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

       cpuset - confine processes to processor and memory node subsets

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

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

       A cpuset defines a list of CPUs and memory nodes.

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

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

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

       When the system begins booting, a single cpuset is defined that includes all CPUs and memory nodes on the  sys-
       tem,  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 administra-
       tor, and processes may be placed in these other cpusets.

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

       Typically, a cpuset is used to manage the CPU and memory-node confinement for a set  of  cooperating  processes
       such  as  a batch scheduler job, and these other mechanisms are used to manage the placement of individual pro-
       cesses 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 contained it) by writing its PID to that cpuset's tasks file
              (with or without a trailing newline.)

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

              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.

              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

              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  non-empty  cpus,
              then their cpus must overlap, because the cpus of any cpuset are always a subset of the cpus of its par-
              ent 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.

              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 that  cpuset's  parent

       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

       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 Mem-
              ory Migration section, below.

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

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

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

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

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

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

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

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

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

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

       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  descen-
       dant, may share any of the same CPUs or memory nodes.

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

       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 hard-
       wall cpusets.

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

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

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

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

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

           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

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

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

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

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

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

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

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

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

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

       The kernel direct reclaim code is entered whenever a process has to satisfy a  memory  page  request  by  first
       finding some other page to repurpose, due to lack of any readily available already free pages.  Dirty file sys-
       tem 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 re-read 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 sec-
       ond, times 1000.

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

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

       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 affect 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 affect, even if it is.
       If cpuset memory spreading is subsequently turned off, the NUMA memory policy most recently specified by  these
       calls is automatically re-applied.

       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, espe-
       cially for jobs that might have just a single thread initializing or reading in the data set.

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

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

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

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

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

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

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

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

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

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

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

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

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

       So,  for  example, if the top cpuset has the flag sched_load_balance enabled, then the scheduler will load bal-
       ance across all CPUs, and the setting of the sched_load_balance flag in other cpusets has no affect,  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  non-
       trivial  amounts of CPU, as such processes may be artificially constrained to some subset of CPUs, depending on
       the particulars of this flag setting in descendant cpusets.  Even if such a process could use spare CPU  cycles
       in  some  other CPUs, the kernel scheduler might not consider the possibility of load balancing that process to
       the underused CPU.

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

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

       On  small  systems,  such  as  those with just a few CPUs, immediate load balancing is useful to improve system
       interactivity and to minimize wasteful idle CPU cycles.   But  on  large  systems,  attempting  immediate  load
       balancing  across  a large number of CPUs can be more costly than it is worth, depending on the particular per-
       formance 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  implementa-
       tion  details  of the kernel scheduler code and on the non-uniform architecture of the hardware.  Both of these
       will evolve over time and vary by system architecture and kernel version.

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

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

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

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

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

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

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

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

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

       Examples of the Mask Format:

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

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


       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  num-
       bers, 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 parent's.

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

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

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

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

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

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

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

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

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

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

   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 mem-

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

           echo 19 > mems

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

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

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

       If hot-plug functionality is used to remove all the CPUs that are currently assigned to a cpuset, then the ker-
       nel 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 reconfig-
       ure 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  con-
       text, 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  permit-

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

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

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

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

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

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

       EACCES Attempted to write(2) a memory_pressure file.

       EACCES Attempted to create a file in a cpuset directory.

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

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

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

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

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

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

       EINVAL Attempted  to change a cpuset, using write(2), in a way that would violate a cpu_exclusive or mem_exclu-
              sive 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

       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.

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

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

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

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

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

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

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

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

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

              Attempted to rename(2) a nonexistent cpuset.

       EPERM  Attempted to remove a file from a cpuset directory.

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

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

       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 pid.

       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 affect.

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

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

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

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

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

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

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

       The following sequence of commands accomplishes this.

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

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

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

           $ cp ../alpha/tasks tasks

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

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

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

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

       The kernel source file Documentation/cpusets.txt.

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2008-11-12                         CPUSET(7)