cpuset

CPUSET(4)                  Linux Programmer's Manual                 CPUSET(4)



NAME
       cpuset - confine tasks to processor and memory node subsets

DESCRIPTION
       The cpuset file system is a pseudo-filesystem interface to the kernel
       cpuset mechanism for controlling the processor and memory placement of
       tasks.  It is commonly mounted at /dev/cpuset.

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

       Every task in the system belongs to exactly one cpuset.  A task is
       confined to only run on the CPUs in the cpuset it belongs to, and to
       allocate memory only on the memory nodes in that cpuset.  When a task
       forks, the child task is placed in the same cpuset as its parent.  With
       sufficient privilege, a task 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, only the top cpuset is defined and all
       tasks are in that cpuset.  During the boot process  or later during
       normal system operation, other cpusets may be created, as sub-
       directories of the top cpuset under the control of the system
       administrator and tasks 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
       task make use of a CPU or memory node that is not allowed by cpusets.
       If changes to a tasks cpuset placement conflict with these other
       mechanisms, then cpuset placement is enforced even if it means
       overriding these other mechanisms.

       Typically, a cpuset is used to manage the CPU and memory node
       confinement for the entire set of tasks in a job, and these other
       mechanisms are used to manage the placement of individual tasks or
       memory regions within a job.

FILES
       Each directory below /dev/cpuset represents a cpuset and contains
       several files describing the state of that cpuset.

       New cpusets are created using the mkdir system call or shell command.
       The properties of a cpuset, such as its flags, allowed CPUs and memory
       nodes, and attached tasks, are queried and modified by reading or
       writing to the appropriate file in that cpusets directory, as listed
       below.

       The files in each cpuset directory are automatically created when the
       cpuset is created, as a result of the mkdir invocation.  It is not
       allowed to add or remove files from a cpuset directory.

       The 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 using ordinary file access routines from programmatic
       languages, such as open(2), read(2), write(2) and close(2) from the 'C'
       library.  These files 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 tasks in that cpuset.  The
              list is formatted as a series of ASCII decimal numbers, each
              followed by a newline.  A task may be added to a cpuset
              (removing it from the cpuset previously containing it) by
              writing its PID to that cpusets tasks file (with or without a
              trailing newline.)

              Beware that 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 considered.

       notify_on_release
              Flag (0 or 1).  If set (1), that cpuset will receive special
              handling whenever its last using task and last child cpuset goes
              away.  See the Notify On Release section, below.

       cpus
              List of CPUs on which tasks 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.  Note however, such a change does not
              take affect until the PIDs of the tasks in the cpuset are
              rewritten to the cpusets tasks file.  See the WARNINGS section,
              below.

       cpu_exclusive
              Flag (0 or 1).  If set (1), the cpuset has exclusive use of its
              CPUs (no sibling or cousin cpuset may overlap CPUs).  By default
              this is off (0).  Newly created cpusets also initially default
              this to off (0).

       mems
              List of memory nodes on which tasks in that cpuset are allowed
              to allocate memory.  See List Format below for a description of
              the format of mems.

       mem_exclusive
              Flag (0 or 1).  If set (1), the cpuset has exclusive use of its
              memory nodes (no sibling or cousin may overlap).  By default
              this is off (0).  Newly created cpusets also initially default
              this to off (0).

       memory_migrate
              Flag (0 or 1).  If set (1), then memory migration is enabled.
              See the Memory Migration section, below.

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

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

       memory_spread_page
              Flag (0 or 1).  If set (1), the kernel page cache (file system
              buffers) are uniformly spread across the cpuset.  See the Memory
              Spread section, below.

       memory_spread_slab
              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.  See the Memory Spread section, below.

       In addition to the above special files in each directory below
       /dev/cpuset, each task under /proc has an added file named cpuset,
       displaying the cpuset name, as the path relative to the root of the
       cpuset file system.

       Also the /proc/<pid>/status file for each task has two added lines,
       displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
       and mems_allowed (on which memory nodes it may obtain memory), in the
       Mask Format (see below) as shown in the following example:

                      Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
                      Mems_allowed:   ffffffff,ffffffff

EXTENDED CAPABILITIES
       In addition to controlling which cpus and mems a task 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 cpu_exclusive has a scheduler (sched) domain
       associated with it.  The sched domain consists of all CPUs in the
       current cpuset that are not part of any exclusive child cpusets.  This
       ensures that the scheduler load balancing code only balances against
       the CPUs that are in the sched domain as defined above and not all of
       the CPUs in the system. This removes any overhead due to load balancing
       code trying to pull tasks outside of the cpu_exclusive cpuset only to
       be prevented by the tasks' cpus_allowed mask.

