2011年（76）
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2011-08-24 17:57:51
cpuset - confine processes to processor and memory node subsets
The cpuset file system is a pseudo-file-system interface to the kernel cpuset
mechanism, which is used to 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 file system (see the EXAMPLE
section below), the administrator can configure the cpusets on a system to
control the processor and memory placement of processes on that system. By
default, if the cpuset configuration on a system is not modified or if the
cpuset file system is not even mounted, then the cpuset mechanism, though
present, has no affect on the system's behavior.
A cpuset defines a list of CPUs and memory nodes.
The CPUs of a system include all the logical processing units on which a
process can execute, including, if present, multiple processor cores within a
package and Hyper-Threads within a processor core. Memory nodes include all
distinct banks of main memory; small and SMP systems typically have just one
memory node that 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-file system,
where the top directory in the hierarchy (/dev/cpuset) represents the entire
system (all online CPUs and memory nodes) and any cpuset that is the child
(descendant) of another parent cpuset contains a subset of that parent's CPUs
and memory nodes. The directories and files representing cpusets have normal
file-system permissions.
Every process in the system belongs to exactly one cpuset. A process is
confined to only run on the CPUs in the cpuset it belongs to, and to allocate
memory only on the memory nodes in that cpuset. When a process 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 scheduling affinity
mechanism and the and 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 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 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 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 , , , and .
The pseudo-files in a cpuset directory represent internal kernel state and do
not have any persistent image on disk. Each of these per-cpuset files is
listed and described below.
tasks List of the process IDs (PIDs) of the processes in that cpuset. The
list is formatted as a series of ASCII decimal numbers, each followed
by a newline. A process may be added to a cpuset (automatically
removing it from the cpuset that previously contained it) by writing
its PID to that cpuset's tasks file (with or without a trailing
newline.)
Warning: only one PID may be written to the tasks file at a time. If a
string is written that contains more than one PID, only the first one
will be used.
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.
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.
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 non-empty cpus, then their cpus must overlap, because the cpus of
any cpuset are always a subset of the cpus of its parent 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.
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 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).
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.
memory_pressure (since Linux 2.6.16)
A measure of how much memory pressure the processes in this cpuset are
causing. See the Memory Pressure section, below. Unless
memory_pressure_enabled is enabled, always has value zero (0). This
file is read-only. See the WARNINGS section, below.
memory_pressure_enabled (since Linux 2.6.16)
Flag (0 or 1). This file is only present in the root cpuset, normally
/dev/cpuset. If set (1), the memory_pressure calculations are enabled
for all cpusets in the system. By default this is off (0). See the
Memory Pressure section, below.
memory_spread_page (since Linux 2.6.17)
Flag (0 or 1). If set (1), pages in the kernel page cache (file-system
buffers) are uniformly spread across the cpuset. By default this is
off (0) in the top cpuset, and inherited from the parent cpuset in
newly created cpusets. See the Memory Spread section, below.
memory_spread_slab (since Linux 2.6.17)
Flag (0 or 1). If set (1), the kernel slab caches for file I/O
(directory and inode structures) are uniformly spread across the
cpuset. By default this is off (0) in the top cpuset, and inherited
from the parent cpuset in newly created cpusets. See the Memory Spread
section, below.
sched_load_balance (since Linux 2.6.24)
Flag (0 or 1). If set (1, the default) the kernel will automatically
load balance processes in that cpuset over the allowed CPUs in that
cpuset. If cleared (0) the kernel will avoid load balancing 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.
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//cpuset , that displays the path of
the process's cpuset directory relative to the root of the cpuset file system.
Also the /proc//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.
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 file system pages, while isolating each job's user
allocation in its own cpuset. To do this, construct a large hardwall cpuset
to hold all the jobs, and construct child cpusets for each individual job
which are not 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.
If the notify_on_release flag is enabled (1) in a cpuset, then whenever the
last process in the cpuset leaves (exits or attaches to some other cpuset) and
the last child cpuset of that cpuset is removed, the kernel will run the
command /sbin/cpuset_release_agent, supplying the pathname (relative to the
mount point of the cpuset file system) of the abandoned cpuset. This enables
automatic removal of abandoned cpusets.
