cpuset(7) — Linux manual page


cpuset(7)           Miscellaneous Information 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 /dev/cpuset.

       On systems with kernels compiled with built in support for
       cpusets, all processes are attached to a cpuset, and cpusets are
       always present.  If a system supports cpusets, then it will have
       the entry nodev cpuset in the file /proc/filesystems.  By
       mounting the cpuset filesystem (see the EXAMPLES 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 permissions.

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

       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 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

       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.

              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.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, below.

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

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

       cpuset.memory_spread_slab (since Linux 2.6.17)
              Flag (0 or 1).  If set (1), the kernel slab caches for
              file I/O (directory and inode structures) are uniformly
              spread across the cpuset.  By 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 details.

       cpuset.sched_relax_domain_level (since Linux 2.6.26)
              Integer, between -1 and a small positive value.  The
              sched_relax_domain_level controls the width of the range
              of CPUs over which the kernel scheduler performs immediate
              rebalancing of runnable tasks across CPUs.  If
              sched_load_balance is 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

   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 cpuset.

       A cpuset that has mem_exclusive or mem_hardwall set is a hardwall
       cpuset.  A hardwall cpuset restricts kernel allocations for page,
       buffer, and other data commonly shared by the kernel across
       multiple users.  All cpusets, whether hardwall or not, restrict
       allocations of memory for user space.

       This enables configuring a system so that several independent
       jobs can share common kernel data, such as filesystem pages,
       while isolating each job's user allocation in its own cpuset.  To
       do this, construct a large hardwall cpuset to hold all the jobs,
       and construct child cpusets for each individual job which are not
       hardwall cpusets.

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

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

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

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

       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 requests.

       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 terminate or reprioritize
       jobs that are trying to use more memory than allowed on the nodes
       assigned them, and with tightly coupled, long-running, massively
       parallel scientific computing jobs that will dramatically fail to
       meet required performance goals if they start to use more memory
       than allowed to them.

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

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

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

       •  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 filesystem pages are repurposed by first writing
       them to disk.  Unmodified filesystem buffer pages are repurposed
       by simply dropping them, though if that page is needed again, it
       will have to be reread from disk.

       The cpuset.memory_pressure file provides an integer number
       representing the recent (half-life of 10 seconds) rate of entries
       to the direct reclaim code caused by any process in the cpuset,
       in units of reclaims attempted per second, times 1000.

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

       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 specified by these calls is
       automatically reapplied.

       Both cpuset.memory_spread_page and cpuset.memory_spread_slab are
       Boolean flag files.  By default, they contain "0", meaning that
       the feature is off for that cpuset.  If a "1" is written to that
       file, that turns the named feature on.

       Cpuset-specified memory spreading behaves similarly to what is
       known (in other contexts) as round-robin or interleave memory

       Cpuset-specified memory spreading can provide substantial
       performance improvements for jobs that:

       •  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

       •  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.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 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 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

       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

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

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

       Of course, processes pinned to a particular CPU can be left in a
       cpuset that disables sched_load_balance as those processes aren't
       going 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
       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
       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="

       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, -1 is the lowest value, overridden by any other
       value, and 0 is the next lowest value.

FORMATS         top

       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 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

       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

       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 parent's.

       •  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 mem_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 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
       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 cpusets.

   Renaming cpusets
       You can use the rename(2) system call to rename cpusets.  Only
       simple renaming is supported; that is, changing the name of a
       cpuset directory is permitted, but moving a directory into a
       different directory is not permitted.

ERRORS         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 parent.

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

       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 processes.

       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

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

       EINVAL Attempted to change a cpuset, using write(2), in a way
              that would violate a cpu_exclusive or mem_exclusive
              attribute of that cpuset or any of its siblings.

       EINVAL Attempted to write(2) an empty cpuset.cpus or cpuset.mems
              list to a cpuset which has attached processes or child

       EINVAL Attempted to write(2) a cpuset.cpus or cpuset.mems list
              which included a range with the second number smaller than
              the first number.

       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

              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 Linux 2.6.12.

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 effect.

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
       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 > 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, 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 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 the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-06-15                      cpuset(7)

Pages that refer to this page: getcpu(2)mbind(2)migrate_pages(2)move_pages(2)sched_setaffinity(2)sched_setattr(2)sched_setscheduler(2)set_mempolicy(2)CPU_SET(3)pthread_attr_setaffinity_np(3)pthread_setaffinity_np(3)proc_pid_cpuset(5)proc_pid_oom_score_adj(5)proc_pid_status(5)proc_sys_vm(5)cgroups(7)numa(7)sched(7)