       A cpuset that is mem_exclusive restricts kernel allocations for page,
       buffer and other data 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, such as file
       system pages, while isolating each jobs 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 typical kernel memory, such as
       requests from interrupt handlers, is allowed to be taken outside even a
       mem_exclusive cpuset.

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

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

       The command /sbin/cpuset_release_agent is invoked, with the name
       (/dev/cpuset relative path) of that cpuset in argv[1].  This supports
       automatic cleanup of abandoned cpusets.

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

                      #!/bin/sh
                      rmdir /dev/cpuset/$1

       By default, notify_on_release is off (0).  Newly created cpusets
       inherit their notify_on_release setting from their parent cpuset.

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

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

       Unless memory pressure calculation is enabled by setting the special
       file /dev/cpuset/memory_pressure_enabled, it is not computed for any
       cpuset, and always reads a value of zero.  See the WARNINGS section,
       below.

       Why a per-cpuset, running average:
          Because this meter is per-cpuset rather than per-task or mm, 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-task or mm, 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 tasks
          in the cpuset.

       A per-cpuset simple digital filter is kept within the kernel, and
       updated by any task attached to that cpuset, if it enters the
       synchronous (direct) page reclaim code.

       A per-cpuset file provides an integer number representing the recent
       (half-life of 10 seconds) rate of direct page reclaims caused by the
       tasks 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 file system buffers and related in
       kernel data structures.  They are called memory_spread_page and
       memory_spread_slab.

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

       If the per-cpuset Boolean flag file memory_spread_slab is set, then the
       kernel will spread some file system related slab caches, such as for
       inodes and directory entries evenly over all the nodes that the
       faulting task is allowed to use, instead of preferring to put those
       pages on the node where the task is running.

       The setting of these flags does not affect anonymous data segment or
       stack segment pages of a task.

       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 task is running.  If that node is not allowed by the tasks
       NUMA mempolicy 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 does have 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 tasks NUMA mempolicy and be spread instead.
       Tasks using mbind() or set_mempolicy() calls to set NUMA mempolicies
       will not notice any change in these calls as a result of their
       containing tasks memory spread settings.  If memory spreading is turned
       off, the currently specified NUMA mempolicy once again applies to
       memory page allocations.

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

       This memory placement policy is also known (in other contexts) as
       round-robin or interleave.

       This policy can provide substantial improvements for jobs that need to
       place thread local data on the corresponding node, but that need to
       access large file system data sets that need to be spread across the
       several nodes in the jobs cpuset in order to fit.  Without this policy,
       especially for jobs that might have one thread reading in the data set,
       the memory allocation across the nodes in the jobs cpuset can become
       very uneven.

   Memory Migration
       Normally, under the default setting (disabled) of memory_migrate, once
       a page is allocated (given a physical page of main memory) then that
       page stays on whatever node it was allocated, so long as it remains
       allocated, even if the cpusets 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 task in the
       cpuset that is on a memory node no longer allowed will be migrated to a
       memory node that is allowed.

       Also if a task 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.

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

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

       It is hexadecimal, using ASCII characters "0" - "9" and "a" - "f". This
       format displays each 32-bit word in hex (zero filled) and for masks
       longer than one word uses a comma separator between words. Words are
       displayed in big-endian order most significant first. And hex digits
       within a word are also in big-endian order.

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

       Examples of the Mask Format:

                      00000001                        # just bit 0 set
                      80000000,00000000,00000000      # just bit 95 set
                      00000001,00000000,00000000      # just bit 64 set
                      000000ff,00000000               # bits 32-39 set
                      00000000,000E3862               # 1,5,6,11-13,17-19 set

       A mask with bits 0, 1, 2, 4, 8, 16, 32 and 64 set displays as
       "00000001,00000001,00010117".  The first "1" is for bit 64, the second
       for bit 32, the third for bit 16, the fourth for bit 8, the fifth for
       bit 4, and the "7" is for bits 2, 1 and 0.

   List Format
       The List Format for cpus and mems is a comma separated list of CPU or
       memory node numbers and ranges of numbers, in ASCII decimal.

       Examples of the List Format:

                      0-4,9           # bits 0, 1, 2, 3, 4 and 9 set
                      0-2,7,12-14     # bits 0, 1, 2, 7, 12, 13 and 14 set

RULES
       The following rules apply to each cpuset:

       * Its CPUs and memory nodes must be a (possibly equal) subset of its
       parents.