The default value of notify_on_release in the root cpuset at system boot is
disabled (0). The default value of other cpusets at creation is the current
value of their parent's notify_on_release setting.
The command /sbin/cpuset_release_agent is invoked, with the name (/dev/cpuset
relative path) of the to-be-released cpuset in argv[1].
The usual contents of the command /sbin/cpuset_release_agent is simply the
shell script:
#!/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.
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 re-prioritize jobs that are trying to
use more memory than allowed on the nodes assigned them, and with tightly
coupled, long-running, massively parallel scientific computing jobs that will
dramatically fail to meet required performance goals if they start to use more
memory than allowed to them.
This mechanism provides a very economical way for the batch manager to monitor
a cpuset for signs of memory pressure. It's up to the batch manager or other
user code to decide what action to take if it detects signs of memory
pressure.
Unless memory pressure calculation is enabled by setting the pseudo-file
/dev/cpuset/memory_pressure_enabled, it is not computed for any cpuset, and
reads from any memory_pressure always return zero, as represented by the ASCII
string "0\n". See the WARNINGS section, below.
A per-cpuset, running average is employed for the following reasons:
* Because this meter is per-cpuset rather than per-process or per virtual
memory region, the system load imposed by a batch scheduler monitoring this
metric is sharply reduced on large systems, because a scan of the tasklist
can be avoided on each set of queries.
* Because this meter is a running average rather than an accumulating
counter, a batch scheduler can detect memory pressure with a single read,
instead of having to read and accumulate results for a period of time.
* Because this meter is per-cpuset rather than per-process, the batch
scheduler can obtain the key information -- memory pressure in a cpuset --
with a single read, rather than having to query and accumulate results over
all the (dynamically changing) set of processes in the cpuset.
The memory_pressure of a cpuset is calculated using a per-cpuset simple
digital filter that is kept within the kernel. For each cpuset, this filter
tracks the recent rate at which processes attached to that cpuset enter the
kernel direct reclaim code.
The kernel direct reclaim code is entered whenever a process has to satisfy a
memory page request by first finding some other page to repurpose, due to lack
of any readily available already free pages. Dirty file system pages are
repurposed by first writing them to disk. Unmodified file system buffer pages
are repurposed by simply dropping them, though if that page is needed again,
it will have to be re-read from disk.
The memory_pressure file provides an integer number representing the recent
(half-life of 10 seconds) rate of entries to the direct reclaim code caused by
any process in the cpuset, in units of reclaims attempted per second, times
1000.
There are two Boolean flag files per cpuset that control where the kernel
allocates pages for the file-system buffers and related in-kernel data
structures. They are called memory_spread_page and memory_spread_slab.
If the per-cpuset Boolean flag file memory_spread_page is set, then the kernel
will spread the file-system buffers (page cache) evenly over all the nodes
that the faulting process is allowed to use, instead of preferring to put
those pages on the node where the process is running.
If the per-cpuset Boolean flag file memory_spread_slab is set, then the kernel
will spread some file-system-related slab caches, such as those for inodes and
directory entries, evenly over all the nodes that the 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 ) or
stack segment pages of a process.
By default, both kinds of memory spreading are off and the kernel prefers to
allocate memory pages on the node local to where the requesting process is
running. If that node is not allowed by the process's NUMA memory policy or
cpuset configuration or if there are insufficient free memory pages on that
node, then the kernel looks for the nearest node that is allowed and has
sufficient free memory.
When new cpusets are created, they inherit the memory spread settings of their
parent.
Setting memory spreading causes allocations for the affected page or slab
caches to ignore the process's NUMA memory policy and be spread instead.
However, the affect of these changes in memory placement caused by cpuset-
specified memory spreading is hidden from the or
calls. These two NUMA memory policy calls always appear to behave as if no
cpuset-specified memory spreading is in affect, even if it is. If cpuset
memory spreading is subsequently turned off, the NUMA memory policy most
recently specified by these calls is automatically re-applied.
