cpuset(7) — Linux manual page


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

NAME         top

       cpuset - confine processes to processor and memory node subsets

DESCRIPTION         top

       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

       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 behavior.

       A cpuset defines a list of CPUs and memory nodes.

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

       Cpusets are represented as directories in a hierarchical pseudo-
       filesystem, 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 filesystem

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

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

FILES         top

       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

       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.

              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, below.

       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,

       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
              calculations are enabled for all cpusets in the system.  By
              default, this is off (0).  See the Memory Pressure section,

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

       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 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 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
       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 descendant, may share any of
       the same CPUs or memory nodes.

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

       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:

           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 addi‐
       tional 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 sci‐
       entific computing jobs that will dramatically fail to meet required
       performance goals if they start to use more memory than allowed to

       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, 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 sched‐
          uler monitoring this metric is sharply reduced on large systems,
          because a scan of the tasklist can be avoided on each set of

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

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

       The memory_pressure of a cpuset is calculated using a per-cpuset sim‐
       ple 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 represent‐
       ing the recent (half-life of 10 seconds) rate of entries to the
       direct reclaim code caused by any process in the cpuset, in units of
       reclaims attempted per second, times 1000.

   Memory spread
       There are two Boolean flag files per cpuset that control where the
       kernel allocates pages for the filesystem buffers and related in-ker‐
       nel 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 running.

       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 speci‐
       fied 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 possible.

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

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

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

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

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

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

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

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

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

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

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

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

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

   Scheduler relax domain level
       The kernel scheduler performs immediate load balancing whenever a CPU
       becomes free or another task becomes runnable.  This load balancing
       works to ensure that as many CPUs as possible are usefully employed
       running tasks.  The kernel also performs periodic load balancing off
       the software clock described in time(7).  The setting of
       sched_relax_domain_level applies only to immediate load balancing.
       Regardless of the sched_relax_domain_level setting, periodic load
       balancing is attempted over all CPUs (unless disabled by turning off
       sched_load_balance.)  In any case, of course, tasks will 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 architec‐
       ture 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
       perform 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
       architecture and kernel version.  This system default value can be
       changed by kernel boot-time "relax_domain_level=" argument.

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

FORMATS         top

       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
       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 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

RULES         top

       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

PERMISSIONS         top

       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
       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 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 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 filesystem path.

       In theory, this means that user code should specify cpusets using
       absolute 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.

WARNINGS         top

   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
       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 > cpuset.mems
           /bin/echo: write error: Invalid argument

EXCEPTIONS         top

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

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

ERRORS         top

       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

       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

       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

       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
              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 cpuset.cpus or
              empty cpuset.mems setting.

       ENOSPC Attempted to write(2) an empty cpuset.cpus or cpuset.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 cpuset.cpus or cpuset.mems list to the kernel
              which included a number too large for the kernel to set in its
              bit masks.

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

VERSIONS         top

       Cpusets appeared in version 2.6.12 of the Linux kernel.

NOTES         top

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

BUGS         top

       cpuset.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

EXAMPLES         top

       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 dif‐
       ferent CPUs and memory nodes in the system, including moving the mem‐
       ory pages currently allocated to that job, perform the following

       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 > 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)
       command, was necessary because only one process PID at a time may be
       written to the tasks file.

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

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

SEE ALSO         top

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

       Documentation/admin-guide/cgroup-v1/cpusets.rst in the Linux kernel
       source tree (or Documentation/cgroup-v1/cpusets.txt before Linux
       4.18, and Documentation/cpusets.txt before Linux 2.6.29)

COLOPHON         top

       This page is part of release 5.08 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                            2020-06-09                        CPUSET(7)

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