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

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

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

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

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

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

       An additional constraint is applied to requests to place some other
       task in a cpuset.  One task may not attach another to a cpuset unless
       it would have permission to send that task a signal.

       A task 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 cpusets directory (execute permissions on the
       encompassing directories) and write the corresponding cpus or mems
       file.

       Note however that since changes to the CPUs of a cpuset don't apply to
       any task in that cpuset until said task is reattached to that cpuset,
       it would normally not be a good idea to arrange the permissions on a
       cpuset so that some task could write the cpus file unless it could also
       write the tasks file to reattach the tasks therein.

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

WARNINGS
   Updating a cpusets cpus
       Changes to a cpusets cpus file do not take affect for any task in that
       cpuset until that tasks process ID (PID) is rewritten to the cpusets
       tasks file.  This unusual requirement is needed to optimize a critical
       code path in the Linux kernel.  Beware that only one PID can be written
       at a time to a cpusets tasks file.  Additional PIDs on a single
       write(2) system call are ignored.  One (unobvious) way to satisfy this
       requirement to rewrite the tasks file after updating the cpus file is
       to use the -u unbuffered option to the sed(1) command, as in the
       following scenario:
              cd /dev/cpuset/foo              # /foo is an existing cpuset
              /bin/echo 3 > cpus              # change /foo's cpus
              sed -un p < tasks > tasks       # rewrite /foo's tasks file

       If one examines the Cpus_allowed value in the /proc/<pid>/status file
       for one of the tasks in cpuset /foo in the above scenario, one will
       notice that the value does not change when the cpus file is written
       (the echo command), but only later, after the tasks file is rewritten
       (the sed command).

   Enabling memory_pressure
       By default, the per-cpuset file memory_pressure always contains zero
       (0).  Unless this feature is enabled by writing "1" to the special file
       /dev/cpuset/memory_pressure_enabled, the kernel does not compute per-
       cpuset memory_pressure.

   Using the echo command
       When using the echo command at the shell prompt to change the values of
       cpuset files, beware that most shell built-in echo commands to not
       display an error message if the write(2) system call fails.  For
       example, if the command:
              echo 19 > mems
       failed because memory node 19 was not allowed (perhaps the current
       system does not have a memory node 19), then the above echo command
       would not display any error.  It is better to use the /bin/echo
       external command to change cpuset file settings, as this command will
       display write(2) errors, as in the example:
              /bin/echo 19 > mems
              /bin/echo: write error: No space left on device

EXCEPTIONS
       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 tasks 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, over
       starving a task that has had all its allowed CPUs or memory nodes taken
       offline.  User code should reconfigure cpusets to only refer to online
       CPUs and memory nodes when using hot-plug to add or remove such
       resources.

       A few kernel critical internal memory allocation requests, marked
       GFP_ATOMIC, must be satisfied, immediately.  The kernel may drop some
       request or malfunction if one of these allocations fail.  If such a
       request cannot be satisfied within the current tasks 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 task context, and are not confined by
       cpusets.

LIMITATIONS
   Kernel limitations updating cpusets
       In order to minimize the impact of cpusets on critical kernel code,
       such as the scheduler, and due to the fact that the kernel does not
       support one task updating the memory placement of another task
       directly, the impact on a task of changing its cpuset CPU or memory
       node placement, or of changing to which cpuset a task is attached, is
       subtle.

       If a cpuset has its memory nodes modified, then for each task attached
       to that cpuset, the next time that the kernel attempts to allocate a
       page of memory for that task, the kernel will notice the change in the
       tasks cpuset, and update its per-task memory placement to remain within
       the new cpusets memory placement.  If the task was using mempolicy
       MPOL_BIND, and the nodes to which it was bound overlap with its new
       cpuset, then the task will continue to use whatever subset of MPOL_BIND
       nodes are still allowed in the new cpuset.  If the task was using
       MPOL_BIND and now none of its MPOL_BIND nodes are allowed in the new
       cpuset, then the task will be essentially treated as if it was
       MPOL_BIND bound to the new cpuset (even though its NUMA placement, as
       queried by get_mempolicy(), doesn't change).  If a task is moved from
       one cpuset to another, then the kernel will adjust the tasks memory
       placement, as above, the next time that the kernel attempts to allocate
       a page of memory for that task.