Both memory_spread_page and memory_spread_slab are Boolean flag files. By
default they contain "0", meaning that the feature is off for that cpuset. If
a "1" is written to that file, that turns the named feature on.
Cpuset-specified memory spreading behaves similarly to what is known (in other
contexts) as round-robin or interleave memory placement.
Cpuset-specified memory spreading can provide substantial performance
improvements for jobs that:
a) need to place thread-local data on memory nodes close to the CPUs which are
running the threads that most frequently access that data; but also
b) need to access large file-system data sets that must to be spread across
the several nodes in the job's cpuset in order to fit.
Without this policy, the memory allocation across the nodes in the job's
cpuset can become very uneven, especially for jobs that might have just a
single thread initializing or reading in the data set.
Normally, under the default setting (disabled) of memory_migrate, once a page
is allocated (given a physical page of main memory) then that page stays on
whatever node it was allocated, so long as it remains allocated, even if the
cpuset's memory-placement policy mems subsequently changes.
When memory migration is enabled in a cpuset, if the mems setting of the
cpuset is changed, then any memory page in use by any process in the cpuset
that is on a memory node that is no longer allowed will be migrated to a
memory node that is allowed.
Furthermore, if a process is moved into a cpuset with memory_migrate enabled,
any memory pages it uses that were on memory nodes allowed in its previous
cpuset, but which are not allowed in its new cpuset, will be migrated to a
memory node allowed in the new cpuset.
The relative placement of a migrated page within the cpuset is preserved
during these migration operations if possible. For example, if the page was
on the second valid node of the prior cpuset, then the page will be placed on
the second valid node of the new cpuset, if possible.
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 .
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
) 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 affect, as we're already fully
load balancing.
Therefore in the above two situations, the flag sched_load_balance should be
disabled in the top cpuset, and only some of the smaller, child cpusets would
have this flag enabled.
When doing this, you don't usually want to leave any unpinned processes in the
top cpuset that might use 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.
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
. The setting of sched_relax_domain_level only applies to immediate
load balancing. Regardless of the sched_relax_domain_level setting, periodic
load balancing is attempted over all CPUs (unless disabled by turning off
sched_load_balance.) In any case, of course, tasks will only be scheduled to
run on CPUs allowed by their cpuset, as modified by
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 only done periodically,
not immediately when a CPU becomes available or another task becomes runnable.
The sched_relax_domain_level value of minus one (-1) always means use the
system default value. The system default value can vary by architecture and
kernel version. This system default value can be changed by kernel boot-time
"relax_domain_level=" argument.
In the case of multiple overlapping cpusets which have conflicting
sched_relax_domain_level values, then the highest such value applies to all
CPUs in any of the overlapping cpusets. In such cases, the value minus one
(-1) is the lowest value, overridden by any other value, and the value zero
(0) is the next lowest value.
The following formats are used to represent sets of CPUs and memory nodes.
The Mask Format is used to represent CPU and memory-node bitmasks in the
/proc//status file.
This format displays each 32-bit word in hexadecimal (using ASCII characters
"0" - "9" and "a" - "f"); words are filled with leading zeros, if required.
For masks longer than one word, a comma separator is used between words.
Words are displayed in big-endian order, which has the most significant bit
first. The hex digits within a word are also in big-endian order.
The number of 32-bit words displayed is the minimum number needed to display
all bits of the bitmask, based on the size of the bitmask.
Examples of the Mask Format:
00000001 # just bit 0 set
40000000,00000000,00000000 # just bit 94 set
00000001,00000000,00000000 # just bit 64 set
000000ff,00000000 # bits 32-39 set
00000000,000E3862 # 1,5,6,11-13,17-19 set
A mask with bits 0, 1, 2, 4, 8, 16, 32, and 64 set displays as:
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.
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 only be marked cpu_exclusive if its parent is.
* It can only be marked mem_exclusive if its parent is.
* If it is cpu_exclusive, its CPUs may not overlap any sibling.
* If it is memory_exclusive, its memory nodes may not overlap any sibling.
