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

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  behav-
       ior.

       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)  repre-
       sents  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  pro-
       cess  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 affin-
       ity 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 over-
       riding 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  pro-
       cess's cpuset.

       Typically,  a cpuset is used to manage the CPU and memory-node confine-
       ment for a set of cooperating processes such as a batch scheduler  job,
       and these other mechanisms are used to manage the placement of individ-
       ual 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  con-
              tained  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  calcula-
              tions  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 automat-
              ically  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  dis-
              abled,  then  the  setting  of sched_relax_domain_level does not
              matter, as no such load balancing is done.   If  sched_load_bal-
              ance   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  sched-
       uled) 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  con-
       figuring  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 indi-
       vidual  job.   Only  a  small amount of kernel memory, such as requests
       from interrupt handlers, is allowed to be placed on memory  nodes  out-
       side 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  aban-
       doned 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  dedi-
       cated  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  rep-
       resented 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 vir-
          tual 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 accu-
          mulate 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 ker-
       nel allocates pages for the file-system buffers and  related  in-kernel
       data   structures.    They   are  called  memory_spread_page  and  mem-
       ory_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  pre-
       ferring 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 pro-
       cess'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 subse-
       quently 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  pre-
       served  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 pos-
       sible.

   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 bal-
       ance 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 Sched-
       uler 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  any-
       where 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 bal-
       ancing is attempted over all  CPUs  (unless  disabled  by  turning  off
       sched_load_balance.)  In any case, of course, tasks will only be sched-
       uled  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 (implementa-
           tion 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 per-
       form 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 archi-
       tecture 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 char-
       acters "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 sib-
          ling.

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 par-
       ent  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 permis-
       sions  are  evaluated and the manner in which normal file-system opera-
       tion permissions are evaluated.  The kernel interprets  relative  path-
       names  starting  at a process's current working directory.  Even if one
       is operating on a cpuset file, relative pathnames are interpreted rela-
       tive  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 cur-
       rent 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 abso-
       lute 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 sys-
       tem  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 cur-
       rently 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 pro-
              cess.

       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 par-
              ent.

       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 pro-
              cesses.

       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 direc-
              tory name is longer than 255 characters.

       ENAMETOOLONG
              Attempted to create, using mkdir(2), a cpuset whose  full  path-
              name,  including the mount point (typically "/dev/cpuset/") pre-
              fix, 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 pro-
              cess 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 differ-
       ent  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)  com-
       mand, was necessary because only one process PID at a time may be writ-
       ten 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_getaffin-
       ity(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 3.25 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/.

Linux                             2008-11-12                         CPUSET(7)
 

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