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


     cpuset - confine processes to processor and memory node subsets


     The  cpuset  filesystem  is a pseudo-filesystem 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 filesystem (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 filesystem is not even mounted, then the
     cpuset mechanism, though present, has no effect on the system's  behav-
     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-
     filesystem, where the top directory in the hierarchy (/dev/cpuset) rep-
     resents  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 filesystem permissions.
     Every process in the system belongs to exactly one cpuset.   A  process
     is confined to run only 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 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
     process'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.


     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.
            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.
            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.
            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.
            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.
            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.
     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).
     cpuset.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,
     cpuset.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.
     cpuset.memory_pressure_enabled (since Linux 2.6.16)
            Flag  (0  or  1).  This file is present only 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.
     cpuset.memory_spread_page (since Linux 2.6.17)
            Flag (0 or 1).  If set (1),  pages  in  the  kernel  page  cache
            (filesystem 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.
     cpuset.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 defaultBy default, is off (0) in the top cpuset, and
            inherited  from the parent cpuset in newly created cpusets.  See
            the Memory Spread section, below.
     cpuset.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.
     cpuset.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 filesystem.
     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.


     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.
     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  filesystem  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 filesystem) 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  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/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,
     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  accumu-
        late 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 filesystem pages are repurposed by first writing  them  to  disk.
     Unmodified  filesystem  buffer  pages are repurposed by simply dropping
     them, though if that page is needed again, it will have  to  be  reread
     from disk.
     The cpuset.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 filesystem buffers and related in-kernel
     data  structures.   They  are  called   cpuset.memory_spread_page   and
     If  the  per-cpuset Boolean flag file cpuset.memory_spread_page is set,
     then the kernel will spread the filesystem 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  run-
     If  the  per-cpuset Boolean flag file cpuset.memory_spread_slab is set,
     then the kernel will spread some filesystem-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  cpuset.memory_spread_page   and   cpuset.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  filesystem 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   cpuset.mem-
     ory_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  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-
 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
     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 applies  only  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 be scheduled
     to  run  only  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
     (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  done  only
     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.


     The  following  formats  are  used to represent sets of CPUs and memory
 Mask format
     The Mask Format is used to represent CPU and memory-node bit  masks  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 bit mask, based on the size of the bit mask.
     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:
     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


     The following rules apply to each cpuset:
  • Its CPUs and memory nodes must be a (possibly equal) subset of its


  • It can be marked cpu_exclusive only if its parent is.
  • It can be marked mem_exclusive only 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-



     The  permissions  of  a cpuset are determined by the permissions of the
     directories and pseudo-files in the cpuset filesystem, 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
     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 filesystem 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 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 filesystem
     In theory, this means that user code should specify cpusets using abso-
     lute  pathnames,  which  requires knowing the mount point of the cpuset
     filesystem (usually, but not necessarily, /dev/cpuset).   In  practice,
     all user level code that this author is aware of simply assumes that if
     the cpuset filesystem 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-filesystem is currently mounted.


 Enabling memory_pressure
     By  default, the per-cpuset file cpuset.memory_pressure always contains
     zero (0).  Unless this feature is enabled by writing "1" to the pseudo-
     file  /dev/cpuset/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 > cpuset.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 > cpuset.mems /bin/echo: write error: Invalid argument


 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
     refer  only  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
 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


     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
     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-
     EACCES Attempted  to  set,  using  write(2),  cpuset.cpu_exclusive   or
            cpuset.mem_exclusive  on  a  cpuset  whose parent lacks the same
     EACCES Attempted to write(2) a cpuset.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-
     EBUSY  Attempted  to  remove,  using  rmdir(2),  a  cpuset  with  child
     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
     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 cpuset.cpus or  cpuset.mems  list
            to a cpuset which has attached processes or child cpusets.
     EINVAL Attempted  to  write(2)  a cpuset.cpus or cpuset.mems list which
            included a range with the second number smaller than  the  first
     EINVAL Attempted  to  write(2)  a cpuset.cpus or cpuset.mems list which
            included an invalid character in the string.
     EINVAL Attempted to write(2) a list to a cpuset.cpus file that did  not
            include any online CPUs.
     EINVAL Attempted  to write(2) a list to a cpuset.mems file that did not
            include any online memory nodes.
     EINVAL Attempted to write(2) a list to a cpuset.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.
            Attempted to read(2) a /proc/<pid>/cpuset file for a cpuset path
            that is longer than the kernel page size.
            Attempted  to create, using mkdir(2), a cpuset whose base direc-
            tory name is longer than 255 characters.
            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
     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
     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 cpuset.cpus or
            empty cpuset.mems setting.
     ENOSPC Attempted to write(2) an empty cpuset.cpus or  cpuset.mems  set-
            ting to a cpuset that has tasks attached.
            Attempted to rename(2) a nonexistent cpuset.
     EPERM  Attempted to remove a file from a cpuset directory.
     ERANGE Specified  a cpuset.cpus or cpuset.mems list to the kernel which
            included a number too large for the kernel to  set  in  its  bit
     ESRCH  Attempted  to  write(2)  the  process  ID (PID) of a nonexistent
            process to a cpuset tasks file.


     Cpusets appeared in version 2.6.12 of the Linux kernel.


     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


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


     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 >
         cpuset.cpus $ /bin/echo 1 > cpuset.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
     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  >
         cpuset.cpus   $  /bin/echo  8-9  >  cpuset.mems  $  /bin/echo  1  >
         cpuset.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,
     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


     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), cgroups(7),  numa(7),  sched(7),  migratepages(8),
     Documentation/cgroup-v1/cpusets.txt in the Linux kernel source tree (or
     Documentation/cpusets.txt before Linux 2.6.29)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2017-09-15 CPUSET(7)

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