       If a cpuset has its CPUs modified, each task using that cpuset does
       _not_ change its behavior automatically.  In order to minimize the
       impact on the critical scheduling code in the kernel, tasks will
       continue to use their prior CPU placement until they are rebound to
       their cpuset, by rewriting their PID to the 'tasks' file of their
       cpuset.  If a task had been bound to some subset of its cpuset using
       the sched_setaffinity() call, and if any of that subset is still
       allowed in its new cpuset settings, then the task will be restricted to
       the intersection of the CPUs it was allowed on before, and its new
       cpuset CPU placement.  If, on the other hand, there is no overlap
       between a tasks prior placement and its new cpuset CPU placement, then
       the task will be allowed to run on any CPU allowed in its new cpuset.
       If a task is moved from one cpuset to another, its CPU placement is
       updated in the same way as if the tasks PID is rewritten to the 'tasks'
       file of its current cpuset.

       In summary, the memory placement of a task whose cpuset is changed is
       updated by the kernel, on the next allocation of a page for that task,
       but the processor placement is not updated, until that tasks PID is
       rewritten to the 'tasks' file of its cpuset.  This is done to avoid
       impacting the scheduler code in the kernel with a check for changes in
       a tasks processor placement.

   Rename limitations
       You can use the rename(2) system call to rename cpusets.  Only simple
       renaming is supported, changing the name of a cpuset directory while
       keeping its same parent.

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

EXAMPLES
       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 setup a cpuset
       named "Charlie", containing just CPUs 2 and 3, and memory node 1, and
       then attach the current shell to that cpuset.

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

   Migrating a job to different memory nodes.
       To migrate a job (the set of tasks 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) Lets 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 task from alpha to beta.

       The following sequence of commands accomplishes this.

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

       The above should move any tasks in alpha to beta, and any memory held
       by these tasks 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 task PID at a time may be
       written to the tasks file.

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

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


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

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

       ENOMEM Insufficient memory is available.

       EBUSY  Attempted to remove a cpuset with attached tasks.

       EBUSY  Attempted to remove a cpuset with child cpusets.

       ENOENT Attempted to create a cpuset in a parent cpuset that doesn't
              exist.

       ENOENT Attempted to access a non-existent file in a cpuset directory.

       EEXIST Attempted to create a cpuset that already exists.

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

       ENOTDIR
              Attempted to rename(2) a non-existent cpuset.

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

       ESRCH  Attempted to write the process ID (PID) of a non-existent task
              to a cpuset tasks file.

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

       EACCESS
              Attempted to write(2) a memory_pressure file.

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

       EINVAL Attempted to change a cpuset in a way that would violate a
              cpu_exclusive or mem_exclusive attribute of that cpuset or any
              of its siblings.

       EINVAL Attempted to write(2) an empty cpus or mems list to the kernel.
              The kernel creates new cpusets (via mkdir(2)) with empty cpus
              and mems.  But the kernel will not allow an empty list to be
              written to the special cpus or mems files of a cpuset.

       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 outside of its current
              directory.

       ENOSPC Attempted to write(2) a list to a cpus file that did not include
              any online CPUs.

       ENOSPC Attempted to write(2) a list to a mems file that did not include
              any online memory nodes.

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

       EACCES Attempted to add a CPU or memory node to a cpuset that is not
              already in its parent.

       EACCES Attempted to set cpu_exclusive or mem_exclusive on a cpuset
              whose parent lacks the same setting.

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

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

       ENAMETOOLONG
              Attempted to read a /proc/<pid>/cpuset file for a cpuset path
              that is longer than the kernel page size.

       ENAMETOOLONG
              Attempted to create a cpuset whose base directory name is longer
              than 255 characters.

       ENAMETOOLONG
              Attempted to create a cpuset whose full pathname including the
              "/dev/cpuset/" prefix is longer than 4095 characters.

       EINVAL Specified a cpus or mems list to the kernel which included a
              range with the second number smaller than the first number.

       EINVAL Specified a cpus or mems list to the kernel which included an
              invalid character in the string.

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

SEE ALSO
       cat(1), echo(1), ls(1), mkdir(1), rmdir(1), sed(1), taskset(1),
       close(2), get_mempolicy(2), mbind(2), mkdir(2), open(2), read(2)
       rmdir(2), sched_getaffinity(2), sched_setaffinity(2), set_mempolicy(2),
       sched_setscheduler(2), taskset(2), write(2), libbitmask(3), proc(5),
       migratepages(8), numactl(8).

HISTORY
       Cpusets appeared in version 2.6.13 of the Linux kernel.

BUGS
       memory_pressure cpuset files can be opened for writing, creation or
       truncation, but then the write(2) fails with errno == EACCESS, and the
       creation and truncation options on open(2) have no affect.

AUTHOR
       This man page was written by Paul Jackson.



Linux 2.6                         2006-05-25                         CPUSET(4)