       operation permissions are evaluated.  The  kernel  interprets  relative
       pathnames  starting  at a process's current working directory.  Even if
       one is operating on a cpuset file, relative pathnames  are  interpreted
       relative  to  the  process's current working directory, not relative to
       the process's current cpuset.  The only ways that cpuset paths relative
       to  a  process's current cpuset can be used are if either the process's
       current working directory is its cpuset (it first did a cd or  chdir(2)
       to its cpuset directory beneath /dev/cpuset, which is a bit unusual) or
       if some user code converts the relative cpuset path  to  a  full  file-
       system path.

       In  theory,  this  means  that  user  code should specify cpusets using
       absolute pathnames, which requires  knowing  the  mount  point  of  the
       cpuset  file  system  (usually,  but not necessarily, /dev/cpuset).  In
       practice, all user level code that  this  author  is  aware  of  simply
       assumes  that  if the cpuset file system is mounted, then it is mounted
       at /dev/cpuset.  Furthermore,  it  is  common  practice  for  carefully
       written   user   code   to  verify  the  presence  of  the  pseudo-file
       /dev/cpuset/tasks in order to verify that the cpuset pseudo-file system
       is currently mounted.

WARNINGS

   Enabling memory_pressure
       By  default,  the  per-cpuset file memory_pressure always contains zero
       (0).  Unless this feature is enabled by writing "1" to the  pseudo-file
       /dev/cpuset/memory_pressure_enabled,  the  kernel does not compute per-
       cpuset memory_pressure.

   Using the echo command
       When using the echo command at the shell prompt to change the values of
       cpuset files, beware that the built-in echo command in some shells does
       not display an error message if the write(2) system  call  fails.   For
       example, if the command:

           echo 19 > mems

       failed  because  memory  node  19  was not allowed (perhaps the current
       system does not have a memory node 19), then the echo command might not
       display  any error.  It is better to use the /bin/echo external command
       to change cpuset file settings, as this command will  display  write(2)
       errors, as in the example:

           /bin/echo 19 > mems
           /bin/echo: write error: Invalid argument

EXCEPTIONS

   Memory placement
       Not  all  allocations  of system memory are constrained by cpusets, for
       the following reasons.

       If hot-plug functionality is used to  remove  all  the  CPUs  that  are
       currently  assigned  to  a  cpuset,  then the kernel will automatically
       update the cpus_allowed of all  processes  attached  to  CPUs  in  that
       cpuset  to  allow  all  CPUs.   When  memory hot-plug functionality for
       removing memory nodes is available, a similar exception is expected  to
       apply  there as well.  In general, the kernel prefers to violate cpuset
       placement, rather than starving a process that has had all its  allowed
       CPUs  or  memory  nodes  taken  offline.   User code should reconfigure
       cpusets to only refer to online CPUs and memory nodes when  using  hot-
       plug to add or remove such resources.

       A  few  kernel-critical,  internal  memory-allocation  requests, marked
       GFP_ATOMIC, must be satisfied immediately.  The kernel  may  drop  some
       request  or  malfunction  if  one of these allocations fail.  If such a
       request cannot be satisfied within the current process's  cpuset,  then
       we relax the cpuset, and look for memory anywhere we can find it.  It's
       better to violate the cpuset than stress the kernel.

       Allocations of memory requested by kernel drivers while  processing  an
       interrupt  lack  any  relevant process context, and are not confined by
       cpusets.

   Renaming cpusets
       You can use the rename(2) system call to rename cpusets.   Only  simple
       renaming is supported; that is, changing the name of a cpuset directory
       is permitted, but moving a directory into a different directory is  not
       permitted.

ERRORS

       The  Linux  kernel  implementation of cpusets sets errno to specify the
       reason for a failed system call affecting cpusets.

       The possible errno settings and their meaning  when  set  on  a  failed
       cpuset call are as listed below.

       E2BIG  Attempted  a  write(2)  on  a  special cpuset file with a length
              larger than some kernel-determined upper limit on the length  of
              such writes.

       EACCES Attempted  to  write(2)  the  process ID (PID) of a process to a
              cpuset tasks  file  when  one  lacks  permission  to  move  that
              process.

       EACCES Attempted  to  add,  using  write(2),  a CPU or memory node to a
              cpuset, when that CPU or memory node  was  not  already  in  its
              parent.

       EACCES Attempted to set, using write(2), cpu_exclusive or mem_exclusive
              on a cpuset whose parent lacks the same setting.

       EACCES Attempted to write(2) a memory_pressure file.

       EACCES Attempted to create a file in a cpuset directory.

       EBUSY  Attempted to remove, using  rmdir(2),  a  cpuset  with  attached
              processes.

       EBUSY  Attempted  to  remove,  using  rmdir(2),  a  cpuset  with  child
              cpusets.

       EBUSY  Attempted to remove a CPU or memory node from a cpuset  that  is
              also in a child of that cpuset.

       EEXIST Attempted  to  create,  using  mkdir(2),  a  cpuset that already
              exists.

       EEXIST Attempted to rename(2) a cpuset to a name that already exists.

       EFAULT Attempted to read(2) or write(2) a cpuset file  using  a  buffer
              that  is outside the writing processes accessible address space.

       EINVAL Attempted to change a cpuset, using  write(2),  in  a  way  that
              would violate a cpu_exclusive or mem_exclusive attribute of that
              cpuset or any of its siblings.

       EINVAL Attempted to write(2) an empty cpus or mems  list  to  a  cpuset
              which has attached processes or child cpusets.

       EINVAL Attempted to write(2) a cpus or mems list which included a range
              with the second number smaller than the first number.

       EINVAL Attempted to write(2) a cpus or  mems  list  which  included  an
              invalid character in the string.

