Provided by: libcpuset1_1.0-1_i386 bug

NAME

       cpuset - confine tasks to processor and memory node subsets

DESCRIPTION

       The  cpuset  file system is a pseudo-filesystem interface to the kernel
       cpuset mechanism for controlling the processor and memory placement  of
       tasks.  It is commonly mounted at /dev/cpuset.

       A  cpuset  defines  a  list  of  CPUs  and  memory  nodes.  Cpusets are
       represented as directories in a hierarchical virtual 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
       parents CPUs and memory nodes.  The directories and files  representing
       cpusets have normal file system permissions.

       Every  task  in  the  system  belongs to exactly one cpuset.  A task 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 task
       forks, the child task is placed in the same cpuset as its parent.  With
       sufficient  privilege,  a  task 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, only the top cpuset is defined and all
       tasks are in that cpuset.  During the boot  process   or  later  during
       normal  system  operation,  other  cpusets  may  be  created,  as  sub-
       directories  of  the  top  cpuset  under  the  control  of  the  system
       administrator and tasks 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
       task make use of a CPU or memory node that is not allowed  by  cpusets.
       If  changes  to  a  tasks  cpuset  placement  conflict with these other
       mechanisms,  then  cpuset  placement  is  enforced  even  if  it  means
       overriding these other mechanisms.

       Typically,  a  cpuset  is  used  to  manage  the  CPU  and  memory node
       confinement for the entire set of tasks  in  a  job,  and  these  other
       mechanisms  are  used  to  manage  the placement of individual tasks or
       memory regions within a job.

FILES

       Each directory below  /dev/cpuset  represents  a  cpuset  and  contains
       several files describing the state of that cpuset.

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

       The  files  in each cpuset directory are automatically created when the
       cpuset is created, as a result of the  mkdir  invocation.   It  is  not
       allowed to add or remove files from a cpuset directory.

       The  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  using  ordinary  file  access  routines from programmatic
       languages, such as open(2), read(2), write(2) and close(2) from the ’C’
       library.   These  files 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 tasks in that cpuset.  The
              list is formatted as a series of  ASCII  decimal  numbers,  each
              followed  by  a  newline.   A  task  may  be  added  to a cpuset
              (removing it  from  the  cpuset  previously  containing  it)  by
              writing  its  PID  to that cpusets tasks file (with or without a
              trailing newline.)

              Beware that 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 considered.

       notify_on_release
              Flag (0 or 1).  If set (1), that  cpuset  will  receive  special
              handling whenever its last using task and last child cpuset goes
              away.  See the Notify On Release section, below.

       cpus
              List of CPUs on which  tasks  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.  Note however, such a change does not
              take affect until the PIDs  of  the  tasks  in  the  cpuset  are
              rewritten  to the cpusets tasks file.  See the WARNINGS section,
              below.

       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).

       mems
              List  of  memory nodes on which tasks in that 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).   By  default
              this  is  off (0).  Newly created cpusets also initially default
              this to off (0).

       memory_migrate
              Flag (0 or 1).  If set (1), then memory  migration  is  enabled.
              See the Memory Migration section, below.

       memory_pressure
              A  measure  of how much memory pressure the tasks 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
              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.  See the
              Memory Pressure section, below.

       memory_spread_page
              Flag  (0  or 1).  If set (1), the kernel page cache (file system
              buffers) are uniformly spread across the cpuset.  See the Memory
              Spread section, below.

       memory_spread_slab
              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.  See the Memory Spread section, below.

       In  addition  to  the  above  special  files  in  each  directory below
       /dev/cpuset, each task under /proc has  an  added  file  named  cpuset,
       displaying  the  cpuset  name,  as the path relative to the root of the
       cpuset file system.

