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NAME | DESCRIPTION | FILES | EXTENDED CAPABILITIES | FORMATS | RULES | PERMISSIONS | WARNINGS | EXCEPTIONS | ERRORS | VERSIONS | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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cpuset(7) Miscellaneous Information Manual cpuset(7)
cpuset - confine processes to processor and memory node subsets
The cpuset filesystem is a pseudo-filesystem interface to the
kernel cpuset mechanism, which is used to control the processor
placement and memory placement of processes. It is commonly
mounted at /dev/cpuset.
On systems with kernels compiled with built in support for
cpusets, all processes are attached to a cpuset, and cpusets are
always present. If a system supports cpusets, then it will have
the entry nodev cpuset in the file /proc/filesystems. By mounting
the cpuset filesystem (see the 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 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.
Each directory below /dev/cpuset represents a cpuset and contains
a fixed set of pseudo-files describing the state of that cpuset.
New cpusets are created using the mkdir(2) system call or the
mkdir(1) command. The properties of a cpuset, such as its flags,
allowed CPUs and memory nodes, and attached processes, are queried
and modified by reading or writing to the appropriate file in that
cpuset's directory, as listed below.
The pseudo-files in each cpuset directory are automatically
created when the cpuset is created, as a result of the mkdir(2)
invocation. It is not possible to directly add or remove these
pseudo-files.
A cpuset directory that contains no child cpuset directories, and
has no attached processes, can be removed using rmdir(2) or
rmdir(1). It is not necessary, or possible, to remove the pseudo-
files inside the directory before removing it.
The pseudo-files in each cpuset directory are small text files
that may be read and written using traditional shell utilities
such as cat(1), and echo(1), or from a program by using file I/O
library functions or system calls, such as open(2), read(2),
write(2), and close(2).
The pseudo-files in a cpuset directory represent internal kernel
state and do not have any persistent image on disk. Each of these
per-cpuset files is listed and described below.
tasks List of the process IDs (PIDs) of the processes in that
cpuset. The list is formatted as a series of ASCII decimal
numbers, each followed by a newline. A process may be
added to a cpuset (automatically removing it from the
cpuset that previously contained it) by writing its PID to
that cpuset's tasks file (with or without a trailing
newline).
Warning: only one PID may be written to the tasks file at a
time. If a string is written that contains more than one
PID, only the first one will be used.
notify_on_release
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.
cpuset.cpus
List of the physical numbers of the CPUs on which processes
in that cpuset are allowed to execute. See List Format
below for a description of the format of cpus.
The CPUs allowed to a cpuset may be changed by writing a
new list to its cpus file.
cpuset.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).
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.
cpuset.mems
List of memory nodes on which processes in this cpuset are
allowed to allocate memory. See List Format below for a
description of the format of mems.
cpuset.mem_exclusive
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, 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
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 cpuset.
Hardwall
A cpuset that has mem_exclusive or mem_hardwall set is a hardwall
cpuset. A hardwall cpuset restricts kernel allocations for page,
buffer, and other data commonly shared by the kernel across
multiple users. All cpusets, whether hardwall or not, restrict
allocations of memory for user space.
This enables configuring a system so that several independent jobs
can share common kernel data, such as filesystem pages, while
isolating each job's user allocation in its own cpuset. To do
this, construct a large hardwall cpuset to hold all the jobs, and
construct child cpusets for each individual job which are not
hardwall cpusets.
Only a small amount of kernel memory, such as requests from
interrupt handlers, is allowed to be taken outside even a hardwall
cpuset.
Notify on release
If the notify_on_release flag is enabled (1) in a cpuset, then
whenever the last process in the cpuset leaves (exits or attaches
to some other cpuset) and the last child cpuset of that cpuset is
removed, the kernel will run the command
/sbin/cpuset_release_agent, supplying the pathname (relative to
the mount point of the cpuset filesystem) of the abandoned cpuset.
This enables automatic removal of abandoned cpusets.
The default value of notify_on_release in the root cpuset at
system boot is disabled (0). The default value of other cpusets
at creation is the current value of their parent's
notify_on_release setting.
The command /sbin/cpuset_release_agent is invoked, with the name
(/dev/cpuset relative path) of the to-be-released cpuset in
argv[1].
The usual contents of the command /sbin/cpuset_release_agent is
simply the shell script:
#!/bin/sh
rmdir /dev/cpuset/$1
As with other flag values below, this flag can be changed by
writing an ASCII number 0 or 1 (with optional trailing newline)
into the file, to clear or set the flag, respectively.