The permissions of a cpuset are determined by the permissions of the
directories and pseudo-files in the cpuset file system, normally mounted at
/dev/cpuset.
For instance, a process can put itself in some other cpuset (than its current
one) if it can write the tasks file for that cpuset. This requires execute
permission on the encompassing directories and write permission on the tasks
file.
An additional constraint is applied to requests to place some other process in
a cpuset. One process may not attach another to a cpuset unless it would have
permission to send that process a signal (see ).
A process may create a child cpuset if it can access and write the parent
cpuset directory. It can modify the CPUs or memory nodes in a cpuset if it
can access that cpuset's directory (execute permissions on the each of the
parent directories) and write the corresponding cpus or mems file.
There is one minor difference between the manner in which these permissions
are evaluated and the manner in which normal file-system operation permissions
are evaluated. The kernel interprets relative pathnames 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 to its cpuset directory beneath /dev/cpuset, which is a bit
unusual) or if some user code converts the relative cpuset path to a full
file-system path.
In theory, this means that user code should specify cpusets using absolute
pathnames, which requires knowing the mount point of the cpuset file system
(usually, but not necessarily, /dev/cpuset). In practice, all user level code
that this author is aware of simply assumes that if the cpuset file system is
mounted, then it is mounted at /dev/cpuset. Furthermore, it is common
practice for carefully written user code to verify the presence of the pseudo-
file /dev/cpuset/tasks in order to verify that the cpuset pseudo-file system
is currently mounted.
By default, the per-cpuset file memory_pressure always contains zero (0).
Unless this feature is enabled by writing "1" to the pseudo-file
/dev/cpuset/memory_pressure_enabled, the kernel does not compute per-cpuset
memory_pressure.
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 system call fails. For example, if the
command:
echo 19 > mems
failed because memory node 19 was not allowed (perhaps the current system does
not have a memory node 19), then the echo command might not display any error.
It is better to use the /bin/echo external command to change cpuset file
settings, as this command will display errors, as in the example:
/bin/echo 19 > mems
/bin/echo: write error: Invalid argument
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 only refer to online CPUs and memory
nodes when using hot-plug to add or remove such resources.
A few kernel-critical, internal memory-allocation requests, marked GFP_ATOMIC,
must be satisfied immediately. The kernel may drop some request or
malfunction if one of these allocations fail. If such a request cannot be
satisfied within the current process's cpuset, then we relax the cpuset, and
look for memory anywhere we can find it. It's better to violate the cpuset
than stress the kernel.
Allocations of memory requested by kernel drivers while processing an
interrupt lack any relevant process context, and are not confined by cpusets.
You can use the 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 on a special cpuset file with a length larger than
some kernel-determined upper limit on the length of such writes.
EACCES Attempted to 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 , 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 , cpu_exclusive or mem_exclusive on a
cpuset whose parent lacks the same setting.
EACCES Attempted to a memory_pressure file.
EACCES Attempted to create a file in a cpuset directory.
EBUSY Attempted to remove, using , a cpuset with attached processes.
EBUSY Attempted to remove, using , 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 , a cpuset that already exists.
EEXIST Attempted to a cpuset to a name that already exists.
EFAULT Attempted to or a cpuset file using a buffer that is
outside the writing processes accessible address space.
EINVAL Attempted to change a cpuset, using , in a way that would
violate a cpu_exclusive or mem_exclusive attribute of that cpuset or
any of its siblings.
EINVAL Attempted to an empty cpus or mems list to a cpuset which has
attached processes or child cpusets.
EINVAL Attempted to a cpus or mems list which included a range with
the second number smaller than the first number.
EINVAL Attempted to a cpus or mems list which included an invalid
character in the string.
EINVAL Attempted to a list to a cpus file that did not include any
online CPUs.
EINVAL Attempted to a list to a mems file that did not include any
online memory nodes.
EINVAL Attempted to a list to a mems file that included a node that
held no memory.
EIO Attempted to a string to a cpuset tasks file that does not
begin with an ASCII decimal integer.
EIO Attempted to a cpuset into a different directory.