       EINVAL Attempted to write(2) a list to a cpus file that did not include
              any online CPUs.

       EINVAL Attempted to write(2) a list to a mems file that did not include
              any online memory nodes.

       EINVAL Attempted to write(2) a list to a mems file that included a node
              that held no memory.

       EIO    Attempted to write(2) a string to a cpuset tasks file that  does
              not begin with an ASCII decimal integer.

       EIO    Attempted to rename(2) a cpuset into a different directory.

       ENAMETOOLONG
              Attempted to read(2) a /proc/<pid>/cpuset file for a cpuset path
              that is longer than the kernel page size.

       ENAMETOOLONG
              Attempted  to  create,  using  mkdir(2),  a  cpuset  whose  base
              directory name is longer than 255 characters.

       ENAMETOOLONG
              Attempted  to  create,  using  mkdir(2),  a  cpuset  whose  full
              pathname, including the mount point  (typically  "/dev/cpuset/")
              prefix, is longer than 4095 characters.

       ENODEV The  cpuset was removed by another process at the same time as a
              write(2) was attempted on one of the pseudo-files in the  cpuset
              directory.

       ENOENT Attempted to create, using mkdir(2), a cpuset in a parent cpuset
              that doesn't exist.

       ENOENT Attempted to access(2) or open(2) a nonexistent file in a cpuset
              directory.

       ENOMEM Insufficient memory is available within the kernel; can occur on
              a variety of system calls affecting cpusets,  but  only  if  the
              system is extremely short of memory.

       ENOSPC Attempted  to  write(2)  the  process ID (PID) of a process to a
              cpuset tasks file when the cpuset had an  empty  cpus  or  empty
              mems setting.

       ENOSPC Attempted  to write(2) an empty cpus or mems setting to a cpuset
              that has tasks attached.

       ENOTDIR
              Attempted to rename(2) a nonexistent cpuset.

       EPERM  Attempted to remove a file from a cpuset directory.

       ERANGE Specified a cpus or mems list to the  kernel  which  included  a
              number too large for the kernel to set in its bitmasks.

       ESRCH  Attempted  to  write(2)  the  process  ID (PID) of a nonexistent
              process to a cpuset tasks file.

VERSIONS

       Cpusets appeared in version 2.6.12 of the Linux kernel.

NOTES

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

BUGS

       memory_pressure  cpuset  files  can be opened for writing, creation, or
       truncation, but then the write(2) fails with errno set to  EACCES,  and
       the creation and truncation options on open(2) have no effect.

EXAMPLE

       The  following examples demonstrate querying and setting cpuset options
       using shell commands.

   Creating and attaching to a cpuset.
       To create a new cpuset and attach the current command shell to it,  the
       steps are:

       1)  mkdir /dev/cpuset (if not already done)
       2)  mount -t cpuset none /dev/cpuset (if not already done)
       3)  Create the new cpuset using mkdir(1).
       4)  Assign CPUs and memory nodes to the new cpuset.
       5)  Attach the shell to the new cpuset.

       For  example,  the  following sequence of commands will set up a cpuset
       named "Charlie", containing just CPUs 2 and 3, and memory node  1,  and
       then attach the current shell to that cpuset.

           $ mkdir /dev/cpuset
           $ mount -t cpuset cpuset /dev/cpuset
           $ cd /dev/cpuset
           $ mkdir Charlie
           $ cd Charlie
           $ /bin/echo 2-3 > cpus
           $ /bin/echo 1 > mems
           $ /bin/echo $$ > tasks
           # The current shell is now running in cpuset Charlie
           # The next line should display '/Charlie'
           $ cat /proc/self/cpuset

   Migrating a job to different memory nodes.
       To  migrate  a  job  (the  set  of  processes  attached to a cpuset) to
       different CPUs and memory nodes in the  system,  including  moving  the
       memory  pages  currently  allocated  to that job, perform the following
       steps.

       1)  Let's say we want to move the job in cpuset  alpha  (CPUs  4-7  and
           memory nodes 2-3) to a new cpuset beta (CPUs 16-19 and memory nodes
           8-9).
       2)  First create the new cpuset beta.
       3)  Then allow CPUs 16-19 and memory nodes 8-9 in beta.
       4)  Then enable memory_migration in beta.
       5)  Then move each process from alpha to beta.

       The following sequence of commands accomplishes this.

           $ cd /dev/cpuset
           $ mkdir beta
           $ cd beta
           $ /bin/echo 16-19 > cpus
           $ /bin/echo 8-9 > mems
           $ /bin/echo 1 > memory_migrate
           $ while read i; do /bin/echo $i; done < ../alpha/tasks > tasks

       The above should move any processes in alpha to beta,  and  any  memory
       held  by  these  processes  on  memory  nodes  2-3 to memory nodes 8-9,
       respectively.

       Notice that the last step of the above sequence did not do:

           $ cp ../alpha/tasks tasks

       The while loop, rather than the  seemingly  easier  use  of  the  cp(1)
       command,  was  necessary  because only one process PID at a time may be
       written to the tasks file.

       The same effect (writing one PID at a time) as the while  loop  can  be
       accomplished  more  efficiently, in fewer keystrokes and in syntax that
       works  on  any  shell,  but  alas  more  obscurely,  by  using  the  -u
       (unbuffered) option of sed(1):

           $ sed -un p < ../alpha/tasks > tasks

SEE ALSO

       taskset(1),        get_mempolicy(2),        getcpu(2),        mbind(2),
       sched_getaffinity(2),   sched_setaffinity(2),    sched_setscheduler(2),
       set_mempolicy(2),   CPU_SET(3),   proc(5),   numa(7),  migratepages(8),
       numactl(8)

       The kernel source file Documentation/cpusets.txt.

COLOPHON

       This page is part of release 3.24 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.