Memory pressure
The memory_pressure of a cpuset provides a simple per-cpuset
running average of the rate that the processes in a cpuset are
attempting to free up in-use memory on the nodes of the cpuset to
satisfy additional memory requests.
This enables batch managers that are monitoring jobs running in
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
reasons:
• Because this meter is per-cpuset rather than per-process or per
virtual memory region, the system load imposed by a batch
scheduler monitoring this metric is sharply reduced on large
systems, because a scan of the tasklist can be avoided on each
set of queries.
• Because this meter is a running average rather than an
accumulating counter, a batch scheduler can detect memory
pressure with a single read, instead of having to read and
accumulate results for a period of time.
• Because this meter is per-cpuset rather than per-process, the
batch scheduler can obtain the key information—memory pressure
in a cpuset—with a single read, rather than having to query and
accumulate results over all the (dynamically changing) set of
processes in the cpuset.
The memory_pressure of a cpuset is calculated using a per-cpuset
simple digital filter that is kept within the kernel. For each
cpuset, this filter tracks the recent rate at which processes
attached to that cpuset enter the kernel direct reclaim code.
The kernel direct reclaim code is entered whenever a process has
to satisfy a memory page request by first finding some other page
to repurpose, due to lack of any readily available already free
pages. Dirty 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
placement.
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 default.)
This default load balancing across all CPUs is not well suited to
the following two situations:
• On large systems, load balancing across many CPUs is expensive.
If the system is managed using cpusets to place independent
jobs on separate sets of CPUs, full load balancing is
unnecessary.
• Systems supporting real-time on some CPUs need to minimize
system overhead on those CPUs, including avoiding process load
balancing if that is not needed.
When the per-cpuset flag sched_load_balance is enabled (the
default setting), it requests load balancing across all the CPUs
in that cpuset's allowed CPUs, ensuring that load balancing can
move a process (not otherwise pinned, as by sched_setaffinity(2))
from any CPU in that cpuset to any other.
When the per-cpuset flag sched_load_balance is disabled, then the
scheduler will avoid load balancing across the CPUs in that
cpuset, except in so far as is necessary because some overlapping
cpuset has sched_load_balance enabled.
So, for example, if the top cpuset has the flag sched_load_balance
enabled, then the scheduler will load balance across all CPUs, and
the setting of the sched_load_balance flag in other cpusets has no
effect, as we're already fully load balancing.
Therefore in the above two situations, the flag sched_load_balance
should be disabled in the top cpuset, and only some of the
smaller, child cpusets would have this flag enabled.
When doing this, you don't usually want to leave any unpinned
processes in the top cpuset that might use nontrivial amounts of
CPU, as such processes may be artificially constrained to some
subset of CPUs, depending on the particulars of this flag setting
in descendant cpusets. Even if such a process could use spare CPU
cycles in some other CPUs, the kernel scheduler might not consider
the possibility of load balancing that process to the underused
CPU.
Of course, processes pinned to a particular CPU can be left in a
cpuset that disables sched_load_balance as those processes aren't
going 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=" 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, -1 is the lowest value, overridden by any other value, and
0 is the next lowest value.
The following formats are used to represent sets of CPUs and
memory nodes.
Mask format
The Mask Format is used to represent CPU and memory-node 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:
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
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
sibling.
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.
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
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.
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
process.
EACCES Attempted to add, using write(2), a CPU or memory node to a
cpuset, when that CPU or memory node was not already in its
parent.
EACCES Attempted to set, using write(2), 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
cpusets.
EBUSY Attempted to remove a CPU or memory node from a cpuset that
is also in a child of that cpuset.
EEXIST Attempted to create, using mkdir(2), a cpuset that already
exists.
EEXIST Attempted to rename(2) a cpuset to a name that already
exists.
EFAULT Attempted to read(2) or write(2) a cpuset file using a
buffer that is outside the writing processes accessible
address space.
EINVAL Attempted to change a cpuset, using write(2), in a way that
would violate a cpu_exclusive or mem_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 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 directory.
ENAMETOOLONG
Attempted to read(2) a /proc/pid/cpuset file for a cpuset
path that is longer than the kernel page size.
ENAMETOOLONG
Attempted to create, using mkdir(2), a cpuset whose base
directory name is longer than 255 characters.
ENAMETOOLONG
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.
ENOTDIR
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.
Cpusets appeared in Linux 2.6.12.
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.
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.
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
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)
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
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. 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
[email protected]
Linux man-pages 6.15 2025-05-17 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)
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