ENAMETOOLONG
Attempted to a /proc//cpuset file for a cpuset path that
is longer than the kernel page size.
ENAMETOOLONG
Attempted to create, using , a cpuset whose base directory name
is longer than 255 characters.
ENAMETOOLONG
Attempted to create, using , 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
was attempted on one of the pseudo-files in the cpuset
directory.
ENOENT Attempted to create, using , a cpuset in a parent cpuset that
doesn't exist.
ENOENT Attempted to or 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 the process ID (PID) of a process to a cpuset
tasks file when the cpuset had an empty cpus or empty mems setting.
ENOSPC Attempted to an empty cpus or mems setting to a cpuset that
has tasks attached.
ENOTDIR
Attempted to a nonexistent cpuset.
EPERM Attempted to remove a file from a cpuset directory.
ERANGE Specified a cpus or mems list to the kernel which included a number too
large for the kernel to set in its bitmasks.
ESRCH Attempted to the process ID (PID) of a nonexistent process to
a cpuset tasks file.
Cpusets appeared in version 2.6.12 of the Linux kernel.
Despite its name, the pid parameter is actually a thread ID, and each thread
in a threaded group can be attached to a different cpuset. The value returned
from a call to can be passed in the argument pid.
memory_pressure cpuset files can be opened for writing, creation, or
truncation, but then the fails with errno set to EACCES, and the
creation and truncation options on have no affect.
The following examples demonstrate querying and setting cpuset options using
shell commands.
To create a new cpuset and attach the current command shell to it, the steps
are:
1) mkdir /dev/cpuset (if not already done)
2) mount -t cpuset none /dev/cpuset (if not already done)
3) Create the new cpuset using mkdir(1).
4) Assign CPUs and memory nodes to the new cpuset.
5) Attach the shell to the new cpuset.
For example, the following sequence of commands will set up a cpuset named
"Charlie", containing just CPUs 2 and 3, and memory node 1, and then attach
the current shell to that cpuset.
$ mkdir /dev/cpuset
$ mount -t cpuset cpuset /dev/cpuset
$ cd /dev/cpuset
$ mkdir Charlie
$ cd Charlie
$ /bin/echo 2-3 > cpus
$ /bin/echo 1 > mems
$ /bin/echo $$ > tasks
# The current shell is now running in cpuset Charlie
# The next line should display '/Charlie'
$ cat /proc/self/cpuset
To migrate a job (the set of processes attached to a cpuset) to different CPUs
and memory nodes in the system, including moving the memory pages currently
allocated to that job, perform the following steps.
1) Let's say we want to move the job in cpuset alpha (CPUs 4-7 and memory
nodes 2-3) to a new cpuset beta (CPUs 16-19 and memory nodes 8-9).
2) First create the new cpuset beta.
3) Then allow CPUs 16-19 and memory nodes 8-9 in beta.
4) Then enable memory_migration in beta.
5) Then move each process from alpha to beta.
The following sequence of commands accomplishes this.
$ cd /dev/cpuset
$ mkdir beta
$ cd beta
$ /bin/echo 16-19 > cpus
$ /bin/echo 8-9 > mems
$ /bin/echo 1 > memory_migrate
$ while read i; do /bin/echo $i; done < ../alpha/tasks > tasks
The above should move any processes in alpha to beta, and any memory held by
these processes on memory nodes 2-3 to memory nodes 8-9, respectively.
Notice that the last step of the above sequence did not do:
$ cp ../alpha/tasks tasks
The while loop, rather than the seemingly easier use of the cp(1) command, was
necessary because only one process PID at a time may be written to the tasks
file.
The same affect (writing one PID at a time) as the while loop can be
accomplished more efficiently, in fewer keystrokes and in syntax that works on
any shell, but alas more obscurely, by using the -u (unbuffered) option of
sed(1):
$ sed -un p < ../alpha/tasks > tasks
taskset(1), , , , ,
, , , ,
, , migratepages(8), numactl(8)
The kernel source file Documentation/cpusets.txt.
This page is part of release 3.21 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at .