       Also the /proc/<pid>/status file for each task  has  two  added  lines,
       displaying  the  tasks cpus_allowed (on which CPUs it may be scheduled)
       and mems_allowed (on which memory nodes it may obtain memory),  in  the
       Mask Format (see below) as shown in the following example:

                      Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
                      Mems_allowed:   ffffffff,ffffffff

EXTENDED CAPABILITIES

       In  addition  to  controlling  which cpus and mems a task 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  cpu_exclusive  has  a  scheduler  (sched)   domain
       associated  with  it.   The  sched  domain  consists of all CPUs in the
       current cpuset that are not part of any exclusive child cpusets.   This
       ensures  that  the  scheduler load balancing code only balances against
       the CPUs that are in the sched domain as defined above and not  all  of
       the CPUs in the system. This removes any overhead due to load balancing
       code trying to pull tasks outside of the cpu_exclusive cpuset  only  to
       be prevented by the tasks’ cpus_allowed mask.

       A  cpuset  that is mem_exclusive restricts kernel allocations for page,
       buffer and other data 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, such as file
       system pages, while isolating each jobs  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 typical kernel memory, such as
       requests from interrupt handlers, is allowed to be taken outside even a
       mem_exclusive cpuset.

   Notify On Release
       If the notify_on_release flag is enabled (1) in a cpuset, then whenever
       the last task 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 parents notify_on_release setting.

       The  command  /sbin/cpuset_release_agent  is  invoked,  with  the  name
       (/dev/cpuset relative path) of that cpuset in argv[1].   This  supports
       automatic cleanup of abandoned cpusets.

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

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

       By default,  notify_on_release  is  off  (0).   Newly  created  cpusets
       inherit their notify_on_release setting from their parent cpuset.

       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 metric of
       the rate that the tasks 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  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 re-prioritize 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  to  do  about  it  and  take
       action.

       Unless  memory  pressure  calculation is enabled by setting the special
       file /dev/cpuset/memory_pressure_enabled, it is not  computed  for  any
       cpuset,  and  always  reads a value of zero.  See the WARNINGS section,
       below.

       Why a per-cpuset, running average:
          Because this meter is per-cpuset rather than  per-task  or  mm,  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-task  or  mm,  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 tasks
          in the cpuset.

       A per-cpuset simple digital filter  is  kept  within  the  kernel,  and
       updated  by  any  task  attached  to  that  cpuset,  if  it  enters the
       synchronous (direct) page reclaim code.

       A per-cpuset file provides an integer number  representing  the  recent
       (half-life  of  10  seconds) rate of direct page reclaims caused by the
       tasks 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  task  is  allowed  to  use,  instead  of
       preferring to put those pages on the node where the task 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  for
       inodes  and  directory  entries  evenly  over  all  the  nodes that the
       faulting task is allowed to use, instead of  preferring  to  put  those
       pages on the node where the task is running.

       The  setting  of  these flags does not affect anonymous data segment or
       stack segment pages of a task.

       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 task is running.  If that node is not allowed by  the  tasks
       NUMA  mempolicy  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 does have 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 tasks NUMA mempolicy and be spread instead.
       Tasks using mbind() or set_mempolicy() calls to  set  NUMA  mempolicies
       will  not  notice  any  change  in  these  calls  as  a result of their
       containing tasks memory spread settings.  If memory spreading is turned
       off,  the  currently  specified  NUMA  mempolicy  once again applies to
       memory page allocations.

       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.

       This  memory  placement  policy  is  also  known (in other contexts) as
       round-robin or interleave.

       This policy can provide substantial improvements for jobs that need  to
       place  thread  local  data  on the corresponding node, but that need to
       access large file system data sets that need to be  spread  across  the
       several nodes in the jobs cpuset in order to fit.  Without this policy,
       especially for jobs that might have one thread reading in the data set,
       the  memory  allocation  across the nodes in the jobs cpuset can become
       very uneven.

   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   cpusets  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 task in the
       cpuset that is on a memory node no longer allowed will be migrated to a
       memory node that is allowed.

       Also  if a task 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.

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.

       It is hexadecimal, using ASCII characters "0" - "9" and "a" - "f". This
       format  displays  each  32-bit  word in hex (zero filled) and for masks
       longer than one word uses a comma separator between  words.  Words  are
       displayed  in  big-endian  order most significant first. And 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
                      80000000,00000000,00000000      # just bit 95 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
        parents.

        * 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
       special files and directories  in  the  cpuset  file  system,  normally
       mounted at /dev/cpuset.

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

       An additional constraint is applied to requests  to  place  some  other
       task  in  a cpuset.  One task may not attach another to a cpuset unless
       it would have permission to send that task a signal.

       A task 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 cpusets directory  (execute  permissions  on  the
       encompassing  directories)  and  write  the  corresponding cpus or mems
       file.

       Note however that since changes to the CPUs of a cpuset don’t apply  to
       any  task  in that cpuset until said task is reattached to that cpuset,
       it would normally not be a good idea to arrange the  permissions  on  a
       cpuset so that some task could write the cpus file unless it could also
       write the tasks file to reattach the tasks therein.

       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  evaluates  relative
       pathnames  starting  at a tasks current working directory.  Even if one
       is operating  on  a  cpuset  file,  relative  pathnames  are  evaluated
       relative  to  the  current  working  directory, not relative to a tasks
       current cpuset.  The only ways that cpuset paths relative  to  a  tasks
       current  cpuset  can  be  used  are if either the tasks current working
       directory is its cpuset (it first did a  cd  or  chdir  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.

WARNINGS

   Updating a cpusets cpus
       Changes to a cpusets cpus file do not take affect for any task in  that
       cpuset  until  that  tasks process ID (PID) is rewritten to the cpusets
       tasks file.  This unusual requirement is needed to optimize a  critical
       code path in the Linux kernel.  Beware that only one PID can be written
       at a time to a  cpusets  tasks  file.   Additional  PIDs  on  a  single
       write(2)  system call are ignored.  One (unobvious) way to satisfy this
       requirement to rewrite the tasks file after updating the cpus  file  is
       to  use  the  -u  unbuffered  option  to  the sed(1) command, as in the
       following scenario:
              cd /dev/cpuset/foo              # /foo is an existing cpuset
              /bin/echo 3 > cpus              # change /foos cpus
              sed -un p < tasks > tasks       # rewrite /foos tasks file

       If one examines the Cpus_allowed value in the  /proc/<pid>/status  file
       for  one  of  the  tasks in cpuset /foo in the above scenario, one will
       notice that the value does not change when the  cpus  file  is  written
       (the  echo  command), but only later, after the tasks file is rewritten
       (the sed command).

   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 special 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 most shell  built-in  echo  commands  to  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 above echo command
       would 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: No space left on device

EXCEPTIONS

       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 tasks 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, over
       starving a task 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 tasks 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 task  context,  and  are  not  confined  by
       cpusets.

LIMITATIONS

   Kernel limitations updating cpusets
       In  order  to  minimize  the impact of cpusets on critical kernel code,
       such as the scheduler, and due to the fact that  the  kernel  does  not
       support  one  task  updating  the  memory  placement  of  another  task
       directly, the impact on a task of changing its  cpuset  CPU  or  memory
       node  placement,  or of changing to which cpuset a task is attached, is
       subtle.

       If a cpuset has its memory nodes modified, then for each task  attached
       to  that  cpuset,  the next time that the kernel attempts to allocate a
       page of memory for that task, the kernel will notice the change in  the
       tasks cpuset, and update its per-task memory placement to remain within
       the new cpusets memory placement.  If  the  task  was  using  mempolicy
       MPOL_BIND,  and  the  nodes  to which it was bound overlap with its new
       cpuset, then the task will continue to use whatever subset of MPOL_BIND
       nodes  are  still  allowed  in  the  new cpuset.  If the task was using
       MPOL_BIND and now none of its MPOL_BIND nodes are allowed  in  the  new
       cpuset,  then  the  task  will  be  essentially  treated  as  if it was
       MPOL_BIND bound to the new cpuset (even though its NUMA  placement,  as
       queried  by  get_mempolicy(), doesn’t change).  If a task is moved from
       one cpuset to another, then the kernel will  adjust  the  tasks  memory
       placement, as above, the next time that the kernel attempts to allocate
       a page of memory for that task.

       If a cpuset has its CPUs modified, each task  using  that  cpuset  does
       _not_  change  its  behavior  automatically.   In order to minimize the
       impact on the critical  scheduling  code  in  the  kernel,  tasks  will
       continue  to  use  their  prior CPU placement until they are rebound to
       their cpuset, by rewriting their PID  to  the  ’tasks’  file  of  their
       cpuset.   If  a  task had been bound to some subset of its cpuset using
       the sched_setaffinity() call, and  if  any  of  that  subset  is  still
       allowed in its new cpuset settings, then the task will be restricted to
       the intersection of the CPUs it was allowed  on  before,  and  its  new
       cpuset  CPU  placement.   If,  on  the  other hand, there is no overlap
       between a tasks prior placement and its new cpuset CPU placement,  then
       the  task  will be allowed to run on any CPU allowed in its new cpuset.
       If a task is moved from one cpuset to another,  its  CPU  placement  is
       updated in the same way as if the tasks PID is rewritten to the ’tasks’
       file of its current cpuset.

       In summary, the memory placement of a task whose cpuset is  changed  is
       updated  by the kernel, on the next allocation of a page for that task,
       but the processor placement is not updated, until  that  tasks  PID  is
       rewritten  to  the  ’tasks’  file of its cpuset.  This is done to avoid
       impacting the scheduler code in the kernel with a check for changes  in
       a tasks processor placement.

   Rename limitations
       You  can  use the rename(2) system call to rename cpusets.  Only simple
       renaming is supported, changing the name of a  cpuset  directory  while
       keeping its same parent.

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.

EXAMPLES

       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  setup  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/Charliecat /proc/self/cpuset

   Migrating a job to different memory nodes.
       To migrate a job (the set of tasks 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) Lets 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 task 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 tasks in alpha to beta, and any  memory  held
       by these tasks 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 task  PID  at  a  time  may  be
       written to the tasks file.

       The  same  affect  (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 sed -u
       [unbuffered] option:

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

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.

       ENOMEM Insufficient memory is available.

       EBUSY  Attempted to remove a cpuset with attached tasks.

       EBUSY  Attempted to remove a cpuset with child cpusets.

       ENOENT Attempted to create a cpuset in a  parent  cpuset  that  doesn’t
              exist.

       ENOENT Attempted to access a non-existent file in a cpuset directory.

       EEXIST Attempted to create a cpuset that already exists.

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

       ENOTDIR
              Attempted to rename(2) a non-existent cpuset.

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

       ESRCH  Attempted  to  write the process ID (PID) of a non-existent task
              to a cpuset tasks file.

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

       EACCESS
              Attempted to write(2) a memory_pressure file.

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

       EINVAL Attempted  to  change  a  cpuset  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 the kernel.
              The kernel creates new cpusets (via mkdir(2))  with  empty  cpus
              and  mems.   But  the  kernel will not allow an empty list to be
              written to the special cpus or mems files of a cpuset.

       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  outside  of  its  current
              directory.

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

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

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

       EACCES Attempted to add a CPU or memory node to a cpuset  that  is  not
              already in its parent.

       EACCES Attempted  to  set  cpu_exclusive  or  mem_exclusive on a cpuset
              whose parent lacks the same setting.

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

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

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

       ENAMETOOLONG
              Attempted to create a cpuset whose base directory name is longer
              than 255 characters.

       ENAMETOOLONG
              Attempted to create a cpuset whose full pathname  including  the
              "/dev/cpuset/" prefix is longer than 4095 characters.

       EINVAL Specified  a  cpus  or  mems list to the kernel which included a
              range with the second number smaller than the first number.

       EINVAL Specified a cpus or mems list to the kernel  which  included  an
              invalid character in the string.

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

SEE ALSO

       cat(1),  echo(1),  ls(1),  mkdir(1),  rmdir(1),   sed(1),   taskset(1),
       close(2),   get_mempolicy(2),   mbind(2),  mkdir(2),  open(2),  read(2)
       rmdir(2), sched_getaffinity(2), sched_setaffinity(2), set_mempolicy(2),
       sched_setscheduler(2),  taskset(2),  write(2),  libbitmask(3), proc(5),
       migratepages(8), numactl(8).

HISTORY

       Cpusets appeared in version 2.6.13 of the Linux kernel.

BUGS

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

AUTHOR

       This man page was written by Paul Jackson.